Re: [PATCH 1/4] kasan: support alloca() poisoning

Andrey Ryabinin
2017-07-10T10:28:14Z

On 07/07/2017 01:01 AM, Greg Hackmann 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.

gcc now supports this too. So I think this patch should enable it.
It's off by default so you'll have to add --param asan-instrument-allocas=1 into cflags
to make it work

>
> __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@google.com>
> ---
> lib/test_kasan.c | 22 ++++++++++++++++++++++

Tests would be better as a separate patch.

> mm/kasan/kasan.c | 26 ++++++++++++++++++++++++++
> mm/kasan/kasan.h | 8 ++++++++
> mm/kasan/report.c | 3 +++
> 4 files changed, 59 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index a25c9763fce1..f774fcafb696 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -473,6 +473,26 @@ static noinline void __init use_after_scope_test(void)
> p[1023] = 1;
> }
>
> +static noinline void __init kasan_alloca_oob_left(void)
> +{
> + volatile int i = 10;
> + char alloca_array[i];
> + char *p = alloca_array - 1;
> +
> + pr_info("out-of-bounds to left on alloca\n");
> + *(volatile char *)p;
> +}
> +
> +static noinline void __init kasan_alloca_oob_right(void)
> +{
> + volatile int i = 10;
> + char alloca_array[i];
> + char *p = alloca_array + round_up(i, 8);

Why round_up() ?

> +
> + pr_info("out-of-bounds to right on alloca\n");
> + *(volatile char *)p;
> +}
> +
> static int __init kmalloc_tests_init(void)
> {
> /*
> @@ -503,6 +523,8 @@ static int __init kmalloc_tests_init(void)
> memcg_accounted_kmem_cache();
> kasan_stack_oob();
> kasan_global_oob();
> + kasan_alloca_oob_left();
> + kasan_alloca_oob_right();
> ksize_unpoisons_memory();
> copy_user_test();
> use_after_scope_test();
> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
> index c81549d5c833..892b626f564b 100644
> --- a/mm/kasan/kasan.c
> +++ b/mm/kasan/kasan.c
> @@ -802,6 +802,32 @@ 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) -
> + round_up(size, KASAN_SHADOW_SCALE_SIZE);
> +
> + const void *left_redzone = (const void *)(addr -
> + KASAN_ALLOCA_REDZONE_SIZE);
> + const void *right_redzone = (const void *)(addr + rounded_up_size);
> +
> + 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);

As Dmitry pointed out, the memory between [addr+size, addr+rounded_up_size) is left
unpoisoned. kasan_alloca_oob_right() without round_up() would have caught this.