LinuxLists.cc - [PATCH v2 0/2] Clean up x86

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Subject: Re: [PATCH v2 0/2] Clean up x86_32 stackprotector

On Sat, Feb 13, 2021 at 8:19 PM Andy Lutomirski <[email protected]> wrote:
>
> x86_32 stackprotector is a maintenance nightmare. Clean it up. This
> disables stackprotector on x86_32 on GCC 8.1 and on all clang
> versions. Some clang people are cc'd.
>
> Changes from v1:
> - Changelog fixes.
> - Comment fixes (mostly from Sean).
> - Fix the !SMP case.
>
> Andy Lutomirski (2):
> x86/stackprotector/32: Make the canary into a regular percpu variable
> x86/entry/32: Remove leftover macros after stackprotector cleanups
>

Thanks Andy for bringing this up as v2.

AFAICS, LLVM bug #47479 was involved here (see [1]).
Maybe Nick can say some words.

After patching my LLVM toolchain I decided to drop this and integrated
the (kernel-side) diff from Nick (see [1]) in my custom patch-series.

Is this your x86/unify_stack_canary Git branch (see [2])?

- Sedat -

[1] https://bugs.llvm.org/show_bug.cgi?id=47479
[2] https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/unify_stack_canary

> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 ++
> arch/x86/entry/entry_32.S | 95 +----------------------
> arch/x86/include/asm/processor.h | 15 +---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 ++-----
> arch/x86/include/asm/stackprotector.h | 79 ++++---------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +-
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 -
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 265 deletions(-)
>
> --
> 2.29.2
>

2021-02-13 19:51:29

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Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Sat, Feb 13, 2021 at 8:19 PM Andy Lutomirski <[email protected]> wrote:
>
> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> percpu storage. It's even nastier because it means that whether %gs
> contains userspace state or kernel state while running kernel code
> depends on whether stackprotector is enabled (this is
> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> that segment selectors work. Supporting both variants is a
> maintenance and testing mess.
>
> Merely rearranging so that percpu and the stack canary
> share the same segment would be messy as the 32-bit percpu address
> layout isn't currently compatible with putting a variable at a fixed
> offset.
>
> Fortunately, GCC 8.1 added options that allow the stack canary to be
> accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
> percpu variable. This lets us get rid of all of the code to manage the
> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>
> (That name is special. We could use any symbol we want for the
> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
> name other than __stack_chk_guard.)
>
> This patch forcibly disables stackprotector on older compilers that
> don't support the new options and makes the stack canary into a
> percpu variable. The "lazy GS" approach is now used for all 32-bit
> configurations.
>
> This patch also makes load_gs_index() work on 32-bit kernels. On
> 64-bit kernels, it loads the GS selector and updates the user
> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> it loads the GS selector and updates GSBASE, which is now
> always the user base. This means that the overall effect is
> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>
> Cc: Sedat Dilek <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 +++
> arch/x86/entry/entry_32.S | 56 ++--------------
> arch/x86/include/asm/processor.h | 15 ++---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 +++------
> arch/x86/include/asm/stackprotector.h | 79 +++++------------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +-----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +--
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 --
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 226 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 21f851179ff0..12d8bf011d08 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -353,10 +353,6 @@ config X86_64_SMP
> def_bool y
> depends on X86_64 && SMP
>
> -config X86_32_LAZY_GS
> - def_bool y
> - depends on X86_32 && !STACKPROTECTOR
> -
> config ARCH_SUPPORTS_UPROBES
> def_bool y
>
> @@ -379,7 +375,8 @@ config CC_HAS_SANE_STACKPROTECTOR
> default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
> help
> We have to make sure stack protector is unconditionally disabled if
> - the compiler produces broken code.
> + the compiler produces broken code or if it does not let us control
> + the segment on 32-bit kernels.
>
> menu "Processor type and features"
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7116da3980be..0b5cd8c49ccb 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -76,6 +76,14 @@ ifeq ($(CONFIG_X86_32),y)
>
> # temporary until string.h is fixed
> KBUILD_CFLAGS += -ffreestanding
> +
> + ifeq ($(CONFIG_STACKPROTECTOR),y)
> + ifeq ($(CONFIG_SMP),y)
> + KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
> + else
> + KBUILD_CFLAGS += -mstack-protector-guard=global
> + endif
> + endif

Just FYI:

According to [1] the following clang compiler flags are available:

-mstack-protector-guard-offset=<arg>
Use the given offset for addressing the stack-protector guard

-mstack-protector-guard-reg=<arg>
Use the given reg for addressing the stack-protector guard

-mstack-protector-guard=<arg>
Use the given guard (global, tls) for addressing the stack-protector guard

Looks like -mstack-protector-guard-symbol=<arg> is not (yet) available.

- Sedat -

[1] https://clang.llvm.org/docs/ClangCommandLineReference.html

> else
> BITS := 64
> UTS_MACHINE := x86_64
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index df8c017e6161..eb0cb662bca5 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -20,7 +20,7 @@
> * 1C(%esp) - %ds
> * 20(%esp) - %es
> * 24(%esp) - %fs
> - * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
> + * 28(%esp) - unused -- was %gs on old stackprotector kernels
> * 2C(%esp) - orig_eax
> * 30(%esp) - %eip
> * 34(%esp) - %cs
> @@ -56,14 +56,9 @@
> /*
> * User gs save/restore
> *
> - * %gs is used for userland TLS and kernel only uses it for stack
> - * canary which is required to be at %gs:20 by gcc. Read the comment
> - * at the top of stackprotector.h for more info.
> - *
> - * Local labels 98 and 99 are used.
> + * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
> + * will remove it entirely.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> -
> /* unfortunately push/pop can't be no-op */
> .macro PUSH_GS
> pushl $0
> @@ -86,49 +81,6 @@
> .macro SET_KERNEL_GS reg
> .endm
>
> -#else /* CONFIG_X86_32_LAZY_GS */
> -
> -.macro PUSH_GS
> - pushl %gs
> -.endm
> -
> -.macro POP_GS pop=0
> -98: popl %gs
> - .if \pop <> 0
> - add $\pop, %esp
> - .endif
> -.endm
> -.macro POP_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, (%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro PTGS_TO_GS
> -98: mov PT_GS(%esp), %gs
> -.endm
> -.macro PTGS_TO_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, PT_GS(%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro GS_TO_REG reg
> - movl %gs, \reg
> -.endm
> -.macro REG_TO_PTGS reg
> - movl \reg, PT_GS(%esp)
> -.endm
> -.macro SET_KERNEL_GS reg
> - movl $(__KERNEL_STACK_CANARY), \reg
> - movl \reg, %gs
> -.endm
> -
> -#endif /* CONFIG_X86_32_LAZY_GS */
>
> /* Unconditionally switch to user cr3 */
> .macro SWITCH_TO_USER_CR3 scratch_reg:req
> @@ -779,7 +731,7 @@ SYM_CODE_START(__switch_to_asm)
>
> #ifdef CONFIG_STACKPROTECTOR
> movl TASK_stack_canary(%edx), %ebx
> - movl %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
> + movl %ebx, PER_CPU_VAR(__stack_chk_guard)
> #endif
>
> #ifdef CONFIG_RETPOLINE
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index c20a52b5534b..c59dff4bbc38 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -441,6 +441,9 @@ struct fixed_percpu_data {
> * GCC hardcodes the stack canary as %gs:40. Since the
> * irq_stack is the object at %gs:0, we reserve the bottom
> * 48 bytes of the irq stack for the canary.
> + *
> + * Once we are willing to require -mstack-protector-guard-symbol=
> + * support for x86_64 stackprotector, we can get rid of this.
> */
> char gs_base[40];
> unsigned long stack_canary;
> @@ -461,17 +464,7 @@ extern asmlinkage void ignore_sysret(void);
> void current_save_fsgs(void);
> #else /* X86_64 */
> #ifdef CONFIG_STACKPROTECTOR
> -/*
> - * Make sure stack canary segment base is cached-aligned:
> - * "For Intel Atom processors, avoid non zero segment base address
> - * that is not aligned to cache line boundary at all cost."
> - * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
> - */
> -struct stack_canary {
> - char __pad[20]; /* canary at %gs:20 */
> - unsigned long canary;
> -};
> -DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> #endif
> /* Per CPU softirq stack pointer */
> DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index d8324a236696..b2c4c12d237c 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -37,7 +37,10 @@ struct pt_regs {
> unsigned short __esh;
> unsigned short fs;
> unsigned short __fsh;
> - /* On interrupt, gs and __gsh store the vector number. */
> + /*
> + * On interrupt, gs and __gsh store the vector number. They never
> + * store gs any more.
> + */
> unsigned short gs;
> unsigned short __gsh;
> /* On interrupt, this is the error code. */
> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> index 7fdd4facfce7..72044026eb3c 100644
> --- a/arch/x86/include/asm/segment.h
> +++ b/arch/x86/include/asm/segment.h
> @@ -95,7 +95,7 @@
> *
> * 26 - ESPFIX small SS
> * 27 - per-cpu [ offset to per-cpu data area ]
> - * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
> + * 28 - unused
> * 29 - unused
> * 30 - unused
> * 31 - TSS for double fault handler
> @@ -118,7 +118,6 @@
>
> #define GDT_ENTRY_ESPFIX_SS 26
> #define GDT_ENTRY_PERCPU 27
> -#define GDT_ENTRY_STACK_CANARY 28
>
> #define GDT_ENTRY_DOUBLEFAULT_TSS 31
>
> @@ -158,12 +157,6 @@
> # define __KERNEL_PERCPU 0
> #endif
>
> -#ifdef CONFIG_STACKPROTECTOR
> -# define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY*8)
> -#else
> -# define __KERNEL_STACK_CANARY 0
> -#endif
> -
> #else /* 64-bit: */
>
> #include <asm/cache.h>
> @@ -364,22 +357,15 @@ static inline void __loadsegment_fs(unsigned short value)
> asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
>
> /*
> - * x86-32 user GS accessors:
> + * x86-32 user GS accessors. This is ugly and could do with some cleaning up.
> */
> #ifdef CONFIG_X86_32
> -# ifdef CONFIG_X86_32_LAZY_GS
> -# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> -# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> -# define task_user_gs(tsk) ((tsk)->thread.gs)
> -# define lazy_save_gs(v) savesegment(gs, (v))
> -# define lazy_load_gs(v) loadsegment(gs, (v))
> -# else /* X86_32_LAZY_GS */
> -# define get_user_gs(regs) (u16)((regs)->gs)
> -# define set_user_gs(regs, v) do { (regs)->gs = (v); } while (0)
> -# define task_user_gs(tsk) (task_pt_regs(tsk)->gs)
> -# define lazy_save_gs(v) do { } while (0)
> -# define lazy_load_gs(v) do { } while (0)
> -# endif /* X86_32_LAZY_GS */
> +# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> +# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> +# define task_user_gs(tsk) ((tsk)->thread.gs)
> +# define lazy_save_gs(v) savesegment(gs, (v))
> +# define lazy_load_gs(v) loadsegment(gs, (v))
> +# define load_gs_index(v) loadsegment(gs, (v))
> #endif /* X86_32 */
>
> #endif /* !__ASSEMBLY__ */
> diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> index 7fb482f0f25b..b6ffe58c70fa 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -5,30 +5,23 @@
> * Stack protector works by putting predefined pattern at the start of
> * the stack frame and verifying that it hasn't been overwritten when
> * returning from the function. The pattern is called stack canary
> - * and unfortunately gcc requires it to be at a fixed offset from %gs.
> - * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64
> - * and x86_32 use segment registers differently and thus handles this
> - * requirement differently.
> + * and unfortunately gcc historically required it to be at a fixed offset
> + * from the percpu segment base. On x86_64, the offset is 40 bytes.
> *
> - * On x86_64, %gs is shared by percpu area and stack canary. All
> - * percpu symbols are zero based and %gs points to the base of percpu
> - * area. The first occupant of the percpu area is always
> - * fixed_percpu_data which contains stack_canary at offset 40. Userland
> - * %gs is always saved and restored on kernel entry and exit using
> - * swapgs, so stack protector doesn't add any complexity there.
> + * The same segment is shared by percpu area and stack canary. On
> + * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
> + * base of percpu area. The first occupant of the percpu area is always
> + * fixed_percpu_data which contains stack_canary at the approproate
> + * offset. On x86_32, the stack canary is just a regular percpu
> + * variable.
> *
> - * On x86_32, it's slightly more complicated. As in x86_64, %gs is
> - * used for userland TLS. Unfortunately, some processors are much
> - * slower at loading segment registers with different value when
> - * entering and leaving the kernel, so the kernel uses %fs for percpu
> - * area and manages %gs lazily so that %gs is switched only when
> - * necessary, usually during task switch.
> + * Putting percpu data in %fs on 32-bit is a minor optimization compared to
> + * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
> + * to load 0 into %fs on exit to usermode, whereas with percpu data in
> + * %gs, we are likely to load a non-null %gs on return to user mode.
> *
> - * As gcc requires the stack canary at %gs:20, %gs can't be managed
> - * lazily if stack protector is enabled, so the kernel saves and
> - * restores userland %gs on kernel entry and exit. This behavior is
> - * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
> - * system.h to hide the details.
> + * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
> + * support, we can remove some of this complexity.
> */
>
> #ifndef _ASM_STACKPROTECTOR_H
> @@ -44,14 +37,6 @@
> #include <linux/random.h>
> #include <linux/sched.h>
>
> -/*
> - * 24 byte read-only segment initializer for stack canary. Linker
> - * can't handle the address bit shifting. Address will be set in
> - * head_32 for boot CPU and setup_per_cpu_areas() for others.
> - */
> -#define GDT_STACK_CANARY_INIT \
> - [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
> -
> /*
> * Initialize the stackprotector canary value.
> *
> @@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void)
> #ifdef CONFIG_X86_64
> this_cpu_write(fixed_percpu_data.stack_canary, canary);
> #else
> - this_cpu_write(stack_canary.canary, canary);
> + this_cpu_write(__stack_chk_guard, canary);
> #endif
> }
>
> @@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> #ifdef CONFIG_X86_64
> per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
> #else
> - per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
> -#endif
> -}
> -
> -static inline void setup_stack_canary_segment(int cpu)
> -{
> -#ifdef CONFIG_X86_32
> - unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
> - struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
> - struct desc_struct desc;
> -
> - desc = gdt_table[GDT_ENTRY_STACK_CANARY];
> - set_desc_base(&desc, canary);
> - write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
> -#endif
> -}
> -
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
> + per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
> #endif
> }
>
> #else /* STACKPROTECTOR */
>
> -#define GDT_STACK_CANARY_INIT
> -
> /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
>
> -static inline void setup_stack_canary_segment(int cpu)
> -{ }
> -
> static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> { }
>
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm volatile ("mov %0, %%gs" : : "r" (0));
> -#endif
> -}
> -
> #endif /* STACKPROTECTOR */
> #endif /* _ASM_STACKPROTECTOR_H */
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index fdbd9d7b7bca..7b132d0312eb 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -13,12 +13,10 @@
> /* image of the saved processor state */
> struct saved_context {
> /*
> - * On x86_32, all segment registers, with the possible exception of
> - * gs, are saved at kernel entry in pt_regs.
> + * On x86_32, all segment registers except gs are saved at kernel
> + * entry in pt_regs.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> u16 gs;
> -#endif
> unsigned long cr0, cr2, cr3, cr4;
> u64 misc_enable;
> bool misc_enable_saved;
> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> index 6e043f295a60..2b411cd00a4e 100644
> --- a/arch/x86/kernel/asm-offsets_32.c
> +++ b/arch/x86/kernel/asm-offsets_32.c
> @@ -53,11 +53,6 @@ void foo(void)
> offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
> offsetofend(struct cpu_entry_area, entry_stack_page.stack));
>
> -#ifdef CONFIG_STACKPROTECTOR
> - BLANK();
> - OFFSET(stack_canary_offset, stack_canary, canary);
> -#endif
> -
> BLANK();
> DEFINE(EFI_svam, offsetof(efi_runtime_services_t, set_virtual_address_map));
> }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 35ad8480c464..f208569d2d3b 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -161,7 +161,6 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
>
> [GDT_ENTRY_ESPFIX_SS] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> [GDT_ENTRY_PERCPU] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> - GDT_STACK_CANARY_INIT
> #endif
> } };
> EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
> @@ -599,7 +598,6 @@ void load_percpu_segment(int cpu)
> __loadsegment_simple(gs, 0);
> wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
> #endif
> - load_stack_canary_segment();
> }
>
> #ifdef CONFIG_X86_32
> @@ -1793,7 +1791,8 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
> EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
>
> #ifdef CONFIG_STACKPROTECTOR
> -DEFINE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> +EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
> #endif
>
> #endif /* CONFIG_X86_64 */
> diff --git a/arch/x86/kernel/doublefault_32.c b/arch/x86/kernel/doublefault_32.c
> index 759d392cbe9f..d1d49e3d536b 100644
> --- a/arch/x86/kernel/doublefault_32.c
> +++ b/arch/x86/kernel/doublefault_32.c
> @@ -100,9 +100,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
> .ss = __KERNEL_DS,
> .ds = __USER_DS,
> .fs = __KERNEL_PERCPU,
> -#ifndef CONFIG_X86_32_LAZY_GS
> - .gs = __KERNEL_STACK_CANARY,
> -#endif
> + .gs = 0,
>
> .__cr3 = __pa_nodebug(swapper_pg_dir),
> },
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 7ed84c282233..67f590425d90 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp)
> movl $(__KERNEL_PERCPU), %eax
> movl %eax,%fs # set this cpu's percpu
>
> - movl $(__KERNEL_STACK_CANARY),%eax
> - movl %eax,%gs
> + xorl %eax,%eax
> + movl %eax,%gs # clear possible garbage in %gs
>
> xorl %eax,%eax # Clear LDT
> lldt %ax
> @@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp)
> */
> __INIT
> setup_once:
> -#ifdef CONFIG_STACKPROTECTOR
> - /*
> - * Configure the stack canary. The linker can't handle this by
> - * relocation. Manually set base address in stack canary
> - * segment descriptor.
> - */
> - movl $gdt_page,%eax
> - movl $stack_canary,%ecx
> - movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
> - movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
> -#endif
> -
> andl $0,setup_once_ref /* Once is enough, thanks */
> ret
>
> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index fd945ce78554..0941d2f44f2a 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -224,7 +224,6 @@ void __init setup_per_cpu_areas(void)
> per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
> per_cpu(cpu_number, cpu) = cpu;
> setup_percpu_segment(cpu);
> - setup_stack_canary_segment(cpu);
> /*
> * Copy data used in early init routines from the
> * initial arrays to the per cpu data areas. These
> diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
> index 64a496a0687f..3c883e064242 100644
> --- a/arch/x86/kernel/tls.c
> +++ b/arch/x86/kernel/tls.c
> @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx,
> savesegment(fs, sel);
> if (sel == modified_sel)
> loadsegment(fs, sel);
> -
> - savesegment(gs, sel);
> - if (sel == modified_sel)
> - load_gs_index(sel);
> #endif
>
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, sel);
> if (sel == modified_sel)
> - loadsegment(gs, sel);
> -#endif
> + load_gs_index(sel);
> } else {
> #ifdef CONFIG_X86_64
> if (p->thread.fsindex == modified_sel)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f923e14e87df..ec39073b4897 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> #ifdef CONFIG_X86_64
> loadsegment(fs, svm->host.fs);
> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> - load_gs_index(svm->host.gs);
> -#else
> -#ifdef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> + load_gs_index(svm->host.gs);
>
> for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> wrmsrl(host_save_user_msrs[i].index,
> @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> } else {
> __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>
> + /* Restore the percpu segment immediately. */
> #ifdef CONFIG_X86_64
> native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
> loadsegment(fs, svm->host.fs);
> -#ifndef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> }
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 4229950a5d78..7f89a091f1fb 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -404,10 +404,6 @@ static short get_segment_selector(struct pt_regs *regs, int seg_reg_idx)
> case INAT_SEG_REG_FS:
> return (unsigned short)(regs->fs & 0xffff);
> case INAT_SEG_REG_GS:
> - /*
> - * GS may or may not be in regs as per CONFIG_X86_32_LAZY_GS.
> - * The macro below takes care of both cases.
> - */
> return get_user_gs(regs);
> case INAT_SEG_REG_IGNORE:
> default:
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 43b4d864817e..afbf0bb252da 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -45,10 +45,8 @@
>
> #define PVH_GDT_ENTRY_CS 1
> #define PVH_GDT_ENTRY_DS 2
> -#define PVH_GDT_ENTRY_CANARY 3
> #define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8)
> #define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8)
> -#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8)
>
> SYM_CODE_START_LOCAL(pvh_start_xen)
> cld
> @@ -109,17 +107,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>
> #else /* CONFIG_X86_64 */
>
> - /* Set base address in stack canary descriptor. */
> - movl $_pa(gdt_start),%eax
> - movl $_pa(canary),%ecx
> - movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
> - movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
> -
> - mov $PVH_CANARY_SEL,%eax
> - mov %eax,%gs
> -
> call mk_early_pgtbl_32
>
> mov $_pa(initial_page_table), %eax
> @@ -163,7 +150,6 @@ SYM_DATA_START_LOCAL(gdt_start)
> .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */
> #endif
> .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */
> - .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */
> SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end)
>
> .balign 16
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index db1378c6ff26..ef4329d67a5f 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
> /*
> * segment registers
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, ctxt->gs);
> -#endif
> #ifdef CONFIG_X86_64
> - savesegment(gs, ctxt->gs);
> savesegment(fs, ctxt->fs);
> savesegment(ds, ctxt->ds);
> savesegment(es, ctxt->es);
> @@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
> #else
> loadsegment(fs, __KERNEL_PERCPU);
> - loadsegment(gs, __KERNEL_STACK_CANARY);
> #endif
>
> /* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
> @@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> */
> wrmsrl(MSR_FS_BASE, ctxt->fs_base);
> wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
> -#elif defined(CONFIG_X86_32_LAZY_GS)
> +#else
> loadsegment(gs, ctxt->gs);
> #endif
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 9a5a50cdaab5..e18235a6390d 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1190,7 +1190,6 @@ static void __init xen_setup_gdt(int cpu)
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
> pv_ops.cpu.load_gdt = xen_load_gdt_boot;
>
> - setup_stack_canary_segment(cpu);
> switch_to_new_gdt(cpu);
>
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
> diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
> index f5c119495254..825c75c5b715 100755
> --- a/scripts/gcc-x86_32-has-stack-protector.sh
> +++ b/scripts/gcc-x86_32-has-stack-protector.sh
> @@ -1,4 +1,8 @@
> #!/bin/sh
> # SPDX-License-Identifier: GPL-2.0
>
> -echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
> +# This requires GCC 8.1 or better. Specifically, we require
> +# -mstack-protector-guard-reg, added by
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
> +
> +echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"
> --
> 2.29.2
>

2021-02-14 14:45:46

[permalink] [raw]

Subject: Re: [PATCH v2 0/2] Clean up x86_32 stackprotector

On Sat, Feb 13, 2021 at 8:19 PM Andy Lutomirski <[email protected]> wrote:
>
> x86_32 stackprotector is a maintenance nightmare. Clean it up. This
> disables stackprotector on x86_32 on GCC 8.1 and on all clang
> versions. Some clang people are cc'd.
>
> Changes from v1:
> - Changelog fixes.
> - Comment fixes (mostly from Sean).
> - Fix the !SMP case.
>
> Andy Lutomirski (2):
> x86/stackprotector/32: Make the canary into a regular percpu variable
> x86/entry/32: Remove leftover macros after stackprotector cleanups
>

Happy Valentine's Day,

I have no rose or pralines for you, Andy :-).

