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From:   Alexander Potapenko <glider@google.com>
Date:   Wed, 13 Jan 2021 17:32:55 +0100
Message-ID: <CAG_fn=X+wtO3tABhJD-Bigx1QHmLu_21_VaEkS9Qsp=EsGS0FQ@mail.gmail.com>
Subject: Re: [PATCH v2 09/14] kasan: adapt kmalloc_uaf2 test to HW_TAGS mode
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jan 13, 2021 at 5:22 PM 'Andrey Konovalov' via kasan-dev
<kasan-dev@googlegroups.com> wrote:
>
> In the kmalloc_uaf2() test, the pointers to the two allocated memory
> blocks might happen to be the same, and the test will fail. With the
> software tag-based mode, the probability of the that is 1/254, so it's
> hard to observe the failure. For the hardware tag-based mode though,
> the probablity is 1/14, which is quite noticable.
>
> Allow up to 16 attempts at generating different tags for the tag-based
> modes.
>
> Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb0743=
4a300bf36388d55
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>

> ---
>  lib/test_kasan.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 283feda9882a..a1a35d75ee1e 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -382,7 +382,9 @@ static void kmalloc_uaf2(struct kunit *test)
>  {
>         char *ptr1, *ptr2;
>         size_t size =3D 43;
> +       int counter =3D 0;
>
> +again:
>         ptr1 =3D kmalloc(size, GFP_KERNEL);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
>
> @@ -391,6 +393,15 @@ static void kmalloc_uaf2(struct kunit *test)
>         ptr2 =3D kmalloc(size, GFP_KERNEL);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
>
> +       /*
> +        * For tag-based KASAN ptr1 and ptr2 tags might happen to be the =
same.
> +        * Allow up to 16 attempts at generating different tags.
> +        */
> +       if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 =3D=3D ptr2 && coun=
ter++ < 16) {
> +               kfree(ptr2);
> +               goto again;
> +       }
> +
>         KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] =3D 'x');
>         KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);
>
> --
> 2.30.0.284.gd98b1dd5eaa7-goog
>
> --
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Alexander Potapenko
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