linux-kernel - Re: [PATCH v2 2/2] kasan: Add strscpy() test to trigger tag fault on arm64

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Message-ID: <CA+fCnZfG79JmGG9rj7KbE=9yX-EM4e8CXDSm5F9=YEmgyX5v3w@mail.gmail.com>
Date: Thu, 20 Mar 2025 18:25:04 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Peter Collingbourne <pcc@...gle.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>, 
	Andy Shevchenko <andy@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	Mark Rutland <mark.rutland@....com>, Vincenzo Frascino <vincenzo.frascino@....com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 2/2] kasan: Add strscpy() test to trigger tag fault on arm64

On Tue, Mar 18, 2025 at 10:41 PM Peter Collingbourne <pcc@...gle.com> wrote:
>
> From: Vincenzo Frascino <vincenzo.frascino@....com>
>
> 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.
>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> Co-developed-by: Peter Collingbourne <pcc@...gle.com>
> Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> Link: https://linux-review.googlesource.com/id/If88e396b9e7c058c1a4b5a252274120e77b1898a
> ---
> v2:
> - rebased
> - fixed test failure
>
>  mm/kasan/kasan_test_c.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index 59d673400085f..c4bb3ee497b54 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -1570,7 +1570,9 @@ static void kasan_memcmp(struct kunit *test)
>  static void kasan_strings(struct kunit *test)
>  {
>         char *ptr;
> -       size_t size = 24;
> +       char *src, *src2;
> +       u8 tag;
> +       size_t size = 2 * KASAN_GRANULE_SIZE;
>
>         /*
>          * str* functions are not instrumented with CONFIG_AMD_MEM_ENCRYPT.
> @@ -1581,6 +1583,33 @@ static void kasan_strings(struct kunit *test)
>         ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>
> +       src = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
> +       strscpy(src, "f0cacc1a00000000f0cacc1a00000000", size);
> +
> +       tag = get_tag(src);
> +
> +       src2 = src + KASAN_GRANULE_SIZE;
> +
> +       /*
> +        * Shorten string and poison the granule after it so that the unaligned
> +        * read in strscpy() triggers a tag mismatch.
> +        */
> +       src[KASAN_GRANULE_SIZE - 1] = '\0';
> +       kasan_poison(src2, KASAN_GRANULE_SIZE, tag + 1, false);
> +
> +       /*
> +        * 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, size));
> +
> +       /* Undo operations above. */
> +       src[KASAN_GRANULE_SIZE - 1] = '0';
> +       kasan_poison(src2, KASAN_GRANULE_SIZE, tag, false);
> +
> +       kfree(src);

I have trouble understanding what this code is doing...

So the goal is to call strcpy with such an address, that the first 8
bytes (partially) cover 2 granules, one accessible and the other is
not?

If so, can we not do something like:

src = kmalloc(KASAN_GRANULE_SIZE, GFP_KERNEL | __GFP_ZERO);
strscpy(src, "aabbcceeddeeffg\0", size);
strscpy(ptr, src + KASAN_GRANULE_SIZE - 2, sizeof(unsigned long));

Otherwise, this code needs more explanatory comments and it's probably
better to move it out to a helper function.

>         kfree(ptr);
>
>         /*
> --
> 2.49.0.395.g12beb8f557-goog
>