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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 2 May 2018 14:31:16 -0700
Message-ID: <CAGXu5jK1_--omGSqz03sfB8twQVzy9ZQR9Xzi-uxseekp+r1jQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] arm64: Clear the stack
To:     Laura Abbott <labbott@redhat.com>
Cc:     Alexander Popov <alex.popov@linux.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, May 2, 2018 at 1:33 PM, Laura Abbott <labbott@redhat.com> wrote:
>
> Implementation of stackleak based heavily on the x86 version

Awesome! Notes below for both you and Alexander, since I think we can
create a common code base instead of having near-duplicates in the
arch/ trees...

>
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> Now written in C instead of a bunch of assembly.
> ---
>  arch/arm64/Kconfig                    |  1 +
>  arch/arm64/include/asm/processor.h    |  6 ++++
>  arch/arm64/kernel/Makefile            |  3 ++
>  arch/arm64/kernel/entry.S             |  6 ++++
>  arch/arm64/kernel/erase.c             | 55 +++++++++++++++++++++++++++++++++++
>  arch/arm64/kernel/process.c           | 16 ++++++++++
>  drivers/firmware/efi/libstub/Makefile |  3 +-
>  scripts/Makefile.gcc-plugins          |  5 +++-
>  8 files changed, 93 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm64/kernel/erase.c
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index eb2cf4938f6d..b0221db95dc9 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -92,6 +92,7 @@ config ARM64
>         select HAVE_ARCH_MMAP_RND_BITS
>         select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>         select HAVE_ARCH_SECCOMP_FILTER
> +       select HAVE_ARCH_STACKLEAK
>         select HAVE_ARCH_THREAD_STRUCT_WHITELIST
>         select HAVE_ARCH_TRACEHOOK
>         select HAVE_ARCH_TRANSPARENT_HUGEPAGE
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 767598932549..d31ab80ff647 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -124,6 +124,12 @@ struct thread_struct {
>         unsigned long           fault_address;  /* fault info */
>         unsigned long           fault_code;     /* ESR_EL1 value */
>         struct debug_info       debug;          /* debugging */
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +       unsigned long           lowest_stack;
> +#ifdef CONFIG_STACKLEAK_METRICS
> +       unsigned long           prev_lowest_stack;
> +#endif
> +#endif

I wonder if x86 and arm64 could include a common struct here that was
empty when the plugin is disabled... it would keep the ifdefs in one
place. Maybe include/linux/stackleak.h could be:

---start---
/* Poison value points to the unused hole in the virtual memory map */
#define STACKLEAK_POISON -0xBEEF
#define STACKLEAK_POISON_CHECK_DEPTH 128

struct stackleak {
#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
       unsigned long           lowest;
#ifdef CONFIG_STACKLEAK_METRICS
       unsigned long           prev_lowest;
#endif
#endif
};

asmlinkage void erase_kstack(void);
---eof---

and arch/*/include/asm/processor.h could do:

@@ -124,6 +124,12 @@ struct thread_struct {
        unsigned long           fault_address;  /* fault info */
        unsigned long           fault_code;     /* ESR_EL1 value */
        struct debug_info       debug;          /* debugging */
+       struct stackleak         stackleak;

and arch/x86/entry/erase.c could move to maybe kernel/stackleak.c?
(Oh, I notice this needs an SPDX line too.)

>  static inline void arch_thread_struct_whitelist(unsigned long *offset,
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bf825f38d206..0ceea613c65b 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
>  arm64-obj-$(CONFIG_CRASH_DUMP)         += crash_dump.o
>  arm64-obj-$(CONFIG_ARM_SDE_INTERFACE)  += sdei.o
>
> +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
> +KASAN_SANITIZE_erase.o := n
> +
>  obj-y                                  += $(arm64-obj-y) vdso/ probes/
>  obj-m                                  += $(arm64-obj-m)
>  head-y                                 := head.o
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ec2ee720e33e..3144f1ebdc18 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -401,6 +401,11 @@ tsk        .req    x28             // current thread_info
>
>         .text
>
> +       .macro  ERASE_KSTACK
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +       bl      erase_kstack
> +#endif
> +       .endm
>  /*
>   * Exception vectors.
>   */
> @@ -906,6 +911,7 @@ ret_to_user:
>         cbnz    x2, work_pending
>  finish_ret_to_user:
>         enable_step_tsk x1, x2
> +       ERASE_KSTACK
>         kernel_exit 0
>  ENDPROC(ret_to_user)

Nice. All of the return paths end up here (I went looking for
ret_from_fork's path). :)

