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

Dmitry Vyukov
2017-07-10T08:44:42Z

On Fri, Jul 7, 2017 at 12:01 AM, Greg Hackmann <ghackmann@google.com> 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@google.com>
> ---
> lib/test_kasan.c | 22 ++++++++++++++++++++++
> 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);
> +
> + 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);

Perhaps s/round_up(size, KASAN_SHADOW_SCALE_SIZE)/rounded_up_size/
because we already calculated that.

> +
> + const void *left_redzone = (const void *)(addr -
> + KASAN_ALLOCA_REDZONE_SIZE);
> + const void *right_redzone = (const void *)(addr + rounded_up_size);

Please check that size is rounded to KASAN_ALLOCA_REDZONE_SIZE. That's
the expectation, right? That can change is clang silently.

> + 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 the unaligned part at the end of the object
from size to rounded_up_size. You can see how we do it for heap
objects.

> +}
> +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)
> +{
> + kasan_unpoison_shadow(stack_top, stack_bottom - stack_top);
> +}
> +EXPORT_SYMBOL(__asan_allocas_unpoison);
> +
> #ifdef CONFIG_MEMORY_HOTPLUG
> static int 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 1229298cce64..b857dc70d6a2 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -23,6 +23,14 @@
> #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 beee0e980e2d..c6a5b7ab9e3a 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -101,6 +101,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
> break;
> case KASAN_USE_AFTER_SCOPE:
> bug_type = "use-after-scope";
> + case KASAN_ALLOCA_LEFT:
> + case KASAN_ALLOCA_RIGHT:
> + bug_type = "alloca-out-of-bounds";
> break;
> }
>
> --
> 2.13.2.725.g09c95d1e9-goog
>