First, I tried to apply your patchset on top of <tip.git#autolatest>
which has shown some conflicts.
AFAICS it conflicts with <tip.git#x86/entry> (or more precisely with
the recent "Merge branch 'x86/paravirt' into x86/entry").
Just wanna let you know.

I have tested this on Debian/testing AMD64 (x86 64bit) with LLVM/Clang
v12.0.0-rc1 and my custom patchset.
Means compile-tested and booted into bare metal and checked build-log
and dmesg-log for warnings.
So far nothing scary.

Feel free to give credits like a Tested-by if you like.
( Roses and pralines are welcome, too, hahaha :-). )

Icecold and sunshiny Greetings from North-West Germany,
- Sedat -

> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 ++
> arch/x86/entry/entry_32.S | 95 +----------------------
> arch/x86/include/asm/processor.h | 15 +---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 ++-----
> arch/x86/include/asm/stackprotector.h | 79 ++++---------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +-
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 -
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 265 deletions(-)
>
> --
> 2.29.2
>

2021-02-16 16:23:50

by Sean Christopherson

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Sat, Feb 13, 2021, Andy Lutomirski wrote:
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f923e14e87df..ec39073b4897 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> #ifdef CONFIG_X86_64
> loadsegment(fs, svm->host.fs);
> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> - load_gs_index(svm->host.gs);
> -#else
> -#ifdef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif

This manually GS crud is gone as of commit e79b91bb3c91 ("KVM: SVM: use
vmsave/vmload for saving/restoring additional host state"), which is queued for
5.12.

> #endif
> + load_gs_index(svm->host.gs);
>
> for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> wrmsrl(host_save_user_msrs[i].index,
> @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> } else {
> __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>
> + /* Restore the percpu segment immediately. */
> #ifdef CONFIG_X86_64
> native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
> loadsegment(fs, svm->host.fs);
> -#ifndef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> }

2021-02-16 18:15:45

by Nick Desaulniers

[permalink] [raw]

Subject: Re: [PATCH v2 0/2] Clean up x86_32 stackprotector

On Sat, Feb 13, 2021 at 11:19 AM Andy Lutomirski <[email protected]> wrote:
>
> x86_32 stackprotector is a maintenance nightmare. Clean it up. This
> disables stackprotector on x86_32 on GCC 8.1 and on all clang
> versions. Some clang people are cc'd.

When in doubt, check the MAINTAINERS entry for LLVM for our mailing
list and listed maintainers.

>
> Changes from v1:
> - Changelog fixes.
> - Comment fixes (mostly from Sean).
> - Fix the !SMP case.
>
> Andy Lutomirski (2):
> x86/stackprotector/32: Make the canary into a regular percpu variable
> x86/entry/32: Remove leftover macros after stackprotector cleanups
>
> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 ++
> arch/x86/entry/entry_32.S | 95 +----------------------
> arch/x86/include/asm/processor.h | 15 +---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 ++-----
> arch/x86/include/asm/stackprotector.h | 79 ++++---------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +-
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 -
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 265 deletions(-)
>
> --
> 2.29.2
>

--
Thanks,
~Nick Desaulniers

2021-02-16 18:48:55

by Nick Desaulniers

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Sat, Feb 13, 2021 at 11:19 AM Andy Lutomirski <[email protected]> wrote:
>
> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> percpu storage. It's even nastier because it means that whether %gs
> contains userspace state or kernel state while running kernel code
> depends on whether stackprotector is enabled (this is
> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> that segment selectors work. Supporting both variants is a
> maintenance and testing mess.
>
> Merely rearranging so that percpu and the stack canary
> share the same segment would be messy as the 32-bit percpu address
> layout isn't currently compatible with putting a variable at a fixed
> offset.
>
> Fortunately, GCC 8.1 added options that allow the stack canary to be
> accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
> percpu variable. This lets us get rid of all of the code to manage the
> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>
> (That name is special. We could use any symbol we want for the
> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
> name other than __stack_chk_guard.)
>
> This patch forcibly disables stackprotector on older compilers that
> don't support the new options and makes the stack canary into a
> percpu variable. The "lazy GS" approach is now used for all 32-bit
> configurations.
>
> This patch also makes load_gs_index() work on 32-bit kernels. On
> 64-bit kernels, it loads the GS selector and updates the user
> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> it loads the GS selector and updates GSBASE, which is now
> always the user base. This means that the overall effect is
> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>
> Cc: Sedat Dilek <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 +++
> arch/x86/entry/entry_32.S | 56 ++--------------
> arch/x86/include/asm/processor.h | 15 ++---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 +++------
> arch/x86/include/asm/stackprotector.h | 79 +++++------------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +-----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +--
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 --
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 226 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 21f851179ff0..12d8bf011d08 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -353,10 +353,6 @@ config X86_64_SMP
> def_bool y
> depends on X86_64 && SMP
>
> -config X86_32_LAZY_GS
> - def_bool y
> - depends on X86_32 && !STACKPROTECTOR
> -
> config ARCH_SUPPORTS_UPROBES
> def_bool y
>
> @@ -379,7 +375,8 @@ config CC_HAS_SANE_STACKPROTECTOR
> default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
> help
> We have to make sure stack protector is unconditionally disabled if
> - the compiler produces broken code.
> + the compiler produces broken code or if it does not let us control
> + the segment on 32-bit kernels.
>
> menu "Processor type and features"
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7116da3980be..0b5cd8c49ccb 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -76,6 +76,14 @@ ifeq ($(CONFIG_X86_32),y)
>
> # temporary until string.h is fixed
> KBUILD_CFLAGS += -ffreestanding
> +
> + ifeq ($(CONFIG_STACKPROTECTOR),y)
> + ifeq ($(CONFIG_SMP),y)
> + KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard

I'm guessing the CC is because this removes stack protector support
with clang, because it does not yet support
-mstack-protector-guard-symbol= (as Sedat notes)? I would like to see
this called out explicitly in the commit message.

(If folks are looking to use various compiler features/flags and find
support missing, please let us know ASAP as it helps give us more time
to triage and plan work around existing schedules. Our ML from
MAINTAINERS <[email protected]> or
[email protected] works.)

For now I've filed:
https://bugs.llvm.org/show_bug.cgi?id=49209

> + else
> + KBUILD_CFLAGS += -mstack-protector-guard=global
> + endif
> + endif
> else
> BITS := 64
> UTS_MACHINE := x86_64
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index df8c017e6161..eb0cb662bca5 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -20,7 +20,7 @@
> * 1C(%esp) - %ds
> * 20(%esp) - %es
> * 24(%esp) - %fs
> - * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
> + * 28(%esp) - unused -- was %gs on old stackprotector kernels
> * 2C(%esp) - orig_eax
> * 30(%esp) - %eip
> * 34(%esp) - %cs
> @@ -56,14 +56,9 @@
> /*
> * User gs save/restore
> *
> - * %gs is used for userland TLS and kernel only uses it for stack
> - * canary which is required to be at %gs:20 by gcc. Read the comment
> - * at the top of stackprotector.h for more info.
> - *
> - * Local labels 98 and 99 are used.
> + * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
> + * will remove it entirely.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> -
> /* unfortunately push/pop can't be no-op */
> .macro PUSH_GS
> pushl $0
> @@ -86,49 +81,6 @@
> .macro SET_KERNEL_GS reg
> .endm
>
> -#else /* CONFIG_X86_32_LAZY_GS */
> -
> -.macro PUSH_GS
> - pushl %gs
> -.endm
> -
> -.macro POP_GS pop=0
> -98: popl %gs
> - .if \pop <> 0
> - add $\pop, %esp
> - .endif
> -.endm
> -.macro POP_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, (%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro PTGS_TO_GS
> -98: mov PT_GS(%esp), %gs
> -.endm
> -.macro PTGS_TO_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, PT_GS(%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro GS_TO_REG reg
> - movl %gs, \reg
> -.endm
> -.macro REG_TO_PTGS reg
> - movl \reg, PT_GS(%esp)
> -.endm
> -.macro SET_KERNEL_GS reg
> - movl $(__KERNEL_STACK_CANARY), \reg
> - movl \reg, %gs
> -.endm
> -
> -#endif /* CONFIG_X86_32_LAZY_GS */
>
> /* Unconditionally switch to user cr3 */
> .macro SWITCH_TO_USER_CR3 scratch_reg:req
> @@ -779,7 +731,7 @@ SYM_CODE_START(__switch_to_asm)
>
> #ifdef CONFIG_STACKPROTECTOR
> movl TASK_stack_canary(%edx), %ebx
> - movl %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
> + movl %ebx, PER_CPU_VAR(__stack_chk_guard)
> #endif
>
> #ifdef CONFIG_RETPOLINE
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index c20a52b5534b..c59dff4bbc38 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -441,6 +441,9 @@ struct fixed_percpu_data {
> * GCC hardcodes the stack canary as %gs:40. Since the
> * irq_stack is the object at %gs:0, we reserve the bottom
> * 48 bytes of the irq stack for the canary.
> + *
> + * Once we are willing to require -mstack-protector-guard-symbol=
> + * support for x86_64 stackprotector, we can get rid of this.
> */
> char gs_base[40];
> unsigned long stack_canary;
> @@ -461,17 +464,7 @@ extern asmlinkage void ignore_sysret(void);
> void current_save_fsgs(void);
> #else /* X86_64 */
> #ifdef CONFIG_STACKPROTECTOR
> -/*
> - * Make sure stack canary segment base is cached-aligned:
> - * "For Intel Atom processors, avoid non zero segment base address
> - * that is not aligned to cache line boundary at all cost."
> - * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
> - */
> -struct stack_canary {
> - char __pad[20]; /* canary at %gs:20 */
> - unsigned long canary;
> -};
> -DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> #endif
> /* Per CPU softirq stack pointer */
> DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index d8324a236696..b2c4c12d237c 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -37,7 +37,10 @@ struct pt_regs {
> unsigned short __esh;
> unsigned short fs;
> unsigned short __fsh;
> - /* On interrupt, gs and __gsh store the vector number. */
> + /*
> + * On interrupt, gs and __gsh store the vector number. They never
> + * store gs any more.
> + */
> unsigned short gs;
> unsigned short __gsh;
> /* On interrupt, this is the error code. */
> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> index 7fdd4facfce7..72044026eb3c 100644
> --- a/arch/x86/include/asm/segment.h
> +++ b/arch/x86/include/asm/segment.h
> @@ -95,7 +95,7 @@
> *
> * 26 - ESPFIX small SS
> * 27 - per-cpu [ offset to per-cpu data area ]
> - * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
> + * 28 - unused
> * 29 - unused
> * 30 - unused
> * 31 - TSS for double fault handler
> @@ -118,7 +118,6 @@
>
> #define GDT_ENTRY_ESPFIX_SS 26
> #define GDT_ENTRY_PERCPU 27
> -#define GDT_ENTRY_STACK_CANARY 28
>
> #define GDT_ENTRY_DOUBLEFAULT_TSS 31
>
> @@ -158,12 +157,6 @@
> # define __KERNEL_PERCPU 0
> #endif
>
> -#ifdef CONFIG_STACKPROTECTOR
> -# define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY*8)
> -#else
> -# define __KERNEL_STACK_CANARY 0
> -#endif
> -
> #else /* 64-bit: */
>
> #include <asm/cache.h>
> @@ -364,22 +357,15 @@ static inline void __loadsegment_fs(unsigned short value)
> asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
>
> /*
> - * x86-32 user GS accessors:
> + * x86-32 user GS accessors. This is ugly and could do with some cleaning up.
> */
> #ifdef CONFIG_X86_32
> -# ifdef CONFIG_X86_32_LAZY_GS
> -# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> -# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> -# define task_user_gs(tsk) ((tsk)->thread.gs)
> -# define lazy_save_gs(v) savesegment(gs, (v))
> -# define lazy_load_gs(v) loadsegment(gs, (v))
> -# else /* X86_32_LAZY_GS */
> -# define get_user_gs(regs) (u16)((regs)->gs)
> -# define set_user_gs(regs, v) do { (regs)->gs = (v); } while (0)
> -# define task_user_gs(tsk) (task_pt_regs(tsk)->gs)
> -# define lazy_save_gs(v) do { } while (0)
> -# define lazy_load_gs(v) do { } while (0)
> -# endif /* X86_32_LAZY_GS */
> +# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> +# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> +# define task_user_gs(tsk) ((tsk)->thread.gs)
> +# define lazy_save_gs(v) savesegment(gs, (v))
> +# define lazy_load_gs(v) loadsegment(gs, (v))
> +# define load_gs_index(v) loadsegment(gs, (v))
> #endif /* X86_32 */
>
> #endif /* !__ASSEMBLY__ */
> diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> index 7fb482f0f25b..b6ffe58c70fa 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -5,30 +5,23 @@
> * Stack protector works by putting predefined pattern at the start of
> * the stack frame and verifying that it hasn't been overwritten when
> * returning from the function. The pattern is called stack canary
> - * and unfortunately gcc requires it to be at a fixed offset from %gs.
> - * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64
> - * and x86_32 use segment registers differently and thus handles this
> - * requirement differently.
> + * and unfortunately gcc historically required it to be at a fixed offset
> + * from the percpu segment base. On x86_64, the offset is 40 bytes.
> *
> - * On x86_64, %gs is shared by percpu area and stack canary. All
> - * percpu symbols are zero based and %gs points to the base of percpu
> - * area. The first occupant of the percpu area is always
> - * fixed_percpu_data which contains stack_canary at offset 40. Userland
> - * %gs is always saved and restored on kernel entry and exit using
> - * swapgs, so stack protector doesn't add any complexity there.
> + * The same segment is shared by percpu area and stack canary. On
> + * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
> + * base of percpu area. The first occupant of the percpu area is always
> + * fixed_percpu_data which contains stack_canary at the approproate
> + * offset. On x86_32, the stack canary is just a regular percpu
> + * variable.
> *
> - * On x86_32, it's slightly more complicated. As in x86_64, %gs is
> - * used for userland TLS. Unfortunately, some processors are much
> - * slower at loading segment registers with different value when
> - * entering and leaving the kernel, so the kernel uses %fs for percpu
> - * area and manages %gs lazily so that %gs is switched only when
> - * necessary, usually during task switch.
> + * Putting percpu data in %fs on 32-bit is a minor optimization compared to
> + * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
> + * to load 0 into %fs on exit to usermode, whereas with percpu data in
> + * %gs, we are likely to load a non-null %gs on return to user mode.
> *
> - * As gcc requires the stack canary at %gs:20, %gs can't be managed
> - * lazily if stack protector is enabled, so the kernel saves and
> - * restores userland %gs on kernel entry and exit. This behavior is
> - * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
> - * system.h to hide the details.
> + * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
> + * support, we can remove some of this complexity.
> */
>
> #ifndef _ASM_STACKPROTECTOR_H
> @@ -44,14 +37,6 @@
> #include <linux/random.h>
> #include <linux/sched.h>
>
> -/*
> - * 24 byte read-only segment initializer for stack canary. Linker
> - * can't handle the address bit shifting. Address will be set in
> - * head_32 for boot CPU and setup_per_cpu_areas() for others.
> - */
> -#define GDT_STACK_CANARY_INIT \
> - [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
> -
> /*
> * Initialize the stackprotector canary value.
> *
> @@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void)
> #ifdef CONFIG_X86_64
> this_cpu_write(fixed_percpu_data.stack_canary, canary);
> #else
> - this_cpu_write(stack_canary.canary, canary);
> + this_cpu_write(__stack_chk_guard, canary);
> #endif
> }
>
> @@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> #ifdef CONFIG_X86_64
> per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
> #else
> - per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
> -#endif
> -}
> -
> -static inline void setup_stack_canary_segment(int cpu)
> -{
> -#ifdef CONFIG_X86_32
> - unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
> - struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
> - struct desc_struct desc;
> -
> - desc = gdt_table[GDT_ENTRY_STACK_CANARY];
> - set_desc_base(&desc, canary);
> - write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
> -#endif
> -}
> -
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
> + per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
> #endif
> }
>
> #else /* STACKPROTECTOR */
>
> -#define GDT_STACK_CANARY_INIT
> -
> /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
>
> -static inline void setup_stack_canary_segment(int cpu)
> -{ }
> -
> static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> { }
>
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm volatile ("mov %0, %%gs" : : "r" (0));
> -#endif
> -}
> -
> #endif /* STACKPROTECTOR */
> #endif /* _ASM_STACKPROTECTOR_H */
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index fdbd9d7b7bca..7b132d0312eb 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -13,12 +13,10 @@
> /* image of the saved processor state */
> struct saved_context {
> /*
> - * On x86_32, all segment registers, with the possible exception of
> - * gs, are saved at kernel entry in pt_regs.
> + * On x86_32, all segment registers except gs are saved at kernel
> + * entry in pt_regs.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> u16 gs;
> -#endif
> unsigned long cr0, cr2, cr3, cr4;
> u64 misc_enable;
> bool misc_enable_saved;
> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> index 6e043f295a60..2b411cd00a4e 100644
> --- a/arch/x86/kernel/asm-offsets_32.c
> +++ b/arch/x86/kernel/asm-offsets_32.c
> @@ -53,11 +53,6 @@ void foo(void)
> offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
> offsetofend(struct cpu_entry_area, entry_stack_page.stack));
>
> -#ifdef CONFIG_STACKPROTECTOR
> - BLANK();
> - OFFSET(stack_canary_offset, stack_canary, canary);
> -#endif
> -
> BLANK();
> DEFINE(EFI_svam, offsetof(efi_runtime_services_t, set_virtual_address_map));
> }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 35ad8480c464..f208569d2d3b 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -161,7 +161,6 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
>
> [GDT_ENTRY_ESPFIX_SS] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> [GDT_ENTRY_PERCPU] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> - GDT_STACK_CANARY_INIT
> #endif
> } };
> EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
> @@ -599,7 +598,6 @@ void load_percpu_segment(int cpu)
> __loadsegment_simple(gs, 0);
> wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
> #endif
> - load_stack_canary_segment();
> }
>
> #ifdef CONFIG_X86_32
> @@ -1793,7 +1791,8 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
> EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
>
> #ifdef CONFIG_STACKPROTECTOR
> -DEFINE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> +EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
> #endif
>
> #endif /* CONFIG_X86_64 */
> diff --git a/arch/x86/kernel/doublefault_32.c b/arch/x86/kernel/doublefault_32.c
> index 759d392cbe9f..d1d49e3d536b 100644
> --- a/arch/x86/kernel/doublefault_32.c
> +++ b/arch/x86/kernel/doublefault_32.c
> @@ -100,9 +100,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
> .ss = __KERNEL_DS,
> .ds = __USER_DS,
> .fs = __KERNEL_PERCPU,
> -#ifndef CONFIG_X86_32_LAZY_GS
> - .gs = __KERNEL_STACK_CANARY,
> -#endif
> + .gs = 0,
>
> .__cr3 = __pa_nodebug(swapper_pg_dir),
> },
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 7ed84c282233..67f590425d90 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp)
> movl $(__KERNEL_PERCPU), %eax
> movl %eax,%fs # set this cpu's percpu
>
> - movl $(__KERNEL_STACK_CANARY),%eax
> - movl %eax,%gs
> + xorl %eax,%eax
> + movl %eax,%gs # clear possible garbage in %gs
>
> xorl %eax,%eax # Clear LDT
> lldt %ax
> @@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp)
> */
> __INIT
> setup_once:
> -#ifdef CONFIG_STACKPROTECTOR
> - /*
> - * Configure the stack canary. The linker can't handle this by
> - * relocation. Manually set base address in stack canary
> - * segment descriptor.
> - */
> - movl $gdt_page,%eax
> - movl $stack_canary,%ecx
> - movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
> - movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
> -#endif
> -
> andl $0,setup_once_ref /* Once is enough, thanks */
> ret
>
> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index fd945ce78554..0941d2f44f2a 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -224,7 +224,6 @@ void __init setup_per_cpu_areas(void)
> per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
> per_cpu(cpu_number, cpu) = cpu;
> setup_percpu_segment(cpu);
> - setup_stack_canary_segment(cpu);
> /*
> * Copy data used in early init routines from the
> * initial arrays to the per cpu data areas. These
> diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
> index 64a496a0687f..3c883e064242 100644
> --- a/arch/x86/kernel/tls.c
> +++ b/arch/x86/kernel/tls.c
> @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx,
> savesegment(fs, sel);
> if (sel == modified_sel)
> loadsegment(fs, sel);
> -
> - savesegment(gs, sel);
> - if (sel == modified_sel)
> - load_gs_index(sel);
> #endif
>
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, sel);
> if (sel == modified_sel)
> - loadsegment(gs, sel);
> -#endif
> + load_gs_index(sel);
> } else {
> #ifdef CONFIG_X86_64
> if (p->thread.fsindex == modified_sel)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f923e14e87df..ec39073b4897 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> #ifdef CONFIG_X86_64
> loadsegment(fs, svm->host.fs);
> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> - load_gs_index(svm->host.gs);
> -#else
> -#ifdef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> + load_gs_index(svm->host.gs);
>
> for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> wrmsrl(host_save_user_msrs[i].index,
> @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> } else {
> __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>
> + /* Restore the percpu segment immediately. */
> #ifdef CONFIG_X86_64
> native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
> loadsegment(fs, svm->host.fs);
> -#ifndef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> }
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 4229950a5d78..7f89a091f1fb 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -404,10 +404,6 @@ static short get_segment_selector(struct pt_regs *regs, int seg_reg_idx)
> case INAT_SEG_REG_FS:
> return (unsigned short)(regs->fs & 0xffff);
> case INAT_SEG_REG_GS:
> - /*
> - * GS may or may not be in regs as per CONFIG_X86_32_LAZY_GS.
> - * The macro below takes care of both cases.
> - */
> return get_user_gs(regs);
> case INAT_SEG_REG_IGNORE:
> default:
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 43b4d864817e..afbf0bb252da 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -45,10 +45,8 @@
>
> #define PVH_GDT_ENTRY_CS 1
> #define PVH_GDT_ENTRY_DS 2
> -#define PVH_GDT_ENTRY_CANARY 3
> #define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8)
> #define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8)
> -#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8)
>
> SYM_CODE_START_LOCAL(pvh_start_xen)
> cld
> @@ -109,17 +107,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>
> #else /* CONFIG_X86_64 */
>
> - /* Set base address in stack canary descriptor. */
> - movl $_pa(gdt_start),%eax
> - movl $_pa(canary),%ecx
> - movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
> - movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
> -
> - mov $PVH_CANARY_SEL,%eax
> - mov %eax,%gs
> -
> call mk_early_pgtbl_32
>
> mov $_pa(initial_page_table), %eax
> @@ -163,7 +150,6 @@ SYM_DATA_START_LOCAL(gdt_start)
> .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */
> #endif
> .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */
> - .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */
> SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end)
>
> .balign 16
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index db1378c6ff26..ef4329d67a5f 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
> /*
> * segment registers
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, ctxt->gs);
> -#endif
> #ifdef CONFIG_X86_64
> - savesegment(gs, ctxt->gs);
> savesegment(fs, ctxt->fs);
> savesegment(ds, ctxt->ds);
> savesegment(es, ctxt->es);
> @@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
> #else
> loadsegment(fs, __KERNEL_PERCPU);
> - loadsegment(gs, __KERNEL_STACK_CANARY);
> #endif
>
> /* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
> @@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> */
> wrmsrl(MSR_FS_BASE, ctxt->fs_base);
> wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
> -#elif defined(CONFIG_X86_32_LAZY_GS)
> +#else
> loadsegment(gs, ctxt->gs);
> #endif
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 9a5a50cdaab5..e18235a6390d 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1190,7 +1190,6 @@ static void __init xen_setup_gdt(int cpu)
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
> pv_ops.cpu.load_gdt = xen_load_gdt_boot;
>
> - setup_stack_canary_segment(cpu);
> switch_to_new_gdt(cpu);
>
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
> diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
> index f5c119495254..825c75c5b715 100755
> --- a/scripts/gcc-x86_32-has-stack-protector.sh
> +++ b/scripts/gcc-x86_32-has-stack-protector.sh
> @@ -1,4 +1,8 @@
> #!/bin/sh
> # SPDX-License-Identifier: GPL-2.0
>
> -echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
> +# This requires GCC 8.1 or better. Specifically, we require
> +# -mstack-protector-guard-reg, added by
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708

And -mstack-protector-guard-symbol= it would seem, since that's what
will cause this to fail with Clang?

