Re: [PATCH v3 2/2] kasan: Add strscpy() test to trigger tag fault on arm64

[Andrey Konovalov]

On Tue, Mar 25, 2025 at 2:56 AM Peter Collingbourne <pcc@xxxxxxxxxx> wrote:
>
> From: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
>
> When we invoke strscpy() with a maximum size of N bytes, it assumes
> that:
> - It can always read N bytes from the source.
> - It always write N bytes (zero-padded) to the destination.
>
> On aarch64 with Memory Tagging Extension enabled if we pass an N that is
> bigger then the source buffer, it triggers an MTE fault.
>
> Implement a KASAN KUnit test that triggers the issue with the current
> implementation of read_word_at_a_time() on aarch64 with MTE enabled.

I think the wording here is confusing, makes it sound like the issue
is not fixed.

> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Co-developed-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
> ---
> v3:
> - simplify test case
>
> v2:
> - rebased
> - fixed test failure
>
>  mm/kasan/kasan_test_c.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index 59d673400085f..b69f66b7eda1d 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -1570,6 +1570,7 @@ static void kasan_memcmp(struct kunit *test)
>  static void kasan_strings(struct kunit *test)
>  {
>         char *ptr;
> +       char *src;
>         size_t size = 24;
>
>         /*
> @@ -1581,6 +1582,18 @@ static void kasan_strings(struct kunit *test)
>         ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>
> +       src = kmalloc(KASAN_GRANULE_SIZE, GFP_KERNEL | __GFP_ZERO);
> +       strscpy(src, "f0cacc1a0000000", KASAN_GRANULE_SIZE);
> +
> +       /*

Please also extend the comment here to say something like: "Make sure
that strscpy() does not trigger KASAN if it overreads into poisoned
memory".

> +        * The expected size does not include the terminator '\0'
> +        * so it is (KASAN_GRANULE_SIZE - 2) ==
> +        * KASAN_GRANULE_SIZE - ("initial removed character" + "\0").
> +        */
> +       KUNIT_EXPECT_EQ(test, KASAN_GRANULE_SIZE - 2,
> +                       strscpy(ptr, src + 1, KASAN_GRANULE_SIZE));
> +
> +       kfree(src);
>         kfree(ptr);
>
>         /*
> --
> 2.49.0.395.g12beb8f557-goog
>

The code looks much better!

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>