Re: [PATCH v2 1/5] kasan: support alloca() poisoning

[Dmitry Vyukov]

/\/\/\/\/\/\On Wed, Nov 29, 2017 at 10:50 PM, Paul Lawrence
<paullawrence@xxxxxxxxxx> wrote:
> clang's AddressSanitizer implementation adds redzones on either side of
> alloca()ed buffers.  These redzones are 32-byte aligned and at least 32
> bytes long.
>
> __asan_alloca_poison() is passed the size and address of the allocated
> buffer, *excluding* the redzones on either side.  The left redzone will
> always be to the immediate left of this buffer; but AddressSanitizer may
> need to add padding between the end of the buffer and the right redzone.
> If there are any 8-byte chunks inside this padding, we should poison
> those too.
>
> __asan_allocas_unpoison() is just passed the top and bottom of the
> dynamic stack area, so unpoisoning is simpler.
>
> Signed-off-by: Greg Hackmann <ghackmann@xxxxxxxxxx>
> Signed-off-by: Paul Lawrence <paullawrence@xxxxxxxxxx>
>
>  mm/kasan/kasan.c  | 32 ++++++++++++++++++++++++++++++++
>  mm/kasan/kasan.h  |  8 ++++++++
>  mm/kasan/report.c |  4 ++++
>  3 files changed, 44 insertions(+)
>
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index 405bba487df5..f86f862f41f8 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -736,6 +736,38 @@ void __asan_unpoison_stack_memory(const void *addr, size_t size)
>  }
>  EXPORT_SYMBOL(__asan_unpoison_stack_memory);
>
> +/* Emitted by compiler to poison alloca()ed objects. */
> +void __asan_alloca_poison(unsigned long addr, size_t size)
> +{
> +       size_t rounded_up_size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
> +       size_t padding_size = round_up(size, KASAN_ALLOCA_REDZONE_SIZE) -
> +                       rounded_up_size;
> +
> +       const void *left_redzone = (const void *)(addr -
> +                       KASAN_ALLOCA_REDZONE_SIZE);
> +       const void *right_redzone = (const void *)(addr + rounded_up_size);
> +
> +       WARN_ON(!IS_ALIGNED(addr, KASAN_ALLOCA_REDZONE_SIZE));
> +
> +       kasan_unpoison_shadow((const void *)addr, size);

/\/\/\/\/\/\

Why do we need this? Stack must be clean. Compiler instrumentation
does not clear shadow for objects in function prologue, if stack is
dirty KASAN would explode.


> +       kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
> +                       KASAN_ALLOCA_LEFT);
> +       kasan_poison_shadow(right_redzone,
> +                       padding_size + KASAN_ALLOCA_REDZONE_SIZE,
> +                       KASAN_ALLOCA_RIGHT);

We also need to poison [size, rounded_up_size) with partial value if
the range is not empty. I.e. we can poison exactly, say, 3 bytes
there.


> +}
> +EXPORT_SYMBOL(__asan_alloca_poison);
> +
> +/* Emitted by compiler to unpoison alloca()ed areas when the stack unwinds. */
> +void __asan_allocas_unpoison(const void *stack_top, const void *stack_bottom)
> +{
> +       if (unlikely(!stack_top || stack_top > stack_bottom))
> +               return;
> +
> +       kasan_unpoison_shadow(stack_top, stack_bottom - stack_top);
> +}
> +EXPORT_SYMBOL(__asan_allocas_unpoison);
> +
>  #ifdef CONFIG_MEMORY_HOTPLUG
>  static int __meminit kasan_mem_notifier(struct notifier_block *nb,
>                         unsigned long action, void *data)
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index c70851a9a6a4..7c0bcd1f4c0d 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -24,6 +24,14 @@
>  #define KASAN_STACK_PARTIAL     0xF4
>  #define KASAN_USE_AFTER_SCOPE   0xF8
>
> +/*
> + * alloca redzone shadow values
> + */
> +#define KASAN_ALLOCA_LEFT      0xCA
> +#define KASAN_ALLOCA_RIGHT     0xCB
> +
> +#define KASAN_ALLOCA_REDZONE_SIZE      32
> +
>  /* Don't break randconfig/all*config builds */
>  #ifndef KASAN_ABI_VERSION
>  #define KASAN_ABI_VERSION 1
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 6bcfb01ba038..25419d426426 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -102,6 +102,10 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
>         case KASAN_USE_AFTER_SCOPE:
>                 bug_type = "use-after-scope";
>                 break;
> +       case KASAN_ALLOCA_LEFT:
> +       case KASAN_ALLOCA_RIGHT:
> +               bug_type = "alloca-out-of-bounds";
> +               break;
>         }
>
>         return bug_type;
> --
> 2.15.0.531.g2ccb3012c9-goog
>