Re: [PATCH v2 14/16] kasan: Remove ksize()-related tests

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, 23 Sept 2022 at 22:28, Kees Cook <keescook@chromium.org> wrote:
>
> In preparation for no longer unpoisoning in ksize(), remove the behavioral
> self-tests for ksize().
>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: kasan-dev@googlegroups.com
> Cc: linux-mm@kvack.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  lib/test_kasan.c  | 42 ------------------------------------------
>  mm/kasan/shadow.c |  4 +---
>  2 files changed, 1 insertion(+), 45 deletions(-)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 58c1b01ccfe2..bdd0ced8f8d7 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -753,46 +753,6 @@ static void kasan_global_oob_left(struct kunit *test)
>         KUNIT_EXPECT_KASAN_FAIL(test, *(volatile char *)p);
>  }
>
> -/* Check that ksize() makes the whole object accessible. */
> -static void ksize_unpoisons_memory(struct kunit *test)
> -{
> -       char *ptr;
> -       size_t size = 123, real_size;
> -
> -       ptr = kmalloc(size, GFP_KERNEL);
> -       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> -       real_size = ksize(ptr);
> -
> -       OPTIMIZER_HIDE_VAR(ptr);
> -
> -       /* This access shouldn't trigger a KASAN report. */
 > -       ptr[size] = 'x';

I would rather keep the tests and update to the new behavior. We had
bugs in ksize, we need test coverage.
I assume ptr[size] access must now produce an error even after ksize.


> -       /* This one must. */
> -       KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[real_size]);
> -
> -       kfree(ptr);
> -}
> -
> -/*
> - * Check that a use-after-free is detected by ksize() and via normal accesses
> - * after it.
> - */
> -static void ksize_uaf(struct kunit *test)
> -{
> -       char *ptr;
> -       int size = 128 - KASAN_GRANULE_SIZE;
> -
> -       ptr = kmalloc(size, GFP_KERNEL);
> -       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> -       kfree(ptr);
> -
> -       OPTIMIZER_HIDE_VAR(ptr);
> -       KUNIT_EXPECT_KASAN_FAIL(test, ksize(ptr));

This is still a bug that should be detected, right? Calling ksize on a
freed pointer is a bug.

> -       KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[0]);
> -       KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
> -}
> -
>  static void kasan_stack_oob(struct kunit *test)
>  {
>         char stack_array[10];
> @@ -1392,8 +1352,6 @@ static struct kunit_case kasan_kunit_test_cases[] = {
>         KUNIT_CASE(kasan_stack_oob),
>         KUNIT_CASE(kasan_alloca_oob_left),
>         KUNIT_CASE(kasan_alloca_oob_right),
> -       KUNIT_CASE(ksize_unpoisons_memory),
> -       KUNIT_CASE(ksize_uaf),
>         KUNIT_CASE(kmem_cache_double_free),
>         KUNIT_CASE(kmem_cache_invalid_free),
>         KUNIT_CASE(kmem_cache_double_destroy),
> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index 0e3648b603a6..0895c73e9b69 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -124,9 +124,7 @@ void kasan_unpoison(const void *addr, size_t size, bool init)
>         addr = kasan_reset_tag(addr);
>
>         /*
> -        * Skip KFENCE memory if called explicitly outside of sl*b. Also note
> -        * that calls to ksize(), where size is not a multiple of machine-word
> -        * size, would otherwise poison the invalid portion of the word.
> +        * Skip KFENCE memory if called explicitly outside of sl*b.
>          */
>         if (is_kfence_address(addr))
>                 return;
> --
> 2.34.1