>
> diff --git a/arch/arm64/kernel/erase.c b/arch/arm64/kernel/erase.c
> new file mode 100644
> index 000000000000..b8b5648d893b
> --- /dev/null
> +++ b/arch/arm64/kernel/erase.c
> @@ -0,0 +1,55 @@
> +#include <linux/bug.h>
> +#include <linux/sched.h>
> +#include <asm/current.h>
> +#include <asm/linkage.h>
> +#include <asm/processor.h>
> +
> +asmlinkage void erase_kstack(void)
> +{
> +       unsigned long p = current->thread.lowest_stack;
> +       unsigned long boundary = p & ~(THREAD_SIZE - 1);
> +       unsigned long poison = 0;
> +       const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
> +                                                       sizeof(unsigned long);
> +
> +       /*
> +        * Let's search for the poison value in the stack.
> +        * Start from the lowest_stack and go to the bottom.
> +        */
> +       while (p > boundary && poison <= check_depth) {
> +               if (*(unsigned long *)p == STACKLEAK_POISON)
> +                       poison++;
> +               else
> +                       poison = 0;
> +
> +               p -= sizeof(unsigned long);
> +       }
> +
> +       /*
> +        * One long int at the bottom of the thread stack is reserved and
> +        * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> +        */
> +       if (p == boundary)
> +               p += sizeof(unsigned long);
> +
> +#ifdef CONFIG_STACKLEAK_METRICS
> +       current->thread.prev_lowest_stack = p;
> +#endif
> +
> +       /*
> +        * So let's write the poison value to the kernel stack.
> +        * Start from the address in p and move up till the new boundary.
> +        */
> +       boundary = current_stack_pointer;

This is the only difference between x86 and arm64 in this code. What
do you think about implementing on_thread_stack() to match x86:

        if (on_thread_stack())
                boundary = current_stack_pointer;
        else
                boundary = current_top_of_stack();

then we could make this common code too instead of having two copies in arch/?

> +       BUG_ON(boundary - p >= THREAD_SIZE);
> +
> +       while (p < boundary) {
> +               *(unsigned long *)p = STACKLEAK_POISON;
> +               p += sizeof(unsigned long);
> +       }
> +
> +       /* Reset the lowest_stack value for the next syscall */
> +       current->thread.lowest_stack = current_stack_pointer;
> +}
> +
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f08a2ed9db0d..156fa0a0da19 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
>         p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
>         p->thread.cpu_context.sp = (unsigned long)childregs;
>
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +       p->thread.lowest_stack = (unsigned long)task_stack_page(p);
> +#endif
>         ptrace_hw_copy_thread(p);
>
>         return 0;
> @@ -493,3 +496,16 @@ void arch_setup_new_exec(void)
>  {
>         current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
>  }
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used check_alloca(unsigned long size)
> +{
> +       unsigned long sp, stack_left;
> +
> +       sp = current_stack_pointer;
> +
> +       stack_left = sp & (THREAD_SIZE - 1);
> +       BUG_ON(stack_left < 256 || size >= stack_left - 256);
> +}
> +EXPORT_SYMBOL(check_alloca);

This is pretty different from x86. Is this just an artifact of ORC, or
something else?

> +#endif
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index a34e9290a699..25dd2a14560d 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB)  += -I$(srctree)/scripts/dtc/libfdt
>  KBUILD_CFLAGS                  := $(cflags-y) -DDISABLE_BRANCH_PROFILING \
>                                    -D__NO_FORTIFY \
>                                    $(call cc-option,-ffreestanding) \
> -                                  $(call cc-option,-fno-stack-protector)
> +                                  $(call cc-option,-fno-stack-protector) \
> +                                  $(DISABLE_STACKLEAK_PLUGIN)
>
>  GCOV_PROFILE                   := n
>  KASAN_SANITIZE                 := n
> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
> index 8d6070fc538f..6cc0e35d3324 100644
> --- a/scripts/Makefile.gcc-plugins
> +++ b/scripts/Makefile.gcc-plugins
> @@ -37,11 +37,14 @@ ifdef CONFIG_GCC_PLUGINS
>
>    gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK)    += stackleak_plugin.so
>    gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK)     += -DSTACKLEAK_PLUGIN -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
> +  ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +    DISABLE_STACKLEAK_PLUGIN           += -fplugin-arg-stackleak_plugin-disable
> +  endif
>
>    GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
>
>    export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR
> -  export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN
> +  export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN DISABLE_STACKLEAK_PLUGIN
>
>    ifneq ($(PLUGINCC),)
>      # SANCOV_PLUGIN can be only in CFLAGS_KCOV because avoid duplication.
> --
> 2.14.3
>

-Kees

-- 
Kees Cook
Pixel Security