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    SubjectRe: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode
    On Tue, Jan 05, 2021 at 07:27PM +0100, Andrey Konovalov wrote:
    > In the kmalloc_uaf2() test, the pointers to the two allocated memory
    > blocks might be the same, and the test will fail. With the software
    > tag-based mode, the probability of the that happening 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 4 attempts at generating different tags for the tag-based
    > modes.
    >
    > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
    > Link: https://linux-review.googlesource.com/id/Ibfa458ef2804ff465d8eb07434a300bf36388d55
    > ---
    > lib/test_kasan.c | 9 +++++++++
    > 1 file changed, 9 insertions(+)
    >
    > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
    > index b5077a47b95a..b67da7f6e17f 100644
    > --- a/lib/test_kasan.c
    > +++ b/lib/test_kasan.c
    > @@ -375,7 +375,9 @@ static void kmalloc_uaf2(struct kunit *test)
    > {
    > char *ptr1, *ptr2;
    > size_t size = 43;
    > + int counter = 0;
    >
    > +again:
    > ptr1 = kmalloc(size, GFP_KERNEL);
    > KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr1);
    >
    > @@ -384,6 +386,13 @@ static void kmalloc_uaf2(struct kunit *test)
    > ptr2 = 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 4 attempts at generating different tags.
    > + */
    > + if (!IS_ENABLED(CONFIG_KASAN_GENERIC) && ptr1 == ptr2 && counter++ < 4)
    > + goto again;
    > +

    Why do we even need a limit? Why not retry until ptr1 != ptr2?

    > KUNIT_EXPECT_KASAN_FAIL(test, ptr1[40] = 'x');
    > KUNIT_EXPECT_PTR_NE(test, ptr1, ptr2);
    >
    > --
    > 2.29.2.729.g45daf8777d-goog
    >

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