/\/\/\/\/\/\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
>
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