What about -mstack-protector-guard=global which can be used for non
SMP? I'm guessing that came out in an earlier version of GCC than
-mstack-protector-guard-reg, so it doesn't need to be checked? FWIW,
some of these were added to Clang in https://reviews.llvm.org/D88631.

> +
> +echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"
> --
> 2.29.2
>

--
Thanks,
~Nick Desaulniers

2021-02-16 20:26:57

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Tue, Feb 16, 2021 at 5:21 PM Sean Christopherson <[email protected]> wrote:
>
> On Sat, Feb 13, 2021, Andy Lutomirski wrote:
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index f923e14e87df..ec39073b4897 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> > #ifdef CONFIG_X86_64
> > loadsegment(fs, svm->host.fs);
> > wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> > - load_gs_index(svm->host.gs);
> > -#else
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > - loadsegment(gs, svm->host.gs);
> > -#endif
>
> This manually GS crud is gone as of commit e79b91bb3c91 ("KVM: SVM: use
> vmsave/vmload for saving/restoring additional host state"), which is queued for
> 5.12.
>

Link to the above KVM patch see [1].

As said the base for this patchset should be changed - for example it
conflicts with [2].

Maybe wait for Linux v5.12-rc1?

- Sedat -

[1] https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?h=tags/kvm-5.12-1&id=e79b91bb3c916a52ce823ab60489c717c925c49f
[2] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tag/?h=x86-entry-2021-02-15

> > #endif
> > + load_gs_index(svm->host.gs);
> >
> > for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> > wrmsrl(host_save_user_msrs[i].index,
> > @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> > } else {
> > __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
> >
> > + /* Restore the percpu segment immediately. */
> > #ifdef CONFIG_X86_64
> > native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> > #else
> > loadsegment(fs, svm->host.fs);
> > -#ifndef CONFIG_X86_32_LAZY_GS
> > - loadsegment(gs, svm->host.gs);
> > -#endif
> > #endif
> > }

2021-02-16 20:31:19

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Tue, Feb 16, 2021 at 7:45 PM Nick Desaulniers
<[email protected]> wrote:
>
> On Sat, Feb 13, 2021 at 11:19 AM Andy Lutomirski <[email protected]> wrote:
> >
> > On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> > percpu storage. It's even nastier because it means that whether %gs
> > contains userspace state or kernel state while running kernel code
> > depends on whether stackprotector is enabled (this is
> > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> > that segment selectors work. Supporting both variants is a
> > maintenance and testing mess.
> >
> > Merely rearranging so that percpu and the stack canary
> > share the same segment would be messy as the 32-bit percpu address
> > layout isn't currently compatible with putting a variable at a fixed
> > offset.
> >
> > Fortunately, GCC 8.1 added options that allow the stack canary to be
> > accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
> > percpu variable. This lets us get rid of all of the code to manage the
> > stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> >
> > (That name is special. We could use any symbol we want for the
> > %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
> > name other than __stack_chk_guard.)
> >
> > This patch forcibly disables stackprotector on older compilers that
> > don't support the new options and makes the stack canary into a
> > percpu variable. The "lazy GS" approach is now used for all 32-bit
> > configurations.
> >
> > This patch also makes load_gs_index() work on 32-bit kernels. On
> > 64-bit kernels, it loads the GS selector and updates the user
> > GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> > it loads the GS selector and updates GSBASE, which is now
> > always the user base. This means that the overall effect is
> > the same on 32-bit and 64-bit, which avoids some ifdeffery.
> >
> > Cc: Sedat Dilek <[email protected]>
> > Cc: Nick Desaulniers <[email protected]>
> > Signed-off-by: Andy Lutomirski <[email protected]>
> > ---
> > arch/x86/Kconfig | 7 +-
> > arch/x86/Makefile | 8 +++
> > arch/x86/entry/entry_32.S | 56 ++--------------
> > arch/x86/include/asm/processor.h | 15 ++---
> > arch/x86/include/asm/ptrace.h | 5 +-
> > arch/x86/include/asm/segment.h | 30 +++------
> > arch/x86/include/asm/stackprotector.h | 79 +++++------------------
> > arch/x86/include/asm/suspend_32.h | 6 +-
> > arch/x86/kernel/asm-offsets_32.c | 5 --
> > arch/x86/kernel/cpu/common.c | 5 +-
> > arch/x86/kernel/doublefault_32.c | 4 +-
> > arch/x86/kernel/head_32.S | 18 +-----
> > arch/x86/kernel/setup_percpu.c | 1 -
> > arch/x86/kernel/tls.c | 8 +--
> > arch/x86/kvm/svm/svm.c | 10 +--
> > arch/x86/lib/insn-eval.c | 4 --
> > arch/x86/platform/pvh/head.S | 14 ----
> > arch/x86/power/cpu.c | 6 +-
> > arch/x86/xen/enlighten_pv.c | 1 -
> > scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> > 20 files changed, 62 insertions(+), 226 deletions(-)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 21f851179ff0..12d8bf011d08 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -353,10 +353,6 @@ config X86_64_SMP
> > def_bool y
> > depends on X86_64 && SMP
> >
> > -config X86_32_LAZY_GS
> > - def_bool y
> > - depends on X86_32 && !STACKPROTECTOR
> > -
> > config ARCH_SUPPORTS_UPROBES
> > def_bool y
> >
> > @@ -379,7 +375,8 @@ config CC_HAS_SANE_STACKPROTECTOR
> > default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
> > help
> > We have to make sure stack protector is unconditionally disabled if
> > - the compiler produces broken code.
> > + the compiler produces broken code or if it does not let us control
> > + the segment on 32-bit kernels.
> >
> > menu "Processor type and features"
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index 7116da3980be..0b5cd8c49ccb 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -76,6 +76,14 @@ ifeq ($(CONFIG_X86_32),y)
> >
> > # temporary until string.h is fixed
> > KBUILD_CFLAGS += -ffreestanding
> > +
> > + ifeq ($(CONFIG_STACKPROTECTOR),y)
> > + ifeq ($(CONFIG_SMP),y)
> > + KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
>
> I'm guessing the CC is because this removes stack protector support
> with clang, because it does not yet support
> -mstack-protector-guard-symbol= (as Sedat notes)? I would like to see
> this called out explicitly in the commit message.
>
> (If folks are looking to use various compiler features/flags and find
> support missing, please let us know ASAP as it helps give us more time
> to triage and plan work around existing schedules. Our ML from
> MAINTAINERS <[email protected]> or
> [email protected] works.)
>
> For now I've filed:
> https://bugs.llvm.org/show_bug.cgi?id=49209
>

Thanks for the bug-report "Bug 49209 - [X86] support for
-mstack-protector-guard-symbol="

> > + else
> > + KBUILD_CFLAGS += -mstack-protector-guard=global
> > + endif
> > + endif
> > else
> > BITS := 64
> > UTS_MACHINE := x86_64
> > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> > index df8c017e6161..eb0cb662bca5 100644
> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -20,7 +20,7 @@
> > * 1C(%esp) - %ds
> > * 20(%esp) - %es
> > * 24(%esp) - %fs
> > - * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
> > + * 28(%esp) - unused -- was %gs on old stackprotector kernels
> > * 2C(%esp) - orig_eax
> > * 30(%esp) - %eip
> > * 34(%esp) - %cs
> > @@ -56,14 +56,9 @@
> > /*
> > * User gs save/restore
> > *
> > - * %gs is used for userland TLS and kernel only uses it for stack
> > - * canary which is required to be at %gs:20 by gcc. Read the comment
> > - * at the top of stackprotector.h for more info.
> > - *
> > - * Local labels 98 and 99 are used.
> > + * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
> > + * will remove it entirely.
> > */
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > -
> > /* unfortunately push/pop can't be no-op */
> > .macro PUSH_GS
> > pushl $0
> > @@ -86,49 +81,6 @@
> > .macro SET_KERNEL_GS reg
> > .endm
> >
> > -#else /* CONFIG_X86_32_LAZY_GS */
> > -
> > -.macro PUSH_GS
> > - pushl %gs
> > -.endm
> > -
> > -.macro POP_GS pop=0
> > -98: popl %gs
> > - .if \pop <> 0
> > - add $\pop, %esp
> > - .endif
> > -.endm
> > -.macro POP_GS_EX
> > -.pushsection .fixup, "ax"
> > -99: movl $0, (%esp)
> > - jmp 98b
> > -.popsection
> > - _ASM_EXTABLE(98b, 99b)
> > -.endm
> > -
> > -.macro PTGS_TO_GS
> > -98: mov PT_GS(%esp), %gs
> > -.endm
> > -.macro PTGS_TO_GS_EX
> > -.pushsection .fixup, "ax"
> > -99: movl $0, PT_GS(%esp)
> > - jmp 98b
> > -.popsection
> > - _ASM_EXTABLE(98b, 99b)
> > -.endm
> > -
> > -.macro GS_TO_REG reg
> > - movl %gs, \reg
> > -.endm
> > -.macro REG_TO_PTGS reg
> > - movl \reg, PT_GS(%esp)
> > -.endm
> > -.macro SET_KERNEL_GS reg
> > - movl $(__KERNEL_STACK_CANARY), \reg
> > - movl \reg, %gs
> > -.endm
> > -
> > -#endif /* CONFIG_X86_32_LAZY_GS */
> >
> > /* Unconditionally switch to user cr3 */
> > .macro SWITCH_TO_USER_CR3 scratch_reg:req
> > @@ -779,7 +731,7 @@ SYM_CODE_START(__switch_to_asm)
> >
> > #ifdef CONFIG_STACKPROTECTOR
> > movl TASK_stack_canary(%edx), %ebx
> > - movl %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
> > + movl %ebx, PER_CPU_VAR(__stack_chk_guard)
> > #endif
> >
> > #ifdef CONFIG_RETPOLINE
> > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > index c20a52b5534b..c59dff4bbc38 100644
> > --- a/arch/x86/include/asm/processor.h
> > +++ b/arch/x86/include/asm/processor.h
> > @@ -441,6 +441,9 @@ struct fixed_percpu_data {
> > * GCC hardcodes the stack canary as %gs:40. Since the
> > * irq_stack is the object at %gs:0, we reserve the bottom
> > * 48 bytes of the irq stack for the canary.
> > + *
> > + * Once we are willing to require -mstack-protector-guard-symbol=
> > + * support for x86_64 stackprotector, we can get rid of this.
> > */
> > char gs_base[40];
> > unsigned long stack_canary;
> > @@ -461,17 +464,7 @@ extern asmlinkage void ignore_sysret(void);
> > void current_save_fsgs(void);
> > #else /* X86_64 */
> > #ifdef CONFIG_STACKPROTECTOR
> > -/*
> > - * Make sure stack canary segment base is cached-aligned:
> > - * "For Intel Atom processors, avoid non zero segment base address
> > - * that is not aligned to cache line boundary at all cost."
> > - * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
> > - */
> > -struct stack_canary {
> > - char __pad[20]; /* canary at %gs:20 */
> > - unsigned long canary;
> > -};
> > -DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> > +DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> > #endif
> > /* Per CPU softirq stack pointer */
> > DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
> > diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> > index d8324a236696..b2c4c12d237c 100644
> > --- a/arch/x86/include/asm/ptrace.h
> > +++ b/arch/x86/include/asm/ptrace.h
> > @@ -37,7 +37,10 @@ struct pt_regs {
> > unsigned short __esh;
> > unsigned short fs;
> > unsigned short __fsh;
> > - /* On interrupt, gs and __gsh store the vector number. */
> > + /*
> > + * On interrupt, gs and __gsh store the vector number. They never
> > + * store gs any more.
> > + */
> > unsigned short gs;
> > unsigned short __gsh;
> > /* On interrupt, this is the error code. */
> > diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> > index 7fdd4facfce7..72044026eb3c 100644
> > --- a/arch/x86/include/asm/segment.h
> > +++ b/arch/x86/include/asm/segment.h
> > @@ -95,7 +95,7 @@
> > *
> > * 26 - ESPFIX small SS
> > * 27 - per-cpu [ offset to per-cpu data area ]
> > - * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
> > + * 28 - unused
> > * 29 - unused
> > * 30 - unused
> > * 31 - TSS for double fault handler
> > @@ -118,7 +118,6 @@
> >
> > #define GDT_ENTRY_ESPFIX_SS 26
> > #define GDT_ENTRY_PERCPU 27
> > -#define GDT_ENTRY_STACK_CANARY 28
> >
> > #define GDT_ENTRY_DOUBLEFAULT_TSS 31
> >
> > @@ -158,12 +157,6 @@
> > # define __KERNEL_PERCPU 0
> > #endif
> >
> > -#ifdef CONFIG_STACKPROTECTOR
> > -# define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY*8)
> > -#else
> > -# define __KERNEL_STACK_CANARY 0
> > -#endif
> > -
> > #else /* 64-bit: */
> >
> > #include <asm/cache.h>
> > @@ -364,22 +357,15 @@ static inline void __loadsegment_fs(unsigned short value)
> > asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
> >
> > /*
> > - * x86-32 user GS accessors:
> > + * x86-32 user GS accessors. This is ugly and could do with some cleaning up.
> > */
> > #ifdef CONFIG_X86_32
> > -# ifdef CONFIG_X86_32_LAZY_GS
> > -# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> > -# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> > -# define task_user_gs(tsk) ((tsk)->thread.gs)
> > -# define lazy_save_gs(v) savesegment(gs, (v))
> > -# define lazy_load_gs(v) loadsegment(gs, (v))
> > -# else /* X86_32_LAZY_GS */
> > -# define get_user_gs(regs) (u16)((regs)->gs)
> > -# define set_user_gs(regs, v) do { (regs)->gs = (v); } while (0)
> > -# define task_user_gs(tsk) (task_pt_regs(tsk)->gs)
> > -# define lazy_save_gs(v) do { } while (0)
> > -# define lazy_load_gs(v) do { } while (0)
> > -# endif /* X86_32_LAZY_GS */
> > +# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> > +# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> > +# define task_user_gs(tsk) ((tsk)->thread.gs)
> > +# define lazy_save_gs(v) savesegment(gs, (v))
> > +# define lazy_load_gs(v) loadsegment(gs, (v))
> > +# define load_gs_index(v) loadsegment(gs, (v))
> > #endif /* X86_32 */
> >
> > #endif /* !__ASSEMBLY__ */
> > diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> > index 7fb482f0f25b..b6ffe58c70fa 100644
> > --- a/arch/x86/include/asm/stackprotector.h
> > +++ b/arch/x86/include/asm/stackprotector.h
> > @@ -5,30 +5,23 @@
> > * Stack protector works by putting predefined pattern at the start of
> > * the stack frame and verifying that it hasn't been overwritten when
> > * returning from the function. The pattern is called stack canary
> > - * and unfortunately gcc requires it to be at a fixed offset from %gs.
> > - * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64
> > - * and x86_32 use segment registers differently and thus handles this
> > - * requirement differently.
> > + * and unfortunately gcc historically required it to be at a fixed offset
> > + * from the percpu segment base. On x86_64, the offset is 40 bytes.
> > *
> > - * On x86_64, %gs is shared by percpu area and stack canary. All
> > - * percpu symbols are zero based and %gs points to the base of percpu
> > - * area. The first occupant of the percpu area is always
> > - * fixed_percpu_data which contains stack_canary at offset 40. Userland
> > - * %gs is always saved and restored on kernel entry and exit using
> > - * swapgs, so stack protector doesn't add any complexity there.
> > + * The same segment is shared by percpu area and stack canary. On
> > + * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
> > + * base of percpu area. The first occupant of the percpu area is always
> > + * fixed_percpu_data which contains stack_canary at the approproate
> > + * offset. On x86_32, the stack canary is just a regular percpu
> > + * variable.
> > *
> > - * On x86_32, it's slightly more complicated. As in x86_64, %gs is
> > - * used for userland TLS. Unfortunately, some processors are much
> > - * slower at loading segment registers with different value when
> > - * entering and leaving the kernel, so the kernel uses %fs for percpu
> > - * area and manages %gs lazily so that %gs is switched only when
> > - * necessary, usually during task switch.
> > + * Putting percpu data in %fs on 32-bit is a minor optimization compared to
> > + * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
> > + * to load 0 into %fs on exit to usermode, whereas with percpu data in
> > + * %gs, we are likely to load a non-null %gs on return to user mode.
> > *
> > - * As gcc requires the stack canary at %gs:20, %gs can't be managed
> > - * lazily if stack protector is enabled, so the kernel saves and
> > - * restores userland %gs on kernel entry and exit. This behavior is
> > - * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
> > - * system.h to hide the details.
> > + * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
> > + * support, we can remove some of this complexity.
> > */
> >
> > #ifndef _ASM_STACKPROTECTOR_H
> > @@ -44,14 +37,6 @@
> > #include <linux/random.h>
> > #include <linux/sched.h>
> >
> > -/*
> > - * 24 byte read-only segment initializer for stack canary. Linker
> > - * can't handle the address bit shifting. Address will be set in
> > - * head_32 for boot CPU and setup_per_cpu_areas() for others.
> > - */
> > -#define GDT_STACK_CANARY_INIT \
> > - [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
> > -
> > /*
> > * Initialize the stackprotector canary value.
> > *
> > @@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void)
> > #ifdef CONFIG_X86_64
> > this_cpu_write(fixed_percpu_data.stack_canary, canary);
> > #else
> > - this_cpu_write(stack_canary.canary, canary);
> > + this_cpu_write(__stack_chk_guard, canary);
> > #endif
> > }
> >
> > @@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> > #ifdef CONFIG_X86_64
> > per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
> > #else
> > - per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
> > -#endif
> > -}
> > -
> > -static inline void setup_stack_canary_segment(int cpu)
> > -{
> > -#ifdef CONFIG_X86_32
> > - unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
> > - struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
> > - struct desc_struct desc;
> > -
> > - desc = gdt_table[GDT_ENTRY_STACK_CANARY];
> > - set_desc_base(&desc, canary);
> > - write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
> > -#endif
> > -}
> > -
> > -static inline void load_stack_canary_segment(void)
> > -{
> > -#ifdef CONFIG_X86_32
> > - asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
> > + per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
> > #endif
> > }
> >
> > #else /* STACKPROTECTOR */
> >
> > -#define GDT_STACK_CANARY_INIT
> > -
> > /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
> >
> > -static inline void setup_stack_canary_segment(int cpu)
> > -{ }
> > -
> > static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> > { }
> >
> > -static inline void load_stack_canary_segment(void)
> > -{
> > -#ifdef CONFIG_X86_32
> > - asm volatile ("mov %0, %%gs" : : "r" (0));
> > -#endif
> > -}
> > -
> > #endif /* STACKPROTECTOR */
> > #endif /* _ASM_STACKPROTECTOR_H */
> > diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> > index fdbd9d7b7bca..7b132d0312eb 100644
> > --- a/arch/x86/include/asm/suspend_32.h
> > +++ b/arch/x86/include/asm/suspend_32.h
> > @@ -13,12 +13,10 @@
> > /* image of the saved processor state */
> > struct saved_context {
> > /*
> > - * On x86_32, all segment registers, with the possible exception of
> > - * gs, are saved at kernel entry in pt_regs.
> > + * On x86_32, all segment registers except gs are saved at kernel
> > + * entry in pt_regs.
> > */
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > u16 gs;
> > -#endif
> > unsigned long cr0, cr2, cr3, cr4;
> > u64 misc_enable;
> > bool misc_enable_saved;
> > diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> > index 6e043f295a60..2b411cd00a4e 100644
> > --- a/arch/x86/kernel/asm-offsets_32.c
> > +++ b/arch/x86/kernel/asm-offsets_32.c
> > @@ -53,11 +53,6 @@ void foo(void)
> > offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
> > offsetofend(struct cpu_entry_area, entry_stack_page.stack));
> >
> > -#ifdef CONFIG_STACKPROTECTOR
> > - BLANK();
> > - OFFSET(stack_canary_offset, stack_canary, canary);
> > -#endif
> > -
> > BLANK();
> > DEFINE(EFI_svam, offsetof(efi_runtime_services_t, set_virtual_address_map));
> > }
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 35ad8480c464..f208569d2d3b 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -161,7 +161,6 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
> >
> > [GDT_ENTRY_ESPFIX_SS] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> > [GDT_ENTRY_PERCPU] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> > - GDT_STACK_CANARY_INIT
> > #endif
> > } };
> > EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
> > @@ -599,7 +598,6 @@ void load_percpu_segment(int cpu)
> > __loadsegment_simple(gs, 0);
> > wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
> > #endif
> > - load_stack_canary_segment();
> > }
> >
> > #ifdef CONFIG_X86_32
> > @@ -1793,7 +1791,8 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
> > EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
> >
> > #ifdef CONFIG_STACKPROTECTOR
> > -DEFINE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> > +DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> > +EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
> > #endif
> >
> > #endif /* CONFIG_X86_64 */
> > diff --git a/arch/x86/kernel/doublefault_32.c b/arch/x86/kernel/doublefault_32.c
> > index 759d392cbe9f..d1d49e3d536b 100644
> > --- a/arch/x86/kernel/doublefault_32.c
> > +++ b/arch/x86/kernel/doublefault_32.c
> > @@ -100,9 +100,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
> > .ss = __KERNEL_DS,
> > .ds = __USER_DS,
> > .fs = __KERNEL_PERCPU,
> > -#ifndef CONFIG_X86_32_LAZY_GS
> > - .gs = __KERNEL_STACK_CANARY,
> > -#endif
> > + .gs = 0,
> >
> > .__cr3 = __pa_nodebug(swapper_pg_dir),
> > },
> > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> > index 7ed84c282233..67f590425d90 100644
> > --- a/arch/x86/kernel/head_32.S
> > +++ b/arch/x86/kernel/head_32.S
> > @@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp)
> > movl $(__KERNEL_PERCPU), %eax
> > movl %eax,%fs # set this cpu's percpu
> >
> > - movl $(__KERNEL_STACK_CANARY),%eax
> > - movl %eax,%gs
> > + xorl %eax,%eax
> > + movl %eax,%gs # clear possible garbage in %gs
> >
> > xorl %eax,%eax # Clear LDT
> > lldt %ax
> > @@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp)
> > */
> > __INIT
> > setup_once:
> > -#ifdef CONFIG_STACKPROTECTOR
> > - /*
> > - * Configure the stack canary. The linker can't handle this by
> > - * relocation. Manually set base address in stack canary
> > - * segment descriptor.
> > - */
> > - movl $gdt_page,%eax
> > - movl $stack_canary,%ecx
> > - movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
> > - shrl $16, %ecx
> > - movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
> > - movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
> > -#endif
> > -
> > andl $0,setup_once_ref /* Once is enough, thanks */
> > ret
> >
> > diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> > index fd945ce78554..0941d2f44f2a 100644
> > --- a/arch/x86/kernel/setup_percpu.c
> > +++ b/arch/x86/kernel/setup_percpu.c
> > @@ -224,7 +224,6 @@ void __init setup_per_cpu_areas(void)
> > per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
> > per_cpu(cpu_number, cpu) = cpu;
> > setup_percpu_segment(cpu);
> > - setup_stack_canary_segment(cpu);
> > /*
> > * Copy data used in early init routines from the
> > * initial arrays to the per cpu data areas. These
> > diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
> > index 64a496a0687f..3c883e064242 100644
> > --- a/arch/x86/kernel/tls.c
> > +++ b/arch/x86/kernel/tls.c
> > @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx,
> > savesegment(fs, sel);
> > if (sel == modified_sel)
> > loadsegment(fs, sel);
> > -
> > - savesegment(gs, sel);
> > - if (sel == modified_sel)
> > - load_gs_index(sel);
> > #endif
> >
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > savesegment(gs, sel);
> > if (sel == modified_sel)
> > - loadsegment(gs, sel);
> > -#endif
> > + load_gs_index(sel);
> > } else {
> > #ifdef CONFIG_X86_64
> > if (p->thread.fsindex == modified_sel)
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index f923e14e87df..ec39073b4897 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> > #ifdef CONFIG_X86_64
> > loadsegment(fs, svm->host.fs);
> > wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> > - load_gs_index(svm->host.gs);
> > -#else
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > - loadsegment(gs, svm->host.gs);
> > -#endif
> > #endif
> > + load_gs_index(svm->host.gs);
> >
> > for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> > wrmsrl(host_save_user_msrs[i].index,
> > @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> > } else {
> > __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
> >
> > + /* Restore the percpu segment immediately. */
> > #ifdef CONFIG_X86_64
> > native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> > #else
> > loadsegment(fs, svm->host.fs);
> > -#ifndef CONFIG_X86_32_LAZY_GS
> > - loadsegment(gs, svm->host.gs);
> > -#endif
> > #endif
> > }
> >
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index 4229950a5d78..7f89a091f1fb 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -404,10 +404,6 @@ static short get_segment_selector(struct pt_regs *regs, int seg_reg_idx)
> > case INAT_SEG_REG_FS:
> > return (unsigned short)(regs->fs & 0xffff);
> > case INAT_SEG_REG_GS:
> > - /*
> > - * GS may or may not be in regs as per CONFIG_X86_32_LAZY_GS.
> > - * The macro below takes care of both cases.
> > - */
> > return get_user_gs(regs);
> > case INAT_SEG_REG_IGNORE:
> > default:
> > diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> > index 43b4d864817e..afbf0bb252da 100644
> > --- a/arch/x86/platform/pvh/head.S
> > +++ b/arch/x86/platform/pvh/head.S
> > @@ -45,10 +45,8 @@
> >
> > #define PVH_GDT_ENTRY_CS 1
> > #define PVH_GDT_ENTRY_DS 2
> > -#define PVH_GDT_ENTRY_CANARY 3
> > #define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8)
> > #define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8)
> > -#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8)
> >
> > SYM_CODE_START_LOCAL(pvh_start_xen)
> > cld
> > @@ -109,17 +107,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
> >
> > #else /* CONFIG_X86_64 */
> >
> > - /* Set base address in stack canary descriptor. */
> > - movl $_pa(gdt_start),%eax
> > - movl $_pa(canary),%ecx
> > - movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
> > - shrl $16, %ecx
> > - movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
> > - movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
> > -
> > - mov $PVH_CANARY_SEL,%eax
> > - mov %eax,%gs
> > -
> > call mk_early_pgtbl_32
> >
> > mov $_pa(initial_page_table), %eax
> > @@ -163,7 +150,6 @@ SYM_DATA_START_LOCAL(gdt_start)
> > .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */
> > #endif
> > .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */
> > - .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */
> > SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end)
> >
> > .balign 16
> > diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> > index db1378c6ff26..ef4329d67a5f 100644
> > --- a/arch/x86/power/cpu.c
> > +++ b/arch/x86/power/cpu.c
> > @@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
> > /*
> > * segment registers
> > */
> > -#ifdef CONFIG_X86_32_LAZY_GS
> > savesegment(gs, ctxt->gs);
> > -#endif
> > #ifdef CONFIG_X86_64
> > - savesegment(gs, ctxt->gs);
> > savesegment(fs, ctxt->fs);
> > savesegment(ds, ctxt->ds);
> > savesegment(es, ctxt->es);
> > @@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> > wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
> > #else
> > loadsegment(fs, __KERNEL_PERCPU);
> > - loadsegment(gs, __KERNEL_STACK_CANARY);
> > #endif
> >
> > /* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
> > @@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> > */
> > wrmsrl(MSR_FS_BASE, ctxt->fs_base);
> > wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
> > -#elif defined(CONFIG_X86_32_LAZY_GS)
> > +#else
> > loadsegment(gs, ctxt->gs);
> > #endif
> >
> > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> > index 9a5a50cdaab5..e18235a6390d 100644
> > --- a/arch/x86/xen/enlighten_pv.c
> > +++ b/arch/x86/xen/enlighten_pv.c
> > @@ -1190,7 +1190,6 @@ static void __init xen_setup_gdt(int cpu)
> > pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
> > pv_ops.cpu.load_gdt = xen_load_gdt_boot;
> >
> > - setup_stack_canary_segment(cpu);
> > switch_to_new_gdt(cpu);
> >
> > pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
> > diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
> > index f5c119495254..825c75c5b715 100755
> > --- a/scripts/gcc-x86_32-has-stack-protector.sh
> > +++ b/scripts/gcc-x86_32-has-stack-protector.sh
> > @@ -1,4 +1,8 @@
> > #!/bin/sh
> > # SPDX-License-Identifier: GPL-2.0
> >
> > -echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
> > +# This requires GCC 8.1 or better. Specifically, we require
> > +# -mstack-protector-guard-reg, added by
> > +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
>
> And -mstack-protector-guard-symbol= it would seem, since that's what
> will cause this to fail with Clang?
>
> What about -mstack-protector-guard=global which can be used for non
> SMP? I'm guessing that came out in an earlier version of GCC than
> -mstack-protector-guard-reg, so it doesn't need to be checked? FWIW,
> some of these were added to Clang in https://reviews.llvm.org/D88631.
>

