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Date:   Tue, 12 Jan 2021 14:39:03 +0100
From:   Marco Elver <elver@google.com>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        kasan-dev@googlegroups.com, linux-arm-kernel@lists.infradead.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 08/11] kasan: adopt kmalloc_uaf2 test to HW_TAGS mode
Message-ID: <X/2md4h0Nki8RNW0@elver.google.com>
References: <cover.1609871239.git.andreyknvl@google.com>
 <9a4f47fe8717b4b249591b307cdd1f26c46dcb82.1609871239.git.andreyknvl@google.com>
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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
>