linux-kernel - Re: [PATCH 2/2] mm/kasan/kunit: extend vmalloc OOB tests to cover vrealloc()

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Message-ID: <CA+fCnZeHdUiQ-k=Cy4bY-DKa7pFow6GfkTsCa2rsYTJNSXYGhw@mail.gmail.com>
Date: Thu, 15 Jan 2026 04:56:18 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Maciej Żenczykowski <maze@...gle.com>, 
	Maciej Wieczor-Retman <m.wieczorretman@...me>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, 
	kasan-dev@...glegroups.com, Uladzislau Rezki <urezki@...il.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm/kasan/kunit: extend vmalloc OOB tests to cover vrealloc()

On Tue, Jan 13, 2026 at 8:16 PM Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
>
> Extend the vmalloc_oob() test to validate OOB detection after
> resizing vmalloc allocations with vrealloc().
>
> The test now verifies that KASAN correctly poisons and unpoisons vmalloc
> memory when allocations are shrunk and expanded, ensuring OOB accesses
> are reliably detected after each resize.
>
> Signed-off-by: Andrey Ryabinin <ryabinin.a.a@...il.com>
> ---
>  mm/kasan/kasan_test_c.c | 50 ++++++++++++++++++++++++++++-------------
>  1 file changed, 35 insertions(+), 15 deletions(-)
>
> diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
> index 2cafca31b092..cc8fc479e13a 100644
> --- a/mm/kasan/kasan_test_c.c
> +++ b/mm/kasan/kasan_test_c.c
> @@ -1840,6 +1840,29 @@ static void vmalloc_helpers_tags(struct kunit *test)
>         vfree(ptr);
>  }
>
> +static void vmalloc_oob_helper(struct kunit *test, char *v_ptr, size_t size)
> +{
> +       /*
> +        * We have to be careful not to hit the guard page in vmalloc tests.
> +        * The MMU will catch that and crash us.
> +        */
> +
> +       /* Make sure in-bounds accesses are valid. */
> +       v_ptr[0] = 0;
> +       v_ptr[size - 1] = 0;
> +
> +       /*
> +        * An unaligned access past the requested vmalloc size.
> +        * Only generic KASAN can precisely detect these.
> +        */
> +       if (IS_ENABLED(CONFIG_KASAN_GENERIC))
> +               KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)v_ptr)[size]);
> +
> +       /* An aligned access into the first out-of-bounds granule. */
> +       size = round_up(size, KASAN_GRANULE_SIZE);
> +       KUNIT_EXPECT_KASAN_FAIL_READ(test, ((volatile char *)v_ptr)[size]);
> +}
> +
>  static void vmalloc_oob(struct kunit *test)
>  {
>         char *v_ptr, *p_ptr;
> @@ -1856,24 +1879,21 @@ static void vmalloc_oob(struct kunit *test)
>
>         OPTIMIZER_HIDE_VAR(v_ptr);
>
> -       /*
> -        * We have to be careful not to hit the guard page in vmalloc tests.
> -        * The MMU will catch that and crash us.
> -        */
> +       vmalloc_oob_helper(test, v_ptr, size);
>
> -       /* Make sure in-bounds accesses are valid. */
> -       v_ptr[0] = 0;
> -       v_ptr[size - 1] = 0;
> +       size--;

Could do size -= KASAN_GRANULE_SIZE + 1: I think this would allow to
also check whole-granule poisoning/unpoisoning logic for tag-based
modes.

> +       v_ptr = vrealloc(v_ptr, size, GFP_KERNEL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, v_ptr);
>
> -       /*
> -        * An unaligned access past the requested vmalloc size.
> -        * Only generic KASAN can precisely detect these.
> -        */
> -       if (IS_ENABLED(CONFIG_KASAN_GENERIC))
> -               KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)v_ptr)[size]);
> +       OPTIMIZER_HIDE_VAR(v_ptr);
>
> -       /* An aligned access into the first out-of-bounds granule. */
> -       KUNIT_EXPECT_KASAN_FAIL_READ(test, ((volatile char *)v_ptr)[size + 5]);
> +       vmalloc_oob_helper(test, v_ptr, size);
> +
> +       size += 2;

And then e.g. size += 2 * KASAN_GRANULE_SIZE + 2 here.

> +       v_ptr = vrealloc(v_ptr, size, GFP_KERNEL);
> +       KUNIT_ASSERT_NOT_ERR_OR_NULL(test, v_ptr);
> +
> +       vmalloc_oob_helper(test, v_ptr, size);
>
>         /* Check that in-bounds accesses to the physical page are valid. */
>         page = vmalloc_to_page(v_ptr);
> --
> 2.52.0
>

Reviewed-by: Andrey Konovalov <andreyknvl@...il.com>