That might be better to check if compiler supports
-mstack-protector-guard-reg=<arg> flag/option.

- Sedat -

> > +
> > +echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"
> > --
> > 2.29.2
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers

2021-03-08 13:15:40

by tip-bot2 for Haifeng Xu

[permalink] [raw]

Subject: [tip: x86/core] x86/entry/32: Remove leftover macros after stackprotector cleanups

The following commit has been merged into the x86/core branch of tip:

Commit-ID: d0962f2b24c99889a386f0658c71535f56358f77
Gitweb: https://git.kernel.org/tip/d0962f2b24c99889a386f0658c71535f56358f77
Author: Andy Lutomirski <[email protected]>
AuthorDate: Sat, 13 Feb 2021 11:19:45 -08:00
Committer: Borislav Petkov <[email protected]>
CommitterDate: Mon, 08 Mar 2021 13:27:31 +01:00

x86/entry/32: Remove leftover macros after stackprotector cleanups

Now that nonlazy-GS mode is gone, remove the macros from entry_32.S
that obfuscated^Wabstracted GS handling. The assembled output is
identical before and after this patch.

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lkml.kernel.org/r/b1543116f0f0e68f1763d90d5f7fcec27885dff5.1613243844.git.luto@kernel.org
---
arch/x86/entry/entry_32.S | 43 +-------------------------------------
1 file changed, 2 insertions(+), 41 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index eb0cb66..bee9101 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -53,35 +53,6 @@

#define PTI_SWITCH_MASK (1 << PAGE_SHIFT)

-/*
- * User gs save/restore
- *
- * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
- * will remove it entirely.
- */
- /* unfortunately push/pop can't be no-op */
-.macro PUSH_GS
- pushl $0
-.endm
-.macro POP_GS pop=0
- addl $(4 + \pop), %esp
-.endm
-.macro POP_GS_EX
-.endm
-
- /* all the rest are no-op */
-.macro PTGS_TO_GS
-.endm
-.macro PTGS_TO_GS_EX
-.endm
-.macro GS_TO_REG reg
-.endm
-.macro REG_TO_PTGS reg
-.endm
-.macro SET_KERNEL_GS reg
-.endm
-
-
/* Unconditionally switch to user cr3 */
.macro SWITCH_TO_USER_CR3 scratch_reg:req
ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
@@ -234,7 +205,7 @@
.macro SAVE_ALL pt_regs_ax=%eax switch_stacks=0 skip_gs=0 unwind_espfix=0
cld
.if \skip_gs == 0
- PUSH_GS
+ pushl $0
.endif
pushl %fs

@@ -259,9 +230,6 @@
movl $(__USER_DS), %edx
movl %edx, %ds
movl %edx, %es
-.if \skip_gs == 0
- SET_KERNEL_GS %edx
-.endif
/* Switch to kernel stack if necessary */
.if \switch_stacks > 0
SWITCH_TO_KERNEL_STACK
@@ -300,7 +268,7 @@
1: popl %ds
2: popl %es
3: popl %fs
- POP_GS \pop
+ addl $(4 + \pop), %esp /* pop the unused "gs" slot */
IRET_FRAME
.pushsection .fixup, "ax"
4: movl $0, (%esp)
@@ -313,7 +281,6 @@
_ASM_EXTABLE(1b, 4b)
_ASM_EXTABLE(2b, 5b)
_ASM_EXTABLE(3b, 6b)
- POP_GS_EX
.endm

.macro RESTORE_ALL_NMI cr3_reg:req pop=0
@@ -928,7 +895,6 @@ SYM_FUNC_START(entry_SYSENTER_32)
movl PT_EIP(%esp), %edx /* pt_regs->ip */
movl PT_OLDESP(%esp), %ecx /* pt_regs->sp */
1: mov PT_FS(%esp), %fs
- PTGS_TO_GS

popl %ebx /* pt_regs->bx */
addl $2*4, %esp /* skip pt_regs->cx and pt_regs->dx */
@@ -964,7 +930,6 @@ SYM_FUNC_START(entry_SYSENTER_32)
jmp 1b
.popsection
_ASM_EXTABLE(1b, 2b)
- PTGS_TO_GS_EX

.Lsysenter_fix_flags:
pushl $X86_EFLAGS_FIXED
@@ -1106,11 +1071,7 @@ SYM_CODE_START_LOCAL_NOALIGN(handle_exception)
SAVE_ALL switch_stacks=1 skip_gs=1 unwind_espfix=1
ENCODE_FRAME_POINTER

- /* fixup %gs */
- GS_TO_REG %ecx
movl PT_GS(%esp), %edi # get the function address
- REG_TO_PTGS %ecx
- SET_KERNEL_GS %ecx

/* fixup orig %eax */
movl PT_ORIG_EAX(%esp), %edx # get the error code

2021-03-08 13:18:54

by tip-bot2 for Haifeng Xu

[permalink] [raw]

Subject: [tip: x86/core] x86/stackprotector/32: Make the canary into a regular percpu variable

The following commit has been merged into the x86/core branch of tip:

Commit-ID: 3fb0fdb3bbe7aed495109b3296b06c2409734023
Gitweb: https://git.kernel.org/tip/3fb0fdb3bbe7aed495109b3296b06c2409734023
Author: Andy Lutomirski <[email protected]>
AuthorDate: Sat, 13 Feb 2021 11:19:44 -08:00
Committer: Borislav Petkov <[email protected]>
CommitterDate: Mon, 08 Mar 2021 13:19:05 +01:00

x86/stackprotector/32: Make the canary into a regular percpu variable

On 32-bit kernels, the stackprotector canary is quite nasty -- it is
stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
percpu storage. It's even nastier because it means that whether %gs
contains userspace state or kernel state while running kernel code
depends on whether stackprotector is enabled (this is
CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
that segment selectors work. Supporting both variants is a
maintenance and testing mess.

Merely rearranging so that percpu and the stack canary
share the same segment would be messy as the 32-bit percpu address
layout isn't currently compatible with putting a variable at a fixed
offset.

Fortunately, GCC 8.1 added options that allow the stack canary to be
accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
percpu variable. This lets us get rid of all of the code to manage the
stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.

(That name is special. We could use any symbol we want for the
%fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
name other than __stack_chk_guard.)

Forcibly disable stackprotector on older compilers that don't support
the new options and turn the stack canary into a percpu variable. The
"lazy GS" approach is now used for all 32-bit configurations.

Also makes load_gs_index() work on 32-bit kernels. On 64-bit kernels,
it loads the GS selector and updates the user GSBASE accordingly. (This
is unchanged.) On 32-bit kernels, it loads the GS selector and updates
GSBASE, which is now always the user base. This means that the overall
effect is the same on 32-bit and 64-bit, which avoids some ifdeffery.

[ bp: Massage commit message. ]

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lkml.kernel.org/r/c0ff7dba14041c7e5d1cae5d4df052f03759bef3.1613243844.git.luto@kernel.org
---
arch/x86/Kconfig | 7 +--
arch/x86/Makefile | 8 ++-
arch/x86/entry/entry_32.S | 56 +---------------
arch/x86/include/asm/processor.h | 15 +---
arch/x86/include/asm/ptrace.h | 5 +-
arch/x86/include/asm/segment.h | 30 ++------
arch/x86/include/asm/stackprotector.h | 79 ++++------------------
arch/x86/include/asm/suspend_32.h | 6 +--
arch/x86/kernel/asm-offsets_32.c | 5 +-
arch/x86/kernel/cpu/common.c | 5 +-
arch/x86/kernel/doublefault_32.c | 4 +-
arch/x86/kernel/head_32.S | 18 +-----
arch/x86/kernel/setup_percpu.c | 1 +-
arch/x86/kernel/tls.c | 8 +--
arch/x86/lib/insn-eval.c | 4 +-
arch/x86/platform/pvh/head.S | 14 +----
arch/x86/power/cpu.c | 6 +--
arch/x86/xen/enlighten_pv.c | 1 +-
scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
19 files changed, 60 insertions(+), 218 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2792879..10cc619 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -360,10 +360,6 @@ config X86_64_SMP
def_bool y
depends on X86_64 && SMP

-config X86_32_LAZY_GS
- def_bool y
- depends on X86_32 && !STACKPROTECTOR
-
config ARCH_SUPPORTS_UPROBES
def_bool y

@@ -386,7 +382,8 @@ config CC_HAS_SANE_STACKPROTECTOR
default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
help
We have to make sure stack protector is unconditionally disabled if
- the compiler produces broken code.
+ the compiler produces broken code or if it does not let us control
+ the segment on 32-bit kernels.

menu "Processor type and features"

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2d6d5a2..952f534 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -79,6 +79,14 @@ ifeq ($(CONFIG_X86_32),y)

# temporary until string.h is fixed
KBUILD_CFLAGS += -ffreestanding
+
+ ifeq ($(CONFIG_STACKPROTECTOR),y)
+ ifeq ($(CONFIG_SMP),y)
+ KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
+ else
+ KBUILD_CFLAGS += -mstack-protector-guard=global
+ endif
+ endif
else
BITS := 64
UTS_MACHINE := x86_64
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index df8c017..eb0cb66 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -20,7 +20,7 @@
* 1C(%esp) - %ds
* 20(%esp) - %es
* 24(%esp) - %fs
- * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
+ * 28(%esp) - unused -- was %gs on old stackprotector kernels
* 2C(%esp) - orig_eax
* 30(%esp) - %eip
* 34(%esp) - %cs
@@ -56,14 +56,9 @@
/*
* User gs save/restore
*
- * %gs is used for userland TLS and kernel only uses it for stack
- * canary which is required to be at %gs:20 by gcc. Read the comment
- * at the top of stackprotector.h for more info.
- *
- * Local labels 98 and 99 are used.
+ * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
+ * will remove it entirely.
*/
-#ifdef CONFIG_X86_32_LAZY_GS
-
/* unfortunately push/pop can't be no-op */
.macro PUSH_GS
pushl $0
@@ -86,49 +81,6 @@
.macro SET_KERNEL_GS reg
.endm

-#else /* CONFIG_X86_32_LAZY_GS */
-
-.macro PUSH_GS
- pushl %gs
-.endm
-
-.macro POP_GS pop=0
-98: popl %gs
- .if \pop <> 0
- add $\pop, %esp
- .endif
-.endm
-.macro POP_GS_EX
-.pushsection .fixup, "ax"
-99: movl $0, (%esp)
- jmp 98b
-.popsection
- _ASM_EXTABLE(98b, 99b)
-.endm
-
-.macro PTGS_TO_GS
-98: mov PT_GS(%esp), %gs
-.endm
-.macro PTGS_TO_GS_EX
-.pushsection .fixup, "ax"
-99: movl $0, PT_GS(%esp)
- jmp 98b
-.popsection
- _ASM_EXTABLE(98b, 99b)
-.endm
-
-.macro GS_TO_REG reg
- movl %gs, \reg
-.endm
-.macro REG_TO_PTGS reg
- movl \reg, PT_GS(%esp)
-.endm
-.macro SET_KERNEL_GS reg
- movl $(__KERNEL_STACK_CANARY), \reg
- movl \reg, %gs
-.endm
-
-#endif /* CONFIG_X86_32_LAZY_GS */

/* Unconditionally switch to user cr3 */
.macro SWITCH_TO_USER_CR3 scratch_reg:req
@@ -779,7 +731,7 @@ SYM_CODE_START(__switch_to_asm)

#ifdef CONFIG_STACKPROTECTOR
movl TASK_stack_canary(%edx), %ebx
- movl %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
+ movl %ebx, PER_CPU_VAR(__stack_chk_guard)
#endif

#ifdef CONFIG_RETPOLINE
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index dc6d149..bac2a42 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -439,6 +439,9 @@ struct fixed_percpu_data {
* GCC hardcodes the stack canary as %gs:40. Since the
* irq_stack is the object at %gs:0, we reserve the bottom
* 48 bytes of the irq stack for the canary.
+ *
+ * Once we are willing to require -mstack-protector-guard-symbol=
+ * support for x86_64 stackprotector, we can get rid of this.
*/
char gs_base[40];
unsigned long stack_canary;
@@ -460,17 +463,7 @@ extern asmlinkage void ignore_sysret(void);
void current_save_fsgs(void);
#else /* X86_64 */
#ifdef CONFIG_STACKPROTECTOR
-/*
- * Make sure stack canary segment base is cached-aligned:
- * "For Intel Atom processors, avoid non zero segment base address
- * that is not aligned to cache line boundary at all cost."
- * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
- */
-struct stack_canary {
- char __pad[20]; /* canary at %gs:20 */
- unsigned long canary;
-};
-DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
+DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
#endif
DECLARE_PER_CPU(struct irq_stack *, hardirq_stack_ptr);
DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index d8324a2..b2c4c12 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -37,7 +37,10 @@ struct pt_regs {
unsigned short __esh;
unsigned short fs;
unsigned short __fsh;
- /* On interrupt, gs and __gsh store the vector number. */
+ /*
+ * On interrupt, gs and __gsh store the vector number. They never
+ * store gs any more.
+ */
unsigned short gs;
unsigned short __gsh;
/* On interrupt, this is the error code. */
diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 7fdd4fa..7204402 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -95,7 +95,7 @@
*
* 26 - ESPFIX small SS
* 27 - per-cpu [ offset to per-cpu data area ]
- * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
+ * 28 - unused
* 29 - unused
* 30 - unused
* 31 - TSS for double fault handler
@@ -118,7 +118,6 @@

