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Subject: Re: [PATCH 2/2] arm64: Clear the stack
To:     Kees Cook <keescook@chromium.org>
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>
References: <20180502203326.9491-1-labbott@redhat.com>
 <20180502203326.9491-3-labbott@redhat.com>
 <CAGXu5jK1_--omGSqz03sfB8twQVzy9ZQR9Xzi-uxseekp+r1jQ@mail.gmail.com>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <4b7e94c1-79c9-0380-25c6-762762ed595f@redhat.com>
Date:   Wed, 2 May 2018 16:07:01 -0700
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On 05/02/2018 02:31 PM, Kees Cook wrote:
> 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
> };
> 

Is this well defined across all compilers if the plugin is off?
This seems to compile with gcc at least but 0 sized structs
make me a little uneasy.

> 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/?
> 

The issue isn't on_thread_stack, it's current_top_of_stack which isn't
defined on arm64. I agree it would be good if the code would be common
but I'm not sure how much we want to start trying to force APIs.

>> +       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?
> 

This was based on the earlier version of x86. I'll confess to
not seeing how the current x86 version ended up with get_stack_info
but I suspect it's either related to ORC unwinding or it's best
practice.

>> +#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
> 

Thanks,
Laura