#define GDT_ENTRY_ESPFIX_SS 26
#define GDT_ENTRY_PERCPU 27
-#define GDT_ENTRY_STACK_CANARY 28

#define GDT_ENTRY_DOUBLEFAULT_TSS 31

@@ -158,12 +157,6 @@
# define __KERNEL_PERCPU 0
#endif

-#ifdef CONFIG_STACKPROTECTOR
-# define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY*8)
-#else
-# define __KERNEL_STACK_CANARY 0
-#endif
-
#else /* 64-bit: */

#include <asm/cache.h>
@@ -364,22 +357,15 @@ static inline void __loadsegment_fs(unsigned short value)
asm("mov %%" #seg ",%0":"=r" (value) : : "memory")

/*
- * x86-32 user GS accessors:
+ * x86-32 user GS accessors. This is ugly and could do with some cleaning up.
*/
#ifdef CONFIG_X86_32
-# ifdef CONFIG_X86_32_LAZY_GS
-# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
-# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
-# define task_user_gs(tsk) ((tsk)->thread.gs)
-# define lazy_save_gs(v) savesegment(gs, (v))
-# define lazy_load_gs(v) loadsegment(gs, (v))
-# else /* X86_32_LAZY_GS */
-# define get_user_gs(regs) (u16)((regs)->gs)
-# define set_user_gs(regs, v) do { (regs)->gs = (v); } while (0)
-# define task_user_gs(tsk) (task_pt_regs(tsk)->gs)
-# define lazy_save_gs(v) do { } while (0)
-# define lazy_load_gs(v) do { } while (0)
-# endif /* X86_32_LAZY_GS */
+# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
+# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
+# define task_user_gs(tsk) ((tsk)->thread.gs)
+# define lazy_save_gs(v) savesegment(gs, (v))
+# define lazy_load_gs(v) loadsegment(gs, (v))
+# define load_gs_index(v) loadsegment(gs, (v))
#endif /* X86_32 */

#endif /* !__ASSEMBLY__ */
diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 7fb482f..b6ffe58 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -5,30 +5,23 @@
* Stack protector works by putting predefined pattern at the start of
* the stack frame and verifying that it hasn't been overwritten when
* returning from the function. The pattern is called stack canary
- * and unfortunately gcc requires it to be at a fixed offset from %gs.
- * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64
- * and x86_32 use segment registers differently and thus handles this
- * requirement differently.
+ * and unfortunately gcc historically required it to be at a fixed offset
+ * from the percpu segment base. On x86_64, the offset is 40 bytes.
*
- * On x86_64, %gs is shared by percpu area and stack canary. All
- * percpu symbols are zero based and %gs points to the base of percpu
- * area. The first occupant of the percpu area is always
- * fixed_percpu_data which contains stack_canary at offset 40. Userland
- * %gs is always saved and restored on kernel entry and exit using
- * swapgs, so stack protector doesn't add any complexity there.
+ * The same segment is shared by percpu area and stack canary. On
+ * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
+ * base of percpu area. The first occupant of the percpu area is always
+ * fixed_percpu_data which contains stack_canary at the approproate
+ * offset. On x86_32, the stack canary is just a regular percpu
+ * variable.
*
- * On x86_32, it's slightly more complicated. As in x86_64, %gs is
- * used for userland TLS. Unfortunately, some processors are much
- * slower at loading segment registers with different value when
- * entering and leaving the kernel, so the kernel uses %fs for percpu
- * area and manages %gs lazily so that %gs is switched only when
- * necessary, usually during task switch.
+ * Putting percpu data in %fs on 32-bit is a minor optimization compared to
+ * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
+ * to load 0 into %fs on exit to usermode, whereas with percpu data in
+ * %gs, we are likely to load a non-null %gs on return to user mode.
*
- * As gcc requires the stack canary at %gs:20, %gs can't be managed
- * lazily if stack protector is enabled, so the kernel saves and
- * restores userland %gs on kernel entry and exit. This behavior is
- * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
- * system.h to hide the details.
+ * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
+ * support, we can remove some of this complexity.
*/

#ifndef _ASM_STACKPROTECTOR_H
@@ -45,14 +38,6 @@
#include <linux/sched.h>

/*
- * 24 byte read-only segment initializer for stack canary. Linker
- * can't handle the address bit shifting. Address will be set in
- * head_32 for boot CPU and setup_per_cpu_areas() for others.
- */
-#define GDT_STACK_CANARY_INIT \
- [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
-
-/*
* Initialize the stackprotector canary value.
*
* NOTE: this must only be called from functions that never return
@@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void)
#ifdef CONFIG_X86_64
this_cpu_write(fixed_percpu_data.stack_canary, canary);
#else
- this_cpu_write(stack_canary.canary, canary);
+ this_cpu_write(__stack_chk_guard, canary);
#endif
}

@@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
#ifdef CONFIG_X86_64
per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
#else
- per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
-#endif
-}
-
-static inline void setup_stack_canary_segment(int cpu)
-{
-#ifdef CONFIG_X86_32
- unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
- struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
- struct desc_struct desc;
-
- desc = gdt_table[GDT_ENTRY_STACK_CANARY];
- set_desc_base(&desc, canary);
- write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
-#endif
-}
-
-static inline void load_stack_canary_segment(void)
-{
-#ifdef CONFIG_X86_32
- asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
+ per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
#endif
}

#else /* STACKPROTECTOR */

-#define GDT_STACK_CANARY_INIT
-
/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */

-static inline void setup_stack_canary_segment(int cpu)
-{ }
-
static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
{ }

-static inline void load_stack_canary_segment(void)
-{
-#ifdef CONFIG_X86_32
- asm volatile ("mov %0, %%gs" : : "r" (0));
-#endif
-}
-
#endif /* STACKPROTECTOR */
#endif /* _ASM_STACKPROTECTOR_H */
diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
index fdbd9d7..7b132d0 100644
--- a/arch/x86/include/asm/suspend_32.h
+++ b/arch/x86/include/asm/suspend_32.h
@@ -13,12 +13,10 @@
/* image of the saved processor state */
struct saved_context {
/*
- * On x86_32, all segment registers, with the possible exception of
- * gs, are saved at kernel entry in pt_regs.
+ * On x86_32, all segment registers except gs are saved at kernel
+ * entry in pt_regs.
*/
-#ifdef CONFIG_X86_32_LAZY_GS
u16 gs;
-#endif
unsigned long cr0, cr2, cr3, cr4;
u64 misc_enable;
bool misc_enable_saved;
diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index 6e043f2..2b411cd 100644
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -53,11 +53,6 @@ void foo(void)
offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
offsetofend(struct cpu_entry_area, entry_stack_page.stack));

-#ifdef CONFIG_STACKPROTECTOR
- BLANK();
- OFFSET(stack_canary_offset, stack_canary, canary);
-#endif
-
BLANK();
DEFINE(EFI_svam, offsetof(efi_runtime_services_t, set_virtual_address_map));
}
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index ab640ab..23cb9d6 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -161,7 +161,6 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {

[GDT_ENTRY_ESPFIX_SS] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
[GDT_ENTRY_PERCPU] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
- GDT_STACK_CANARY_INIT
#endif
} };
EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
@@ -599,7 +598,6 @@ void load_percpu_segment(int cpu)
__loadsegment_simple(gs, 0);
wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
#endif
- load_stack_canary_segment();
}

#ifdef CONFIG_X86_32
@@ -1796,7 +1794,8 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);

#ifdef CONFIG_STACKPROTECTOR
-DEFINE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
+DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
+EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
#endif

#endif /* CONFIG_X86_64 */
diff --git a/arch/x86/kernel/doublefault_32.c b/arch/x86/kernel/doublefault_32.c
index 759d392..d1d49e3 100644
--- a/arch/x86/kernel/doublefault_32.c
+++ b/arch/x86/kernel/doublefault_32.c
@@ -100,9 +100,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
.ss = __KERNEL_DS,
.ds = __USER_DS,
.fs = __KERNEL_PERCPU,
-#ifndef CONFIG_X86_32_LAZY_GS
- .gs = __KERNEL_STACK_CANARY,
-#endif
+ .gs = 0,

.__cr3 = __pa_nodebug(swapper_pg_dir),
},
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 7ed84c2..67f5904 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp)
movl $(__KERNEL_PERCPU), %eax
movl %eax,%fs # set this cpu's percpu

- movl $(__KERNEL_STACK_CANARY),%eax
- movl %eax,%gs
+ xorl %eax,%eax
+ movl %eax,%gs # clear possible garbage in %gs

xorl %eax,%eax # Clear LDT
lldt %ax
@@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp)
*/
__INIT
setup_once:
-#ifdef CONFIG_STACKPROTECTOR
- /*
- * Configure the stack canary. The linker can't handle this by
- * relocation. Manually set base address in stack canary
- * segment descriptor.
- */
- movl $gdt_page,%eax
- movl $stack_canary,%ecx
- movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
- shrl $16, %ecx
- movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
- movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
-#endif
-
andl $0,setup_once_ref /* Once is enough, thanks */
ret

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index fd945ce..0941d2f 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -224,7 +224,6 @@ void __init setup_per_cpu_areas(void)
per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
per_cpu(cpu_number, cpu) = cpu;
setup_percpu_segment(cpu);
- setup_stack_canary_segment(cpu);
/*
* Copy data used in early init routines from the
* initial arrays to the per cpu data areas. These
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 64a496a..3c883e0 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx,
savesegment(fs, sel);
if (sel == modified_sel)
loadsegment(fs, sel);
-
- savesegment(gs, sel);
- if (sel == modified_sel)
- load_gs_index(sel);
#endif

-#ifdef CONFIG_X86_32_LAZY_GS
savesegment(gs, sel);
if (sel == modified_sel)
- loadsegment(gs, sel);
-#endif
+ load_gs_index(sel);
} else {
#ifdef CONFIG_X86_64
if (p->thread.fsindex == modified_sel)
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 4229950..7f89a09 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -404,10 +404,6 @@ static short get_segment_selector(struct pt_regs *regs, int seg_reg_idx)
case INAT_SEG_REG_FS:
return (unsigned short)(regs->fs & 0xffff);
case INAT_SEG_REG_GS:
- /*
- * GS may or may not be in regs as per CONFIG_X86_32_LAZY_GS.
- * The macro below takes care of both cases.
- */
return get_user_gs(regs);
case INAT_SEG_REG_IGNORE:
default:
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index d2ccadc..b049070 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -46,10 +46,8 @@

#define PVH_GDT_ENTRY_CS 1
#define PVH_GDT_ENTRY_DS 2
-#define PVH_GDT_ENTRY_CANARY 3
#define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8)
#define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8)
-#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8)

SYM_CODE_START_LOCAL(pvh_start_xen)
cld
@@ -111,17 +109,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen)

#else /* CONFIG_X86_64 */

- /* Set base address in stack canary descriptor. */
- movl $_pa(gdt_start),%eax
- movl $_pa(canary),%ecx
- movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
- shrl $16, %ecx
- movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
- movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
-
- mov $PVH_CANARY_SEL,%eax
- mov %eax,%gs
-
call mk_early_pgtbl_32

mov $_pa(initial_page_table), %eax
@@ -165,7 +152,6 @@ SYM_DATA_START_LOCAL(gdt_start)
.quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */
#endif
.quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */
- .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */
SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end)

.balign 16
diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index db1378c..ef4329d 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
/*
* segment registers
*/
-#ifdef CONFIG_X86_32_LAZY_GS
savesegment(gs, ctxt->gs);
-#endif
#ifdef CONFIG_X86_64
- savesegment(gs, ctxt->gs);
savesegment(fs, ctxt->fs);
savesegment(ds, ctxt->ds);
savesegment(es, ctxt->es);
@@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
#else
loadsegment(fs, __KERNEL_PERCPU);
- loadsegment(gs, __KERNEL_STACK_CANARY);
#endif

/* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
@@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
*/
wrmsrl(MSR_FS_BASE, ctxt->fs_base);
wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
-#elif defined(CONFIG_X86_32_LAZY_GS)
+#else
loadsegment(gs, ctxt->gs);
#endif

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index dc0a337..33e797b 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1204,7 +1204,6 @@ static void __init xen_setup_gdt(int cpu)
pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
pv_ops.cpu.load_gdt = xen_load_gdt_boot;

- setup_stack_canary_segment(cpu);
switch_to_new_gdt(cpu);

pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
index f5c1194..825c75c 100755
--- a/scripts/gcc-x86_32-has-stack-protector.sh
+++ b/scripts/gcc-x86_32-has-stack-protector.sh
@@ -1,4 +1,8 @@
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0

-echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
+# This requires GCC 8.1 or better. Specifically, we require
+# -mstack-protector-guard-reg, added by
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
+
+echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"

2022-09-29 14:59:52

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

+Cc: Ferry

On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> percpu storage. It's even nastier because it means that whether %gs
> contains userspace state or kernel state while running kernel code
> depends on whether stackprotector is enabled (this is
> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> that segment selectors work. Supporting both variants is a
> maintenance and testing mess.
>
> Merely rearranging so that percpu and the stack canary
> share the same segment would be messy as the 32-bit percpu address
> layout isn't currently compatible with putting a variable at a fixed
> offset.
>
> Fortunately, GCC 8.1 added options that allow the stack canary to be
> accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
> percpu variable. This lets us get rid of all of the code to manage the
> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>
> (That name is special. We could use any symbol we want for the
> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
> name other than __stack_chk_guard.)
>
> This patch forcibly disables stackprotector on older compilers that
> don't support the new options and makes the stack canary into a
> percpu variable. The "lazy GS" approach is now used for all 32-bit
> configurations.
>
> This patch also makes load_gs_index() work on 32-bit kernels. On
> 64-bit kernels, it loads the GS selector and updates the user
> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> it loads the GS selector and updates GSBASE, which is now
> always the user base. This means that the overall effect is
> the same on 32-bit and 64-bit, which avoids some ifdeffery.

This patch broke 32-bit boot on Intel Merrifield

git bisect start
# good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
# bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
git bisect bad 62fb9874f5da54fdb243003b386128037319b219
# bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch 'md-fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md into block-5.13
git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
# good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag 'regulator-v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
git bisect good ca62e9090d229926f43f20291bb44d67897baab7
# bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
# good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26] drm/amd/display: Change input parameter for set_drr
git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
# good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda: fix macroblocks count control usage
git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
# bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag 'x86_core_for_v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
# good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch 'work.inode-type-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
git bisect good d1466bc583a81830cef2399a4b8a514398351b40
# good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag 'afs-netfs-lib-20210426' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
# bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve a semantic conflict
git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
# bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d] x86/tools/insn_decoder_test: Convert to insn_decode()
git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
# bad: [514ef77607b9ff184c11b88e8f100bc27f07460d] x86/boot/compressed/sev-es: Convert to insn_decode()
git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
# bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn: Rename insn_decode() to insn_decode_from_regs()
git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
# bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32: Remove leftover macros after stackprotector cleanups
git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
# bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable
git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
# first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable

Any suggestions how to fix are welcome!

Configuration is based on in-tree i386_defconfig with some drivers enabled
on top (no core stuff was altered, but if you wish to check, it's here:
https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).

> Cc: Sedat Dilek <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> arch/x86/Kconfig | 7 +-
> arch/x86/Makefile | 8 +++
> arch/x86/entry/entry_32.S | 56 ++--------------
> arch/x86/include/asm/processor.h | 15 ++---
> arch/x86/include/asm/ptrace.h | 5 +-
> arch/x86/include/asm/segment.h | 30 +++------
> arch/x86/include/asm/stackprotector.h | 79 +++++------------------
> arch/x86/include/asm/suspend_32.h | 6 +-
> arch/x86/kernel/asm-offsets_32.c | 5 --
> arch/x86/kernel/cpu/common.c | 5 +-
> arch/x86/kernel/doublefault_32.c | 4 +-
> arch/x86/kernel/head_32.S | 18 +-----
> arch/x86/kernel/setup_percpu.c | 1 -
> arch/x86/kernel/tls.c | 8 +--
> arch/x86/kvm/svm/svm.c | 10 +--
> arch/x86/lib/insn-eval.c | 4 --
> arch/x86/platform/pvh/head.S | 14 ----
> arch/x86/power/cpu.c | 6 +-
> arch/x86/xen/enlighten_pv.c | 1 -
> scripts/gcc-x86_32-has-stack-protector.sh | 6 +-
> 20 files changed, 62 insertions(+), 226 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 21f851179ff0..12d8bf011d08 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -353,10 +353,6 @@ config X86_64_SMP
> def_bool y
> depends on X86_64 && SMP
>
> -config X86_32_LAZY_GS
> - def_bool y
> - depends on X86_32 && !STACKPROTECTOR
> -
> config ARCH_SUPPORTS_UPROBES
> def_bool y
>
> @@ -379,7 +375,8 @@ config CC_HAS_SANE_STACKPROTECTOR
> default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
> help
> We have to make sure stack protector is unconditionally disabled if
> - the compiler produces broken code.
> + the compiler produces broken code or if it does not let us control
> + the segment on 32-bit kernels.
>
> menu "Processor type and features"
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7116da3980be..0b5cd8c49ccb 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -76,6 +76,14 @@ ifeq ($(CONFIG_X86_32),y)
>
> # temporary until string.h is fixed
> KBUILD_CFLAGS += -ffreestanding
> +
> + ifeq ($(CONFIG_STACKPROTECTOR),y)
> + ifeq ($(CONFIG_SMP),y)
> + KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
> + else
> + KBUILD_CFLAGS += -mstack-protector-guard=global
> + endif
> + endif
> else
> BITS := 64
> UTS_MACHINE := x86_64
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index df8c017e6161..eb0cb662bca5 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -20,7 +20,7 @@
> * 1C(%esp) - %ds
> * 20(%esp) - %es
> * 24(%esp) - %fs
> - * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
> + * 28(%esp) - unused -- was %gs on old stackprotector kernels
> * 2C(%esp) - orig_eax
> * 30(%esp) - %eip
> * 34(%esp) - %cs
> @@ -56,14 +56,9 @@
> /*
> * User gs save/restore
> *
> - * %gs is used for userland TLS and kernel only uses it for stack
> - * canary which is required to be at %gs:20 by gcc. Read the comment
> - * at the top of stackprotector.h for more info.
> - *
> - * Local labels 98 and 99 are used.
> + * This is leftover junk from CONFIG_X86_32_LAZY_GS. A subsequent patch
> + * will remove it entirely.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> -
> /* unfortunately push/pop can't be no-op */
> .macro PUSH_GS
> pushl $0
> @@ -86,49 +81,6 @@
> .macro SET_KERNEL_GS reg
> .endm
>
> -#else /* CONFIG_X86_32_LAZY_GS */
> -
> -.macro PUSH_GS
> - pushl %gs
> -.endm
> -
> -.macro POP_GS pop=0
> -98: popl %gs
> - .if \pop <> 0
> - add $\pop, %esp
> - .endif
> -.endm
> -.macro POP_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, (%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro PTGS_TO_GS
> -98: mov PT_GS(%esp), %gs
> -.endm
> -.macro PTGS_TO_GS_EX
> -.pushsection .fixup, "ax"
> -99: movl $0, PT_GS(%esp)
> - jmp 98b
> -.popsection
> - _ASM_EXTABLE(98b, 99b)
> -.endm
> -
> -.macro GS_TO_REG reg
> - movl %gs, \reg
> -.endm
> -.macro REG_TO_PTGS reg
> - movl \reg, PT_GS(%esp)
> -.endm
> -.macro SET_KERNEL_GS reg
> - movl $(__KERNEL_STACK_CANARY), \reg
> - movl \reg, %gs
> -.endm
> -
> -#endif /* CONFIG_X86_32_LAZY_GS */
>
> /* Unconditionally switch to user cr3 */
> .macro SWITCH_TO_USER_CR3 scratch_reg:req
> @@ -779,7 +731,7 @@ SYM_CODE_START(__switch_to_asm)
>
> #ifdef CONFIG_STACKPROTECTOR
> movl TASK_stack_canary(%edx), %ebx
> - movl %ebx, PER_CPU_VAR(stack_canary)+stack_canary_offset
> + movl %ebx, PER_CPU_VAR(__stack_chk_guard)
> #endif
>
> #ifdef CONFIG_RETPOLINE
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index c20a52b5534b..c59dff4bbc38 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -441,6 +441,9 @@ struct fixed_percpu_data {
> * GCC hardcodes the stack canary as %gs:40. Since the
> * irq_stack is the object at %gs:0, we reserve the bottom
> * 48 bytes of the irq stack for the canary.
> + *
> + * Once we are willing to require -mstack-protector-guard-symbol=
> + * support for x86_64 stackprotector, we can get rid of this.
> */
> char gs_base[40];
> unsigned long stack_canary;
> @@ -461,17 +464,7 @@ extern asmlinkage void ignore_sysret(void);
> void current_save_fsgs(void);
> #else /* X86_64 */
> #ifdef CONFIG_STACKPROTECTOR
> -/*
> - * Make sure stack canary segment base is cached-aligned:
> - * "For Intel Atom processors, avoid non zero segment base address
> - * that is not aligned to cache line boundary at all cost."
> - * (Optim Ref Manual Assembly/Compiler Coding Rule 15.)
> - */
> -struct stack_canary {
> - char __pad[20]; /* canary at %gs:20 */
> - unsigned long canary;
> -};
> -DECLARE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DECLARE_PER_CPU(unsigned long, __stack_chk_guard);
> #endif
> /* Per CPU softirq stack pointer */
> DECLARE_PER_CPU(struct irq_stack *, softirq_stack_ptr);
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index d8324a236696..b2c4c12d237c 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -37,7 +37,10 @@ struct pt_regs {
> unsigned short __esh;
> unsigned short fs;
> unsigned short __fsh;
> - /* On interrupt, gs and __gsh store the vector number. */
> + /*
> + * On interrupt, gs and __gsh store the vector number. They never
> + * store gs any more.
> + */
> unsigned short gs;
> unsigned short __gsh;
> /* On interrupt, this is the error code. */
> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> index 7fdd4facfce7..72044026eb3c 100644
> --- a/arch/x86/include/asm/segment.h
> +++ b/arch/x86/include/asm/segment.h
> @@ -95,7 +95,7 @@
> *
> * 26 - ESPFIX small SS
> * 27 - per-cpu [ offset to per-cpu data area ]
> - * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
> + * 28 - unused
> * 29 - unused
> * 30 - unused
> * 31 - TSS for double fault handler
> @@ -118,7 +118,6 @@
>
> #define GDT_ENTRY_ESPFIX_SS 26
> #define GDT_ENTRY_PERCPU 27
> -#define GDT_ENTRY_STACK_CANARY 28
>
> #define GDT_ENTRY_DOUBLEFAULT_TSS 31
>
> @@ -158,12 +157,6 @@
> # define __KERNEL_PERCPU 0
> #endif
>
> -#ifdef CONFIG_STACKPROTECTOR
> -# define __KERNEL_STACK_CANARY (GDT_ENTRY_STACK_CANARY*8)
> -#else
> -# define __KERNEL_STACK_CANARY 0
> -#endif
> -
> #else /* 64-bit: */
>
> #include <asm/cache.h>
> @@ -364,22 +357,15 @@ static inline void __loadsegment_fs(unsigned short value)
> asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
>
> /*
> - * x86-32 user GS accessors:
> + * x86-32 user GS accessors. This is ugly and could do with some cleaning up.
> */
> #ifdef CONFIG_X86_32
> -# ifdef CONFIG_X86_32_LAZY_GS
> -# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> -# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> -# define task_user_gs(tsk) ((tsk)->thread.gs)
> -# define lazy_save_gs(v) savesegment(gs, (v))
> -# define lazy_load_gs(v) loadsegment(gs, (v))
> -# else /* X86_32_LAZY_GS */
> -# define get_user_gs(regs) (u16)((regs)->gs)
> -# define set_user_gs(regs, v) do { (regs)->gs = (v); } while (0)
> -# define task_user_gs(tsk) (task_pt_regs(tsk)->gs)
> -# define lazy_save_gs(v) do { } while (0)
> -# define lazy_load_gs(v) do { } while (0)
> -# endif /* X86_32_LAZY_GS */
> +# define get_user_gs(regs) (u16)({ unsigned long v; savesegment(gs, v); v; })
> +# define set_user_gs(regs, v) loadsegment(gs, (unsigned long)(v))
> +# define task_user_gs(tsk) ((tsk)->thread.gs)
> +# define lazy_save_gs(v) savesegment(gs, (v))
> +# define lazy_load_gs(v) loadsegment(gs, (v))
> +# define load_gs_index(v) loadsegment(gs, (v))
> #endif /* X86_32 */
>
> #endif /* !__ASSEMBLY__ */
> diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> index 7fb482f0f25b..b6ffe58c70fa 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -5,30 +5,23 @@
> * Stack protector works by putting predefined pattern at the start of
> * the stack frame and verifying that it hasn't been overwritten when
> * returning from the function. The pattern is called stack canary
> - * and unfortunately gcc requires it to be at a fixed offset from %gs.
> - * On x86_64, the offset is 40 bytes and on x86_32 20 bytes. x86_64
> - * and x86_32 use segment registers differently and thus handles this
> - * requirement differently.
> + * and unfortunately gcc historically required it to be at a fixed offset
> + * from the percpu segment base. On x86_64, the offset is 40 bytes.
> *
> - * On x86_64, %gs is shared by percpu area and stack canary. All
> - * percpu symbols are zero based and %gs points to the base of percpu
> - * area. The first occupant of the percpu area is always
> - * fixed_percpu_data which contains stack_canary at offset 40. Userland
> - * %gs is always saved and restored on kernel entry and exit using
> - * swapgs, so stack protector doesn't add any complexity there.
> + * The same segment is shared by percpu area and stack canary. On
> + * x86_64, percpu symbols are zero based and %gs (64-bit) points to the
> + * base of percpu area. The first occupant of the percpu area is always
> + * fixed_percpu_data which contains stack_canary at the approproate
> + * offset. On x86_32, the stack canary is just a regular percpu
> + * variable.
> *
> - * On x86_32, it's slightly more complicated. As in x86_64, %gs is
> - * used for userland TLS. Unfortunately, some processors are much
> - * slower at loading segment registers with different value when
> - * entering and leaving the kernel, so the kernel uses %fs for percpu
> - * area and manages %gs lazily so that %gs is switched only when
> - * necessary, usually during task switch.
> + * Putting percpu data in %fs on 32-bit is a minor optimization compared to
> + * using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
> + * to load 0 into %fs on exit to usermode, whereas with percpu data in
> + * %gs, we are likely to load a non-null %gs on return to user mode.
> *
> - * As gcc requires the stack canary at %gs:20, %gs can't be managed
> - * lazily if stack protector is enabled, so the kernel saves and
> - * restores userland %gs on kernel entry and exit. This behavior is
> - * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
> - * system.h to hide the details.
> + * Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
> + * support, we can remove some of this complexity.
> */
>
> #ifndef _ASM_STACKPROTECTOR_H
> @@ -44,14 +37,6 @@
> #include <linux/random.h>
> #include <linux/sched.h>
>
> -/*
> - * 24 byte read-only segment initializer for stack canary. Linker
> - * can't handle the address bit shifting. Address will be set in
> - * head_32 for boot CPU and setup_per_cpu_areas() for others.
> - */
> -#define GDT_STACK_CANARY_INIT \
> - [GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),
> -
> /*
> * Initialize the stackprotector canary value.
> *
> @@ -86,7 +71,7 @@ static __always_inline void boot_init_stack_canary(void)
> #ifdef CONFIG_X86_64
> this_cpu_write(fixed_percpu_data.stack_canary, canary);
> #else
> - this_cpu_write(stack_canary.canary, canary);
> + this_cpu_write(__stack_chk_guard, canary);
> #endif
> }
>
> @@ -95,48 +80,16 @@ static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> #ifdef CONFIG_X86_64
> per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
> #else
> - per_cpu(stack_canary.canary, cpu) = idle->stack_canary;
> -#endif
> -}
> -
> -static inline void setup_stack_canary_segment(int cpu)
> -{
> -#ifdef CONFIG_X86_32
> - unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu);
> - struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu);
> - struct desc_struct desc;
> -
> - desc = gdt_table[GDT_ENTRY_STACK_CANARY];
> - set_desc_base(&desc, canary);
> - write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S);
> -#endif
> -}
> -
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory");
> + per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
> #endif
> }
>
> #else /* STACKPROTECTOR */
>
> -#define GDT_STACK_CANARY_INIT
> -
> /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
>
> -static inline void setup_stack_canary_segment(int cpu)
> -{ }
> -
> static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
> { }
>
> -static inline void load_stack_canary_segment(void)
> -{
> -#ifdef CONFIG_X86_32
> - asm volatile ("mov %0, %%gs" : : "r" (0));
> -#endif
> -}
> -
> #endif /* STACKPROTECTOR */
> #endif /* _ASM_STACKPROTECTOR_H */
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index fdbd9d7b7bca..7b132d0312eb 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -13,12 +13,10 @@
> /* image of the saved processor state */
> struct saved_context {
> /*
> - * On x86_32, all segment registers, with the possible exception of
> - * gs, are saved at kernel entry in pt_regs.
> + * On x86_32, all segment registers except gs are saved at kernel
> + * entry in pt_regs.
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> u16 gs;
> -#endif
> unsigned long cr0, cr2, cr3, cr4;
> u64 misc_enable;
> bool misc_enable_saved;
> diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
> index 6e043f295a60..2b411cd00a4e 100644
> --- a/arch/x86/kernel/asm-offsets_32.c
> +++ b/arch/x86/kernel/asm-offsets_32.c
> @@ -53,11 +53,6 @@ void foo(void)
> offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
> offsetofend(struct cpu_entry_area, entry_stack_page.stack));
>
> -#ifdef CONFIG_STACKPROTECTOR
> - BLANK();
> - OFFSET(stack_canary_offset, stack_canary, canary);
> -#endif
> -
> BLANK();
> DEFINE(EFI_svam, offsetof(efi_runtime_services_t, set_virtual_address_map));
> }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 35ad8480c464..f208569d2d3b 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -161,7 +161,6 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
>
> [GDT_ENTRY_ESPFIX_SS] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> [GDT_ENTRY_PERCPU] = GDT_ENTRY_INIT(0xc092, 0, 0xfffff),
> - GDT_STACK_CANARY_INIT
> #endif
> } };
> EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
> @@ -599,7 +598,6 @@ void load_percpu_segment(int cpu)
> __loadsegment_simple(gs, 0);
> wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
> #endif
> - load_stack_canary_segment();
> }
>
> #ifdef CONFIG_X86_32
> @@ -1793,7 +1791,8 @@ DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
> EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
>
> #ifdef CONFIG_STACKPROTECTOR
> -DEFINE_PER_CPU_ALIGNED(struct stack_canary, stack_canary);
> +DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> +EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
> #endif
>
> #endif /* CONFIG_X86_64 */
> diff --git a/arch/x86/kernel/doublefault_32.c b/arch/x86/kernel/doublefault_32.c
> index 759d392cbe9f..d1d49e3d536b 100644
> --- a/arch/x86/kernel/doublefault_32.c
> +++ b/arch/x86/kernel/doublefault_32.c
> @@ -100,9 +100,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack) = {
> .ss = __KERNEL_DS,
> .ds = __USER_DS,
> .fs = __KERNEL_PERCPU,
> -#ifndef CONFIG_X86_32_LAZY_GS
> - .gs = __KERNEL_STACK_CANARY,
> -#endif
> + .gs = 0,
>
> .__cr3 = __pa_nodebug(swapper_pg_dir),
> },
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 7ed84c282233..67f590425d90 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp)
> movl $(__KERNEL_PERCPU), %eax
> movl %eax,%fs # set this cpu's percpu
>
> - movl $(__KERNEL_STACK_CANARY),%eax
> - movl %eax,%gs
> + xorl %eax,%eax
> + movl %eax,%gs # clear possible garbage in %gs
>
> xorl %eax,%eax # Clear LDT
> lldt %ax
> @@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp)
> */
> __INIT
> setup_once:
> -#ifdef CONFIG_STACKPROTECTOR
> - /*
> - * Configure the stack canary. The linker can't handle this by
> - * relocation. Manually set base address in stack canary
> - * segment descriptor.
> - */
> - movl $gdt_page,%eax
> - movl $stack_canary,%ecx
> - movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax)
> - movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax)
> -#endif
> -
> andl $0,setup_once_ref /* Once is enough, thanks */
> ret
>
> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index fd945ce78554..0941d2f44f2a 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -224,7 +224,6 @@ void __init setup_per_cpu_areas(void)
> per_cpu(this_cpu_off, cpu) = per_cpu_offset(cpu);
> per_cpu(cpu_number, cpu) = cpu;
> setup_percpu_segment(cpu);
> - setup_stack_canary_segment(cpu);
> /*
> * Copy data used in early init routines from the
> * initial arrays to the per cpu data areas. These
> diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
> index 64a496a0687f..3c883e064242 100644
> --- a/arch/x86/kernel/tls.c
> +++ b/arch/x86/kernel/tls.c
> @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx,
> savesegment(fs, sel);
> if (sel == modified_sel)
> loadsegment(fs, sel);
> -
> - savesegment(gs, sel);
> - if (sel == modified_sel)
> - load_gs_index(sel);
> #endif
>
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, sel);
> if (sel == modified_sel)
> - loadsegment(gs, sel);
> -#endif
> + load_gs_index(sel);
> } else {
> #ifdef CONFIG_X86_64
> if (p->thread.fsindex == modified_sel)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f923e14e87df..ec39073b4897 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1467,12 +1467,8 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> #ifdef CONFIG_X86_64
> loadsegment(fs, svm->host.fs);
> wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> - load_gs_index(svm->host.gs);
> -#else
> -#ifdef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> + load_gs_index(svm->host.gs);
>
> for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
> wrmsrl(host_save_user_msrs[i].index,
> @@ -3705,13 +3701,11 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> } else {
> __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>
> + /* Restore the percpu segment immediately. */
> #ifdef CONFIG_X86_64
> native_wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
> loadsegment(fs, svm->host.fs);
> -#ifndef CONFIG_X86_32_LAZY_GS
> - loadsegment(gs, svm->host.gs);
> -#endif
> #endif
> }
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 4229950a5d78..7f89a091f1fb 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -404,10 +404,6 @@ static short get_segment_selector(struct pt_regs *regs, int seg_reg_idx)
> case INAT_SEG_REG_FS:
> return (unsigned short)(regs->fs & 0xffff);
> case INAT_SEG_REG_GS:
> - /*
> - * GS may or may not be in regs as per CONFIG_X86_32_LAZY_GS.
> - * The macro below takes care of both cases.
> - */
> return get_user_gs(regs);
> case INAT_SEG_REG_IGNORE:
> default:
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 43b4d864817e..afbf0bb252da 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -45,10 +45,8 @@
>
> #define PVH_GDT_ENTRY_CS 1
> #define PVH_GDT_ENTRY_DS 2
> -#define PVH_GDT_ENTRY_CANARY 3
> #define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8)
> #define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8)
> -#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8)
>
> SYM_CODE_START_LOCAL(pvh_start_xen)
> cld
> @@ -109,17 +107,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>
> #else /* CONFIG_X86_64 */
>
> - /* Set base address in stack canary descriptor. */
> - movl $_pa(gdt_start),%eax
> - movl $_pa(canary),%ecx
> - movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
> - shrl $16, %ecx
> - movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
> - movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
> -
> - mov $PVH_CANARY_SEL,%eax
> - mov %eax,%gs
> -
> call mk_early_pgtbl_32
>
> mov $_pa(initial_page_table), %eax
> @@ -163,7 +150,6 @@ SYM_DATA_START_LOCAL(gdt_start)
> .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */
> #endif
> .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */
> - .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */
> SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end)
>
> .balign 16
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index db1378c6ff26..ef4329d67a5f 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
> /*
> * segment registers
> */
> -#ifdef CONFIG_X86_32_LAZY_GS
> savesegment(gs, ctxt->gs);
> -#endif
> #ifdef CONFIG_X86_64
> - savesegment(gs, ctxt->gs);
> savesegment(fs, ctxt->fs);
> savesegment(ds, ctxt->ds);
> savesegment(es, ctxt->es);
> @@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
> #else
> loadsegment(fs, __KERNEL_PERCPU);
> - loadsegment(gs, __KERNEL_STACK_CANARY);
> #endif
>
> /* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
> @@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
> */
> wrmsrl(MSR_FS_BASE, ctxt->fs_base);
> wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
> -#elif defined(CONFIG_X86_32_LAZY_GS)
> +#else
> loadsegment(gs, ctxt->gs);
> #endif
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 9a5a50cdaab5..e18235a6390d 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1190,7 +1190,6 @@ static void __init xen_setup_gdt(int cpu)
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry_boot;
> pv_ops.cpu.load_gdt = xen_load_gdt_boot;
>
> - setup_stack_canary_segment(cpu);
> switch_to_new_gdt(cpu);
>
> pv_ops.cpu.write_gdt_entry = xen_write_gdt_entry;
> diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
> index f5c119495254..825c75c5b715 100755
> --- a/scripts/gcc-x86_32-has-stack-protector.sh
> +++ b/scripts/gcc-x86_32-has-stack-protector.sh
> @@ -1,4 +1,8 @@
> #!/bin/sh
> # SPDX-License-Identifier: GPL-2.0
>
> -echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
> +# This requires GCC 8.1 or better. Specifically, we require
> +# -mstack-protector-guard-reg, added by
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
> +
> +echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"
> --
> 2.29.2
>
>

--
With Best Regards,
Andy Shevchenko

2022-09-29 15:03:18

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> +Cc: Ferry
>
> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> > On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> > percpu storage. It's even nastier because it means that whether %gs
> > contains userspace state or kernel state while running kernel code
> > depends on whether stackprotector is enabled (this is
> > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> > that segment selectors work. Supporting both variants is a
> > maintenance and testing mess.
> >
> > Merely rearranging so that percpu and the stack canary
> > share the same segment would be messy as the 32-bit percpu address
> > layout isn't currently compatible with putting a variable at a fixed
> > offset.
> >
> > Fortunately, GCC 8.1 added options that allow the stack canary to be
> > accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
> > percpu variable. This lets us get rid of all of the code to manage the
> > stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> >
> > (That name is special. We could use any symbol we want for the
> > %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
> > name other than __stack_chk_guard.)
> >
> > This patch forcibly disables stackprotector on older compilers that
> > don't support the new options and makes the stack canary into a
> > percpu variable. The "lazy GS" approach is now used for all 32-bit
> > configurations.
> >
> > This patch also makes load_gs_index() work on 32-bit kernels. On
> > 64-bit kernels, it loads the GS selector and updates the user
> > GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> > it loads the GS selector and updates GSBASE, which is now
> > always the user base. This means that the overall effect is
> > the same on 32-bit and 64-bit, which avoids some ifdeffery.
>
> This patch broke 32-bit boot on Intel Merrifield
>
> git bisect start
> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch 'md-fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md into block-5.13
> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag 'regulator-v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26] drm/amd/display: Change input parameter for set_drr
> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda: fix macroblocks count control usage
> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag 'x86_core_for_v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch 'work.inode-type-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag 'afs-netfs-lib-20210426' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve a semantic conflict
> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d] x86/tools/insn_decoder_test: Convert to insn_decode()
> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d] x86/boot/compressed/sev-es: Convert to insn_decode()
> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn: Rename insn_decode() to insn_decode_from_regs()
> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32: Remove leftover macros after stackprotector cleanups
> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable
> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable
>
> Any suggestions how to fix are welcome!
>
> Configuration is based on in-tree i386_defconfig with some drivers enabled
> on top (no core stuff was altered, but if you wish to check, it's here:
> https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).

For the record (and preventing some questions) the v6.0-rc7 still has this issue.

I can't test reverts, because it's huge pile of changes in that area happened
for the last year or so.

--
With Best Regards,
Andy Shevchenko

2022-09-30 21:08:55

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

Hi,

Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
>> +Cc: Ferry
>>
>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
>>> percpu storage. It's even nastier because it means that whether %gs
>>> contains userspace state or kernel state while running kernel code
>>> depends on whether stackprotector is enabled (this is
>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
>>> that segment selectors work. Supporting both variants is a
>>> maintenance and testing mess.
>>>
>>> Merely rearranging so that percpu and the stack canary
>>> share the same segment would be messy as the 32-bit percpu address
>>> layout isn't currently compatible with putting a variable at a fixed
>>> offset.
>>>
>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
>>> accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
>>> percpu variable. This lets us get rid of all of the code to manage the
>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>>>
>>> (That name is special. We could use any symbol we want for the
>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
>>> name other than __stack_chk_guard.)
>>>
>>> This patch forcibly disables stackprotector on older compilers that
>>> don't support the new options and makes the stack canary into a
>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
>>> configurations.
>>>
>>> This patch also makes load_gs_index() work on 32-bit kernels. On
>>> 64-bit kernels, it loads the GS selector and updates the user
>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
>>> it loads the GS selector and updates GSBASE, which is now
>>> always the user base. This means that the overall effect is
>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>> This patch broke 32-bit boot on Intel Merrifield
>>
>> git bisect start
>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch 'md-fixes' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md into block-5.13
>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag 'regulator-v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26] drm/amd/display: Change input parameter for set_drr
>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda: fix macroblocks count control usage
>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag 'x86_core_for_v5.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch 'work.inode-type-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag 'afs-netfs-lib-20210426' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve a semantic conflict
>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d] x86/tools/insn_decoder_test: Convert to insn_decode()
>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d] x86/boot/compressed/sev-es: Convert to insn_decode()
>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn: Rename insn_decode() to insn_decode_from_regs()
>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32: Remove leftover macros after stackprotector cleanups
>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable
>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023] x86/stackprotector/32: Make the canary into a regular percpu variable

With the bad commit the last words in dmesg are:

mem auto-init: stack:off, heap alloc:off, heap free:off
Initializing HighMem for node 0 (00036ffe:0003f500)
Initializing Movable for node 0 (00000000:00000000)
Checking if this processor honours the WP bit even in supervisor mode...Ok.
Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
136200K highmem)
SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
trace event string verifier disabled
Dynamic Preempt: voluntary
rcu: Preemptible hierarchical RCU implementation.
rcu: RCU event tracing is enabled.
rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
Trampoline variant of Tasks RCU enabled.
Tracing variant of Tasks RCU enabled.
rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0

without the bad commit dmesg continues:

random: get_random_bytes called from start_kernel+0x492/0x65a with
crng_init=0
Console: colour dummy device 80x25
printk: console [tty0] enabled
printk: bootconsole [uart0] disabled

....

>> Any suggestions how to fix are welcome!
>>
>> Configuration is based on in-tree i386_defconfig with some drivers enabled
>> on top (no core stuff was altered, but if you wish to check, it's here:
>> https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).
> For the record (and preventing some questions) the v6.0-rc7 still has this issue.
>
> I can't test reverts, because it's huge pile of changes in that area happened
> for the last year or so.
>
I just tested this by reverting 3fb0fdb3 "x86/stackprotector/32: Make
the canary into a regular percpu variable" and it's prerequisite
d0962f2b "x86/entry/32: Remove leftover macros after stackprotector
cleanups" on top of v5.13 and indeed this resolves the boot issue.

I can also confirm the 2 reverts will not apply on top of v6.0-rc7.

2022-09-30 22:31:11

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

Hi,

Op 30-09-2022 om 22:30 schreef Ferry Toth:
> Hi,
>
> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
>>> +Cc: Ferry
>>>
>>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
>>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
>>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
>>>> percpu storage. It's even nastier because it means that whether %gs
>>>> contains userspace state or kernel state while running kernel code
>>>> depends on whether stackprotector is enabled (this is
>>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
>>>> that segment selectors work. Supporting both variants is a
>>>> maintenance and testing mess.
>>>>
>>>> Merely rearranging so that percpu and the stack canary
>>>> share the same segment would be messy as the 32-bit percpu address
>>>> layout isn't currently compatible with putting a variable at a fixed
>>>> offset.
>>>>
>>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
>>>> accessed as %fs:__stack_chk_guard, effectively turning it into an
>>>> ordinary
>>>> percpu variable. This lets us get rid of all of the code to manage
>>>> the
>>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>>>>
>>>> (That name is special. We could use any symbol we want for the
>>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us
>>>> use any
>>>> name other than __stack_chk_guard.)
>>>>
>>>> This patch forcibly disables stackprotector on older compilers that
>>>> don't support the new options and makes the stack canary into a
>>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
>>>> configurations.
>>>>
>>>> This patch also makes load_gs_index() work on 32-bit kernels. On
>>>> 64-bit kernels, it loads the GS selector and updates the user
>>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
>>>> it loads the GS selector and updates GSBASE, which is now
>>>> always the user base. This means that the overall effect is
>>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>>> This patch broke 32-bit boot on Intel Merrifield
>>>
>>> git bisect start
>>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
>>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
>>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
>>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
>>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
>>> 'md-fixes' of
>>> https://git.kernel.org/pub/scm/linux/kernel/git/song/md into block-5.13
>>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
>>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
>>> 'regulator-v5.13' of
>>> git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
>>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
>>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
>>> 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
>>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
>>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26] drm/amd/display:
>>> Change input parameter for set_drr
>>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
>>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda: fix
>>> macroblocks count control usage
>>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
>>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
>>> 'x86_core_for_v5.13' of
>>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
>>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
>>> 'work.inode-type-fixes' of
>>> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
>>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
>>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
>>> 'afs-netfs-lib-20210426' of
>>> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
>>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
>>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
>>> 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve a
>>> semantic conflict
>>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
>>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
>>> x86/tools/insn_decoder_test: Convert to insn_decode()
>>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
>>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
>>> x86/boot/compressed/sev-es: Convert to insn_decode()
>>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
>>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn: Rename
>>> insn_decode() to insn_decode_from_regs()
>>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
>>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
>>> Remove leftover macros after stackprotector cleanups
>>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
>>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>> x86/stackprotector/32: Make the canary into a regular percpu variable
>>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
>>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>> x86/stackprotector/32: Make the canary into a regular percpu variable
>
> With the bad commit the last words in dmesg are:
>
> mem auto-init: stack:off, heap alloc:off, heap free:off
> Initializing HighMem for node 0 (00036ffe:0003f500)
> Initializing Movable for node 0 (00000000:00000000)
> Checking if this processor honours the WP bit even in supervisor
> mode...Ok.
> Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> 136200K highmem)
> SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> trace event string verifier disabled
> Dynamic Preempt: voluntary
> rcu: Preemptible hierarchical RCU implementation.
> rcu: RCU event tracing is enabled.
> rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> Trampoline variant of Tasks RCU enabled.
> Tracing variant of Tasks RCU enabled.
> rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
>
> without the bad commit dmesg continues:
>
> random: get_random_bytes called from start_kernel+0x492/0x65a with
> crng_init=0
> Console: colour dummy device 80x25
> printk: console [tty0] enabled
> printk: bootconsole [uart0] disabled
>
> ....
>
>>> Any suggestions how to fix are welcome!
>>>
Interesting. I added the following fragment to the kernel config:

# CONFIG_STACKPROTECTOR is not set

And this resolves the boot issue (tested with v5.17 i686 on Intel
Merrifield)

>>> Configuration is based on in-tree i386_defconfig with some drivers
>>> enabled
>>> on top (no core stuff was altered, but if you wish to check, it's here:
>>> https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).
>>>
>> For the record (and preventing some questions) the v6.0-rc7 still has
>> this issue.
>>
>> I can't test reverts, because it's huge pile of changes in that area
>> happened
>> for the last year or so.
>>
> I just tested this by reverting 3fb0fdb3 "x86/stackprotector/32: Make
> the canary into a regular percpu variable" and it's prerequisite
> d0962f2b "x86/entry/32: Remove leftover macros after stackprotector
> cleanups" on top of v5.13 and indeed this resolves the boot issue.
>
> I can also confirm the 2 reverts will not apply on top of v6.0-rc7.
>

2022-11-09 15:56:28

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
> Op 30-09-2022 om 22:30 schreef Ferry Toth:
> > Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> > > On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> > > > On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> > > > > On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> > > > > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> > > > > percpu storage.? It's even nastier because it means that whether %gs
> > > > > contains userspace state or kernel state while running kernel code
> > > > > depends on whether stackprotector is enabled (this is
> > > > > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> > > > > that segment selectors work.? Supporting both variants is a
> > > > > maintenance and testing mess.
> > > > >
> > > > > Merely rearranging so that percpu and the stack canary
> > > > > share the same segment would be messy as the 32-bit percpu address
> > > > > layout isn't currently compatible with putting a variable at a fixed
> > > > > offset.
> > > > >
> > > > > Fortunately, GCC 8.1 added options that allow the stack canary to be
> > > > > accessed as %fs:__stack_chk_guard, effectively turning it
> > > > > into an ordinary
> > > > > percpu variable.? This lets us get rid of all of the code to
> > > > > manage the
> > > > > stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> > > > >
> > > > > (That name is special.? We could use any symbol we want for the
> > > > > ? %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
> > > > > let us use any
> > > > > ? name other than __stack_chk_guard.)
> > > > >
> > > > > This patch forcibly disables stackprotector on older compilers that
> > > > > don't support the new options and makes the stack canary into a
> > > > > percpu variable.? The "lazy GS" approach is now used for all 32-bit
> > > > > configurations.
> > > > >
> > > > > This patch also makes load_gs_index() work on 32-bit kernels.? On
> > > > > 64-bit kernels, it loads the GS selector and updates the user
> > > > > GSBASE accordingly.? (This is unchanged.)? On 32-bit kernels,
> > > > > it loads the GS selector and updates GSBASE, which is now
> > > > > always the user base.? This means that the overall effect is
> > > > > the same on 32-bit and 64-bit, which avoids some ifdeffery.
> > > > This patch broke 32-bit boot on Intel Merrifield
> > > >
> > > > git bisect start
> > > > # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> > > > git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> > > > # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> > > > git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> > > > # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
> > > > 'md-fixes' of
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
> > > > block-5.13
> > > > git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> > > > # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
> > > > 'regulator-v5.13' of
> > > > git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> > > > git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> > > > # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
> > > > 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> > > > git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> > > > # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
> > > > drm/amd/display: Change input parameter for set_drr
> > > > git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> > > > # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
> > > > fix macroblocks count control usage
> > > > git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> > > > # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
> > > > 'x86_core_for_v5.13' of
> > > > git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> > > > git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> > > > # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
> > > > 'work.inode-type-fixes' of
> > > > git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> > > > git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> > > > # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
> > > > 'afs-netfs-lib-20210426' of
> > > > git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> > > > git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> > > > # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
> > > > 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
> > > > a semantic conflict
> > > > git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> > > > # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
> > > > x86/tools/insn_decoder_test: Convert to insn_decode()
> > > > git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> > > > # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
> > > > x86/boot/compressed/sev-es: Convert to insn_decode()
> > > > git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> > > > # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
> > > > Rename insn_decode() to insn_decode_from_regs()
> > > > git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> > > > # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
> > > > Remove leftover macros after stackprotector cleanups
> > > > git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> > > > # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > variable
> > > > git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> > > > # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > variable
> >
> > With the bad commit the last words in dmesg are:
> >
> > mem auto-init: stack:off, heap alloc:off, heap free:off
> > Initializing HighMem for node 0 (00036ffe:0003f500)
> > Initializing Movable for node 0 (00000000:00000000)
> > Checking if this processor honours the WP bit even in supervisor
> > mode...Ok.
> > Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> > 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> > 136200K highmem)
> > SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> > trace event string verifier disabled
> > Dynamic Preempt: voluntary
> > rcu: Preemptible hierarchical RCU implementation.
> > rcu:???? RCU event tracing is enabled.
> > rcu:???? RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> > ?Trampoline variant of Tasks RCU enabled.
> > ?Tracing variant of Tasks RCU enabled.
> > rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> > rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> > NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
> >
> > without the bad commit dmesg continues:
> >
> > random: get_random_bytes called from start_kernel+0x492/0x65a with
> > crng_init=0
> > Console: colour dummy device 80x25
> > printk: console [tty0] enabled
> > printk: bootconsole [uart0] disabled
> >
> > ....
> >
> > > > Any suggestions how to fix are welcome!
> > > >
> Interesting. I added the following fragment to the kernel config:
>
> # CONFIG_STACKPROTECTOR is not set
>
> And this resolves the boot issue (tested with v5.17 i686 on Intel
> Merrifield)

I'm not sure that's the correct approach.
Any answer from the Andy Lutomirski?

And in general to x86 maintainers, do we support all features on x86 32-bit? If
no, can it be said explicitly, please?

> > > > Configuration is based on in-tree i386_defconfig with some
> > > > drivers enabled
> > > > on top (no core stuff was altered, but if you wish to check, it's here:
> > > > https://github.com/andy-shev/linux/blob/eds-acpi/arch/x86/configs/i386_defconfig).
> > > >
> > > For the record (and preventing some questions) the v6.0-rc7 still
> > > has this issue.
> > >
> > > I can't test reverts, because it's huge pile of changes in that area
> > > happened
> > > for the last year or so.
> > >
> > I just tested this by reverting 3fb0fdb3 "x86/stackprotector/32: Make
> > the canary into a regular percpu variable" and it's prerequisite
> > d0962f2b "x86/entry/32: Remove leftover macros after stackprotector
> > cleanups" on top of v5.13 and indeed this resolves the boot issue.
> >
> > I can also confirm the 2 reverts will not apply on top of v6.0-rc7.

--
With Best Regards,
Andy Shevchenko

2022-11-09 23:45:16

by Brian Gerst

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
<[email protected]> wrote:
>
> On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
> > Op 30-09-2022 om 22:30 schreef Ferry Toth:
> > > Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> > > > On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> > > > > On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> > > > > > On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> > > > > > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> > > > > > percpu storage. It's even nastier because it means that whether %gs
> > > > > > contains userspace state or kernel state while running kernel code
> > > > > > depends on whether stackprotector is enabled (this is
> > > > > > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> > > > > > that segment selectors work. Supporting both variants is a
> > > > > > maintenance and testing mess.
> > > > > >
> > > > > > Merely rearranging so that percpu and the stack canary
> > > > > > share the same segment would be messy as the 32-bit percpu address
> > > > > > layout isn't currently compatible with putting a variable at a fixed
> > > > > > offset.
> > > > > >
> > > > > > Fortunately, GCC 8.1 added options that allow the stack canary to be
> > > > > > accessed as %fs:__stack_chk_guard, effectively turning it
> > > > > > into an ordinary
> > > > > > percpu variable. This lets us get rid of all of the code to
> > > > > > manage the
> > > > > > stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> > > > > >
> > > > > > (That name is special. We could use any symbol we want for the
> > > > > > %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
> > > > > > let us use any
> > > > > > name other than __stack_chk_guard.)
> > > > > >
> > > > > > This patch forcibly disables stackprotector on older compilers that
> > > > > > don't support the new options and makes the stack canary into a
> > > > > > percpu variable. The "lazy GS" approach is now used for all 32-bit
> > > > > > configurations.
> > > > > >
> > > > > > This patch also makes load_gs_index() work on 32-bit kernels. On
> > > > > > 64-bit kernels, it loads the GS selector and updates the user
> > > > > > GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> > > > > > it loads the GS selector and updates GSBASE, which is now
> > > > > > always the user base. This means that the overall effect is
> > > > > > the same on 32-bit and 64-bit, which avoids some ifdeffery.
> > > > > This patch broke 32-bit boot on Intel Merrifield
> > > > >
> > > > > git bisect start
> > > > > # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> > > > > git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> > > > > # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> > > > > git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> > > > > # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
> > > > > 'md-fixes' of
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
> > > > > block-5.13
> > > > > git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> > > > > # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
> > > > > 'regulator-v5.13' of
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> > > > > git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> > > > > # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
> > > > > 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> > > > > git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> > > > > # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
> > > > > drm/amd/display: Change input parameter for set_drr
> > > > > git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> > > > > # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
> > > > > fix macroblocks count control usage
> > > > > git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> > > > > # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
> > > > > 'x86_core_for_v5.13' of
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> > > > > git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> > > > > # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
> > > > > 'work.inode-type-fixes' of
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> > > > > git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> > > > > # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
> > > > > 'afs-netfs-lib-20210426' of
> > > > > git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> > > > > git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> > > > > # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
> > > > > 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
> > > > > a semantic conflict
> > > > > git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> > > > > # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
> > > > > x86/tools/insn_decoder_test: Convert to insn_decode()
> > > > > git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> > > > > # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
> > > > > x86/boot/compressed/sev-es: Convert to insn_decode()
> > > > > git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> > > > > # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
> > > > > Rename insn_decode() to insn_decode_from_regs()
> > > > > git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> > > > > # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
> > > > > Remove leftover macros after stackprotector cleanups
> > > > > git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> > > > > # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > > variable
> > > > > git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> > > > > # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > > variable
> > >
> > > With the bad commit the last words in dmesg are:
> > >
> > > mem auto-init: stack:off, heap alloc:off, heap free:off
> > > Initializing HighMem for node 0 (00036ffe:0003f500)
> > > Initializing Movable for node 0 (00000000:00000000)
> > > Checking if this processor honours the WP bit even in supervisor
> > > mode...Ok.
> > > Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> > > 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> > > 136200K highmem)
> > > SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> > > trace event string verifier disabled
> > > Dynamic Preempt: voluntary
> > > rcu: Preemptible hierarchical RCU implementation.
> > > rcu: RCU event tracing is enabled.
> > > rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> > > Trampoline variant of Tasks RCU enabled.
> > > Tracing variant of Tasks RCU enabled.
> > > rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> > > rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> > > NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
> > >
> > > without the bad commit dmesg continues:
> > >
> > > random: get_random_bytes called from start_kernel+0x492/0x65a with
> > > crng_init=0
> > > Console: colour dummy device 80x25
> > > printk: console [tty0] enabled
> > > printk: bootconsole [uart0] disabled
> > >
> > > ....
> > >
> > > > > Any suggestions how to fix are welcome!
> > > > >
> > Interesting. I added the following fragment to the kernel config:
> >
> > # CONFIG_STACKPROTECTOR is not set
> >
> > And this resolves the boot issue (tested with v5.17 i686 on Intel
> > Merrifield)
>
> I'm not sure that's the correct approach.
> Any answer from the Andy Lutomirski?
>
> And in general to x86 maintainers, do we support all features on x86 32-bit? If
> no, can it be said explicitly, please?

What compiler version are you using?

--
Brian Gerst

2022-11-10 14:33:36

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Wed, Nov 09, 2022 at 05:33:24PM -0500, Brian Gerst wrote:
> On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
> <[email protected]> wrote:
> > On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
> > > Op 30-09-2022 om 22:30 schreef Ferry Toth:
> > > > Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> > > > > On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> > > > > > On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> > > > > > > On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> > > > > > > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> > > > > > > percpu storage. It's even nastier because it means that whether %gs
> > > > > > > contains userspace state or kernel state while running kernel code
> > > > > > > depends on whether stackprotector is enabled (this is
> > > > > > > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> > > > > > > that segment selectors work. Supporting both variants is a
> > > > > > > maintenance and testing mess.
> > > > > > >
> > > > > > > Merely rearranging so that percpu and the stack canary
> > > > > > > share the same segment would be messy as the 32-bit percpu address
> > > > > > > layout isn't currently compatible with putting a variable at a fixed
> > > > > > > offset.
> > > > > > >
> > > > > > > Fortunately, GCC 8.1 added options that allow the stack canary to be
> > > > > > > accessed as %fs:__stack_chk_guard, effectively turning it
> > > > > > > into an ordinary
> > > > > > > percpu variable. This lets us get rid of all of the code to
> > > > > > > manage the
> > > > > > > stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> > > > > > >
> > > > > > > (That name is special. We could use any symbol we want for the
> > > > > > > %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
> > > > > > > let us use any
> > > > > > > name other than __stack_chk_guard.)
> > > > > > >
> > > > > > > This patch forcibly disables stackprotector on older compilers that
> > > > > > > don't support the new options and makes the stack canary into a
> > > > > > > percpu variable. The "lazy GS" approach is now used for all 32-bit
> > > > > > > configurations.
> > > > > > >
> > > > > > > This patch also makes load_gs_index() work on 32-bit kernels. On
> > > > > > > 64-bit kernels, it loads the GS selector and updates the user
> > > > > > > GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> > > > > > > it loads the GS selector and updates GSBASE, which is now
> > > > > > > always the user base. This means that the overall effect is
> > > > > > > the same on 32-bit and 64-bit, which avoids some ifdeffery.
> > > > > > This patch broke 32-bit boot on Intel Merrifield
> > > > > >
> > > > > > git bisect start
> > > > > > # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> > > > > > git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> > > > > > # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> > > > > > git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> > > > > > # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
> > > > > > 'md-fixes' of
> > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
> > > > > > block-5.13
> > > > > > git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> > > > > > # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
> > > > > > 'regulator-v5.13' of
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> > > > > > git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> > > > > > # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
> > > > > > 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> > > > > > git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> > > > > > # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
> > > > > > drm/amd/display: Change input parameter for set_drr
> > > > > > git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> > > > > > # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
> > > > > > fix macroblocks count control usage
> > > > > > git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> > > > > > # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
> > > > > > 'x86_core_for_v5.13' of
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> > > > > > git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> > > > > > # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
> > > > > > 'work.inode-type-fixes' of
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> > > > > > git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> > > > > > # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
> > > > > > 'afs-netfs-lib-20210426' of
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> > > > > > git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> > > > > > # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
> > > > > > 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
> > > > > > a semantic conflict
> > > > > > git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> > > > > > # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
> > > > > > x86/tools/insn_decoder_test: Convert to insn_decode()
> > > > > > git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> > > > > > # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
> > > > > > x86/boot/compressed/sev-es: Convert to insn_decode()
> > > > > > git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> > > > > > # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
> > > > > > Rename insn_decode() to insn_decode_from_regs()
> > > > > > git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> > > > > > # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
> > > > > > Remove leftover macros after stackprotector cleanups
> > > > > > git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> > > > > > # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > > > variable
> > > > > > git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> > > > > > # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> > > > > > x86/stackprotector/32: Make the canary into a regular percpu
> > > > > > variable
> > > >
> > > > With the bad commit the last words in dmesg are:
> > > >
> > > > mem auto-init: stack:off, heap alloc:off, heap free:off
> > > > Initializing HighMem for node 0 (00036ffe:0003f500)
> > > > Initializing Movable for node 0 (00000000:00000000)
> > > > Checking if this processor honours the WP bit even in supervisor
> > > > mode...Ok.
> > > > Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> > > > 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> > > > 136200K highmem)
> > > > SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> > > > trace event string verifier disabled
> > > > Dynamic Preempt: voluntary
> > > > rcu: Preemptible hierarchical RCU implementation.
> > > > rcu: RCU event tracing is enabled.
> > > > rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> > > > Trampoline variant of Tasks RCU enabled.
> > > > Tracing variant of Tasks RCU enabled.
> > > > rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> > > > rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> > > > NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
> > > >
> > > > without the bad commit dmesg continues:
> > > >
> > > > random: get_random_bytes called from start_kernel+0x492/0x65a with
> > > > crng_init=0
> > > > Console: colour dummy device 80x25
> > > > printk: console [tty0] enabled
> > > > printk: bootconsole [uart0] disabled
> > > >
> > > > ....
> > > >
> > > > > > Any suggestions how to fix are welcome!
> > > > > >
> > > Interesting. I added the following fragment to the kernel config:
> > >
> > > # CONFIG_STACKPROTECTOR is not set
> > >
> > > And this resolves the boot issue (tested with v5.17 i686 on Intel
> > > Merrifield)
> >
> > I'm not sure that's the correct approach.
> > Any answer from the Andy Lutomirski?
> >
> > And in general to x86 maintainers, do we support all features on x86 32-bit? If
> > no, can it be said explicitly, please?
>
> What compiler version are you using?

I guess it was gcc 11 and now gcc 12 (as of Debian unstable distro).

Currently
gcc (Debian 12.2.0-3) 12.2.0

--
With Best Regards,
Andy Shevchenko

2022-11-10 19:56:57

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

Hi,

Op 09-11-2022 om 23:33 schreef Brian Gerst:
> On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
> <[email protected]> wrote:
>> On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
>>> Op 30-09-2022 om 22:30 schreef Ferry Toth:
>>>> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
>>>>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
>>>>>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
>>>>>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
>>>>>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
>>>>>>> percpu storage. It's even nastier because it means that whether %gs
>>>>>>> contains userspace state or kernel state while running kernel code
>>>>>>> depends on whether stackprotector is enabled (this is
>>>>>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
>>>>>>> that segment selectors work. Supporting both variants is a
>>>>>>> maintenance and testing mess.
>>>>>>>
>>>>>>> Merely rearranging so that percpu and the stack canary
>>>>>>> share the same segment would be messy as the 32-bit percpu address
>>>>>>> layout isn't currently compatible with putting a variable at a fixed
>>>>>>> offset.
>>>>>>>
>>>>>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
>>>>>>> accessed as %fs:__stack_chk_guard, effectively turning it
>>>>>>> into an ordinary
>>>>>>> percpu variable. This lets us get rid of all of the code to
>>>>>>> manage the
>>>>>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>>>>>>>
>>>>>>> (That name is special. We could use any symbol we want for the
>>>>>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
>>>>>>> let us use any
>>>>>>> name other than __stack_chk_guard.)
>>>>>>>
>>>>>>> This patch forcibly disables stackprotector on older compilers that
>>>>>>> don't support the new options and makes the stack canary into a
>>>>>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
>>>>>>> configurations.
>>>>>>>
>>>>>>> This patch also makes load_gs_index() work on 32-bit kernels. On
>>>>>>> 64-bit kernels, it loads the GS selector and updates the user
>>>>>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
>>>>>>> it loads the GS selector and updates GSBASE, which is now
>>>>>>> always the user base. This means that the overall effect is
>>>>>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>>>>>> This patch broke 32-bit boot on Intel Merrifield
>>>>>>
>>>>>> git bisect start
>>>>>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
>>>>>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
>>>>>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
>>>>>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
>>>>>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
>>>>>> 'md-fixes' of
>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
>>>>>> block-5.13
>>>>>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
>>>>>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
>>>>>> 'regulator-v5.13' of
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
>>>>>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
>>>>>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
>>>>>> 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
>>>>>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
>>>>>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
>>>>>> drm/amd/display: Change input parameter for set_drr
>>>>>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
>>>>>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
>>>>>> fix macroblocks count control usage
>>>>>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
>>>>>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
>>>>>> 'x86_core_for_v5.13' of
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>>>>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
>>>>>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
>>>>>> 'work.inode-type-fixes' of
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
>>>>>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
>>>>>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
>>>>>> 'afs-netfs-lib-20210426' of
>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
>>>>>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
>>>>>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
>>>>>> 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
>>>>>> a semantic conflict
>>>>>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
>>>>>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
>>>>>> x86/tools/insn_decoder_test: Convert to insn_decode()
>>>>>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
>>>>>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
>>>>>> x86/boot/compressed/sev-es: Convert to insn_decode()
>>>>>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
>>>>>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
>>>>>> Rename insn_decode() to insn_decode_from_regs()
>>>>>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
>>>>>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
>>>>>> Remove leftover macros after stackprotector cleanups
>>>>>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
>>>>>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>>>>> x86/stackprotector/32: Make the canary into a regular percpu
>>>>>> variable
>>>>>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
>>>>>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>>>>> x86/stackprotector/32: Make the canary into a regular percpu
>>>>>> variable
>>>> With the bad commit the last words in dmesg are:
>>>>
>>>> mem auto-init: stack:off, heap alloc:off, heap free:off
>>>> Initializing HighMem for node 0 (00036ffe:0003f500)
>>>> Initializing Movable for node 0 (00000000:00000000)
>>>> Checking if this processor honours the WP bit even in supervisor
>>>> mode...Ok.
>>>> Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
>>>> 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
>>>> 136200K highmem)
>>>> SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
>>>> trace event string verifier disabled
>>>> Dynamic Preempt: voluntary
>>>> rcu: Preemptible hierarchical RCU implementation.
>>>> rcu: RCU event tracing is enabled.
>>>> rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
>>>> Trampoline variant of Tasks RCU enabled.
>>>> Tracing variant of Tasks RCU enabled.
>>>> rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
>>>> rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
>>>> NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
>>>>
>>>> without the bad commit dmesg continues:
>>>>
>>>> random: get_random_bytes called from start_kernel+0x492/0x65a with
>>>> crng_init=0
>>>> Console: colour dummy device 80x25
>>>> printk: console [tty0] enabled
>>>> printk: bootconsole [uart0] disabled
>>>>
>>>> ....
>>>>
>>>>>> Any suggestions how to fix are welcome!
>>>>>>
>>> Interesting. I added the following fragment to the kernel config:
>>>
>>> # CONFIG_STACKPROTECTOR is not set
>>>
>>> And this resolves the boot issue (tested with v5.17 i686 on Intel
>>> Merrifield)
>> I'm not sure that's the correct approach.
I didn't intend as a resolution, merely as a workaround. And since
revert was not possible, as proof issue is localized in stack protector.
>> Any answer from the Andy Lutomirski?
>>
>> And in general to x86 maintainers, do we support all features on x86 32-bit? If
>> no, can it be said explicitly, please?
> What compiler version are you using?

I built with Yocto Honister which builds it's own cross-compiler gcc
11.2. For completeness:

root@yuna:~# uname -a
Linux yuna 5.17.0-edison-acpi-standard #1 SMP PREEMPT Sun Mar 20
20:14:17 UTC 2022 i686 i686 i386 GNU/Linux

root@yuna:~# cat /proc/version
Linux version 5.17.0-edison-acpi-standard (oe-user@oe-host)
(i686-poky-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37.20210721)
#1 SMP PREEMPT Sun Mar 20 20:14:17 UTC 2022

root@yuna:~# cat /etc/os-release
ID=poky-edison
NAME="Poky (Yocto Project Reference Distro)"
VERSION="3.4.4 (honister)"
VERSION_ID=3.4.4
PRETTY_NAME="Poky (Yocto Project Reference Distro) 3.4.4 (honister)"

> --
> Brian Gerst

2022-11-14 22:06:48

by Brian Gerst

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

On Thu, Nov 10, 2022 at 2:36 PM Ferry Toth <[email protected]> wrote:
>
> Hi,
>
> Op 09-11-2022 om 23:33 schreef Brian Gerst:
> > On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
> > <[email protected]> wrote:
> >> On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
> >>> Op 30-09-2022 om 22:30 schreef Ferry Toth:
> >>>> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
> >>>>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
> >>>>>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
> >>>>>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
> >>>>>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
> >>>>>>> percpu storage. It's even nastier because it means that whether %gs
> >>>>>>> contains userspace state or kernel state while running kernel code
> >>>>>>> depends on whether stackprotector is enabled (this is
> >>>>>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
> >>>>>>> that segment selectors work. Supporting both variants is a
> >>>>>>> maintenance and testing mess.
> >>>>>>>
> >>>>>>> Merely rearranging so that percpu and the stack canary
> >>>>>>> share the same segment would be messy as the 32-bit percpu address
> >>>>>>> layout isn't currently compatible with putting a variable at a fixed
> >>>>>>> offset.
> >>>>>>>
> >>>>>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
> >>>>>>> accessed as %fs:__stack_chk_guard, effectively turning it
> >>>>>>> into an ordinary
> >>>>>>> percpu variable. This lets us get rid of all of the code to
> >>>>>>> manage the
> >>>>>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
> >>>>>>>
> >>>>>>> (That name is special. We could use any symbol we want for the
> >>>>>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
> >>>>>>> let us use any
> >>>>>>> name other than __stack_chk_guard.)
> >>>>>>>
> >>>>>>> This patch forcibly disables stackprotector on older compilers that
> >>>>>>> don't support the new options and makes the stack canary into a
> >>>>>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
> >>>>>>> configurations.
> >>>>>>>
> >>>>>>> This patch also makes load_gs_index() work on 32-bit kernels. On
> >>>>>>> 64-bit kernels, it loads the GS selector and updates the user
> >>>>>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
> >>>>>>> it loads the GS selector and updates GSBASE, which is now
> >>>>>>> always the user base. This means that the overall effect is
> >>>>>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
> >>>>>> This patch broke 32-bit boot on Intel Merrifield
> >>>>>>
> >>>>>> git bisect start
> >>>>>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
> >>>>>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> >>>>>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
> >>>>>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
> >>>>>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
> >>>>>> 'md-fixes' of
> >>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
> >>>>>> block-5.13
> >>>>>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
> >>>>>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
> >>>>>> 'regulator-v5.13' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
> >>>>>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
> >>>>>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
> >>>>>> 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
> >>>>>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
> >>>>>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
> >>>>>> drm/amd/display: Change input parameter for set_drr
> >>>>>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
> >>>>>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
> >>>>>> fix macroblocks count control usage
> >>>>>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
> >>>>>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
> >>>>>> 'x86_core_for_v5.13' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
> >>>>>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
> >>>>>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
> >>>>>> 'work.inode-type-fixes' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
> >>>>>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
> >>>>>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
> >>>>>> 'afs-netfs-lib-20210426' of
> >>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
> >>>>>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
> >>>>>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
> >>>>>> 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
> >>>>>> a semantic conflict
> >>>>>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
> >>>>>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
> >>>>>> x86/tools/insn_decoder_test: Convert to insn_decode()
> >>>>>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
> >>>>>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
> >>>>>> x86/boot/compressed/sev-es: Convert to insn_decode()
> >>>>>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
> >>>>>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
> >>>>>> Rename insn_decode() to insn_decode_from_regs()
> >>>>>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
> >>>>>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
> >>>>>> Remove leftover macros after stackprotector cleanups
> >>>>>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
> >>>>>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> >>>>>> x86/stackprotector/32: Make the canary into a regular percpu
> >>>>>> variable
> >>>>>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
> >>>>>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
> >>>>>> x86/stackprotector/32: Make the canary into a regular percpu
> >>>>>> variable
> >>>> With the bad commit the last words in dmesg are:
> >>>>
> >>>> mem auto-init: stack:off, heap alloc:off, heap free:off
> >>>> Initializing HighMem for node 0 (00036ffe:0003f500)
> >>>> Initializing Movable for node 0 (00000000:00000000)
> >>>> Checking if this processor honours the WP bit even in supervisor
> >>>> mode...Ok.
> >>>> Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
> >>>> 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
> >>>> 136200K highmem)
> >>>> SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
> >>>> trace event string verifier disabled
> >>>> Dynamic Preempt: voluntary
> >>>> rcu: Preemptible hierarchical RCU implementation.
> >>>> rcu: RCU event tracing is enabled.
> >>>> rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
> >>>> Trampoline variant of Tasks RCU enabled.
> >>>> Tracing variant of Tasks RCU enabled.
> >>>> rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
> >>>> rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
> >>>> NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
> >>>>
> >>>> without the bad commit dmesg continues:
> >>>>
> >>>> random: get_random_bytes called from start_kernel+0x492/0x65a with
> >>>> crng_init=0
> >>>> Console: colour dummy device 80x25
> >>>> printk: console [tty0] enabled
> >>>> printk: bootconsole [uart0] disabled
> >>>>
> >>>> ....
> >>>>
> >>>>>> Any suggestions how to fix are welcome!
> >>>>>>
> >>> Interesting. I added the following fragment to the kernel config:
> >>>
> >>> # CONFIG_STACKPROTECTOR is not set
> >>>
> >>> And this resolves the boot issue (tested with v5.17 i686 on Intel
> >>> Merrifield)
> >> I'm not sure that's the correct approach.
> I didn't intend as a resolution, merely as a workaround. And since
> revert was not possible, as proof issue is localized in stack protector.
> >> Any answer from the Andy Lutomirski?
> >>
> >> And in general to x86 maintainers, do we support all features on x86 32-bit? If
> >> no, can it be said explicitly, please?
> > What compiler version are you using?
>
> I built with Yocto Honister which builds it's own cross-compiler gcc
> 11.2. For completeness:
>
> root@yuna:~# uname -a
> Linux yuna 5.17.0-edison-acpi-standard #1 SMP PREEMPT Sun Mar 20
> 20:14:17 UTC 2022 i686 i686 i386 GNU/Linux
>
> root@yuna:~# cat /proc/version
> Linux version 5.17.0-edison-acpi-standard (oe-user@oe-host)
> (i686-poky-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37.20210721)
> #1 SMP PREEMPT Sun Mar 20 20:14:17 UTC 2022
>
> root@yuna:~# cat /etc/os-release
> ID=poky-edison
> NAME="Poky (Yocto Project Reference Distro)"
> VERSION="3.4.4 (honister)"
> VERSION_ID=3.4.4
> PRETTY_NAME="Poky (Yocto Project Reference Distro) 3.4.4 (honister)"
>
> > --
> > Brian Gerst

What exactly happens when it fails (hang/reboot/oops)?

Does removing the call to boot_init_stack_canary() in init/main.c fix
the problem?

--
Brian Gerst

2022-11-14 22:34:53

[permalink] [raw]

Subject: Re: [PATCH v2 1/2] x86/stackprotector/32: Make the canary into a regular percpu variable

Hi

Op 14-11-2022 om 22:43 schreef Brian Gerst:
> On Thu, Nov 10, 2022 at 2:36 PM Ferry Toth <[email protected]> wrote:
>> Hi,
>>
>> Op 09-11-2022 om 23:33 schreef Brian Gerst:
>>> On Wed, Nov 9, 2022 at 9:50 AM Andy Shevchenko
>>> <[email protected]> wrote:
>>>> On Fri, Sep 30, 2022 at 11:18:51PM +0200, Ferry Toth wrote:
>>>>> Op 30-09-2022 om 22:30 schreef Ferry Toth:
>>>>>> Op 29-09-2022 om 16:20 schreef Andy Shevchenko:
>>>>>>> On Thu, Sep 29, 2022 at 04:56:07PM +0300, Andy Shevchenko wrote:
>>>>>>>> On Sat, Feb 13, 2021 at 11:19:44AM -0800, Andy Lutomirski wrote:
>>>>>>>>> On 32-bit kernels, the stackprotector canary is quite nasty -- it is
>>>>>>>>> stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
>>>>>>>>> percpu storage. It's even nastier because it means that whether %gs
>>>>>>>>> contains userspace state or kernel state while running kernel code
>>>>>>>>> depends on whether stackprotector is enabled (this is
>>>>>>>>> CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
>>>>>>>>> that segment selectors work. Supporting both variants is a
>>>>>>>>> maintenance and testing mess.
>>>>>>>>>
>>>>>>>>> Merely rearranging so that percpu and the stack canary
>>>>>>>>> share the same segment would be messy as the 32-bit percpu address
>>>>>>>>> layout isn't currently compatible with putting a variable at a fixed
>>>>>>>>> offset.
>>>>>>>>>
>>>>>>>>> Fortunately, GCC 8.1 added options that allow the stack canary to be
>>>>>>>>> accessed as %fs:__stack_chk_guard, effectively turning it
>>>>>>>>> into an ordinary
>>>>>>>>> percpu variable. This lets us get rid of all of the code to
>>>>>>>>> manage the
>>>>>>>>> stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
>>>>>>>>>
>>>>>>>>> (That name is special. We could use any symbol we want for the
>>>>>>>>> %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to
>>>>>>>>> let us use any
>>>>>>>>> name other than __stack_chk_guard.)
>>>>>>>>>
>>>>>>>>> This patch forcibly disables stackprotector on older compilers that
>>>>>>>>> don't support the new options and makes the stack canary into a
>>>>>>>>> percpu variable. The "lazy GS" approach is now used for all 32-bit
>>>>>>>>> configurations.
>>>>>>>>>
>>>>>>>>> This patch also makes load_gs_index() work on 32-bit kernels. On
>>>>>>>>> 64-bit kernels, it loads the GS selector and updates the user
>>>>>>>>> GSBASE accordingly. (This is unchanged.) On 32-bit kernels,
>>>>>>>>> it loads the GS selector and updates GSBASE, which is now
>>>>>>>>> always the user base. This means that the overall effect is
>>>>>>>>> the same on 32-bit and 64-bit, which avoids some ifdeffery.
>>>>>>>> This patch broke 32-bit boot on Intel Merrifield
>>>>>>>>
>>>>>>>> git bisect start
>>>>>>>> # good: [9f4ad9e425a1d3b6a34617b8ea226d56a119a717] Linux 5.12
>>>>>>>> git bisect good 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
>>>>>>>> # bad: [62fb9874f5da54fdb243003b386128037319b219] Linux 5.13
>>>>>>>> git bisect bad 62fb9874f5da54fdb243003b386128037319b219
>>>>>>>> # bad: [85f3f17b5db2dd9f8a094a0ddc665555135afd22] Merge branch
>>>>>>>> 'md-fixes' of
>>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/song/md into
>>>>>>>> block-5.13
>>>>>>>> git bisect bad 85f3f17b5db2dd9f8a094a0ddc665555135afd22
>>>>>>>> # good: [ca62e9090d229926f43f20291bb44d67897baab7] Merge tag
>>>>>>>> 'regulator-v5.13' of
>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regulator
>>>>>>>> git bisect good ca62e9090d229926f43f20291bb44d67897baab7
>>>>>>>> # bad: [68a32ba14177d4a21c4a9a941cf1d7aea86d436f] Merge tag
>>>>>>>> 'drm-next-2021-04-28' of git://anongit.freedesktop.org/drm/drm
>>>>>>>> git bisect bad 68a32ba14177d4a21c4a9a941cf1d7aea86d436f
>>>>>>>> # good: [49c70ece54b0d1c51bc31b2b0c1070777c992c26]
>>>>>>>> drm/amd/display: Change input parameter for set_drr
>>>>>>>> git bisect good 49c70ece54b0d1c51bc31b2b0c1070777c992c26
>>>>>>>> # good: [0b276e470a4d43e1365d3eb53c608a3d208cabd4] media: coda:
>>>>>>>> fix macroblocks count control usage
>>>>>>>> git bisect good 0b276e470a4d43e1365d3eb53c608a3d208cabd4
>>>>>>>> # bad: [c6536676c7fe3f572ba55842e59c3c71c01e7fb3] Merge tag
>>>>>>>> 'x86_core_for_v5.13' of
>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>>>>>>> git bisect bad c6536676c7fe3f572ba55842e59c3c71c01e7fb3
>>>>>>>> # good: [d1466bc583a81830cef2399a4b8a514398351b40] Merge branch
>>>>>>>> 'work.inode-type-fixes' of
>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs
>>>>>>>> git bisect good d1466bc583a81830cef2399a4b8a514398351b40
>>>>>>>> # good: [fafe1e39ed213221c0bce6b0b31669334368dc97] Merge tag
>>>>>>>> 'afs-netfs-lib-20210426' of
>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs
>>>>>>>> git bisect good fafe1e39ed213221c0bce6b0b31669334368dc97
>>>>>>>> # bad: [b1f480bc0686e65d5413c035bd13af2ea4888784] Merge branch
>>>>>>>> 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve
>>>>>>>> a semantic conflict
>>>>>>>> git bisect bad b1f480bc0686e65d5413c035bd13af2ea4888784
>>>>>>>> # bad: [0c925c61dae18ee3cb93a61cc9dd9562a066034d]
>>>>>>>> x86/tools/insn_decoder_test: Convert to insn_decode()
>>>>>>>> git bisect bad 0c925c61dae18ee3cb93a61cc9dd9562a066034d
>>>>>>>> # bad: [514ef77607b9ff184c11b88e8f100bc27f07460d]
>>>>>>>> x86/boot/compressed/sev-es: Convert to insn_decode()
>>>>>>>> git bisect bad 514ef77607b9ff184c11b88e8f100bc27f07460d
>>>>>>>> # bad: [9e761296c52dcdb1aaa151b65bd39accb05740d9] x86/insn:
>>>>>>>> Rename insn_decode() to insn_decode_from_regs()
>>>>>>>> git bisect bad 9e761296c52dcdb1aaa151b65bd39accb05740d9
>>>>>>>> # bad: [d0962f2b24c99889a386f0658c71535f56358f77] x86/entry/32:
>>>>>>>> Remove leftover macros after stackprotector cleanups
>>>>>>>> git bisect bad d0962f2b24c99889a386f0658c71535f56358f77
>>>>>>>> # bad: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>>>>>>> x86/stackprotector/32: Make the canary into a regular percpu
>>>>>>>> variable
>>>>>>>> git bisect bad 3fb0fdb3bbe7aed495109b3296b06c2409734023
>>>>>>>> # first bad commit: [3fb0fdb3bbe7aed495109b3296b06c2409734023]
>>>>>>>> x86/stackprotector/32: Make the canary into a regular percpu
>>>>>>>> variable
>>>>>> With the bad commit the last words in dmesg are:
>>>>>>
>>>>>> mem auto-init: stack:off, heap alloc:off, heap free:off
>>>>>> Initializing HighMem for node 0 (00036ffe:0003f500)
>>>>>> Initializing Movable for node 0 (00000000:00000000)
>>>>>> Checking if this processor honours the WP bit even in supervisor
>>>>>> mode...Ok.
>>>>>> Memory: 948444K/1004124K available (12430K kernel code, 2167K rwdata,
>>>>>> 4948K rodata, 716K init, 716K bss, 55680K reserved, 0K cma-reserved,
>>>>>> 136200K highmem)
>>>>>> SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
>>>>>> trace event string verifier disabled
>>>>>> Dynamic Preempt: voluntary
>>>>>> rcu: Preemptible hierarchical RCU implementation.
>>>>>> rcu: RCU event tracing is enabled.
>>>>>> rcu: RCU restricting CPUs from NR_CPUS=8 to nr_cpu_ids=2.
>>>>>> Trampoline variant of Tasks RCU enabled.
>>>>>> Tracing variant of Tasks RCU enabled.
>>>>>> rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
>>>>>> rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
>>>>>> NR_IRQS: 2304, nr_irqs: 512, preallocated irqs: 0
>>>>>>
>>>>>> without the bad commit dmesg continues:
>>>>>>
>>>>>> random: get_random_bytes called from start_kernel+0x492/0x65a with
>>>>>> crng_init=0
>>>>>> Console: colour dummy device 80x25
>>>>>> printk: console [tty0] enabled
>>>>>> printk: bootconsole [uart0] disabled
>>>>>>
>>>>>> ....
>>>>>>
>>>>>>>> Any suggestions how to fix are welcome!
>>>>>>>>
>>>>> Interesting. I added the following fragment to the kernel config:
>>>>>
>>>>> # CONFIG_STACKPROTECTOR is not set
>>>>>
>>>>> And this resolves the boot issue (tested with v5.17 i686 on Intel
>>>>> Merrifield)
>>>> I'm not sure that's the correct approach.
>> I didn't intend as a resolution, merely as a workaround. And since
>> revert was not possible, as proof issue is localized in stack protector.
>>>> Any answer from the Andy Lutomirski?
>>>>
>>>> And in general to x86 maintainers, do we support all features on x86 32-bit? If
>>>> no, can it be said explicitly, please?
>>> What compiler version are you using?
>> I built with Yocto Honister which builds it's own cross-compiler gcc
>> 11.2. For completeness:
>>
>> root@yuna:~# uname -a
>> Linux yuna 5.17.0-edison-acpi-standard #1 SMP PREEMPT Sun Mar 20
>> 20:14:17 UTC 2022 i686 i686 i386 GNU/Linux
>>
>> root@yuna:~# cat /proc/version
>> Linux version 5.17.0-edison-acpi-standard (oe-user@oe-host)
>> (i686-poky-linux-gcc (GCC) 11.2.0, GNU ld (GNU Binutils) 2.37.20210721)
>> #1 SMP PREEMPT Sun Mar 20 20:14:17 UTC 2022
>>
>> root@yuna:~# cat /etc/os-release
>> ID=poky-edison
>> NAME="Poky (Yocto Project Reference Distro)"
>> VERSION="3.4.4 (honister)"
>> VERSION_ID=3.4.4
>> PRETTY_NAME="Poky (Yocto Project Reference Distro) 3.4.4 (honister)"
>>
>>> --
>>> Brian Gerst
> What exactly happens when it fails (hang/reboot/oops)?

It just hangs. Rebooting into another kernel with CONFIG_STACKPROTECTOR
not set and displaying journal of the failed boot show the last words
mentioned above.

> Does removing the call to boot_init_stack_canary() in init/main.c fix
> the problem?
I will try tomorrow.
> --
> Brian Gerst

2022-11-17 19:46:50