linux-hardening - Re: [PATCH 1/2] kasan: test: use underlying string helpers

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Date:   Thu, 14 Oct 2021 10:12:54 +0200
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Arnd Bergmann <arnd@...nel.org>, linux-hardening@...r.kernel.org,
        Kees Cook <keescook@...mium.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com
Cc:     Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Patricia Alfonso <trishalfonso@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kasan: test: use underlying string helpers



On 10/13/21 5:00 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Calling memcmp() and memchr() with an intentional buffer overflow
> is now caught at compile time:
> 
> In function 'memcmp',
>     inlined from 'kasan_memcmp' at lib/test_kasan.c:897:2:
> include/linux/fortify-string.h:263:25: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
>   263 |                         __read_overflow();
>       |                         ^~~~~~~~~~~~~~~~~
> In function 'memchr',
>     inlined from 'kasan_memchr' at lib/test_kasan.c:872:2:
> include/linux/fortify-string.h:277:17: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
>   277 |                 __read_overflow();
>       |                 ^~~~~~~~~~~~~~~~~
> 
> Change the kasan tests to wrap those inside of a noinline function
> to prevent the compiler from noticing the bug and let kasan find
> it at runtime.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>

> ---
>  lib/test_kasan.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 67ed689a0b1b..903215e944f1 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -852,6 +852,21 @@ static void kmem_cache_invalid_free(struct kunit *test)
>  	kmem_cache_destroy(cache);
>  }
>  
> +/*
> + * noinline wrappers to prevent the compiler from noticing the overflow
> + * at compile time rather than having kasan catch it.
> + * */
> +static noinline void *__kasan_memchr(const void *s, int c, size_t n)
> +{
> +	return memchr(s, c, n);
> +}
> +
> +static noinline int __kasan_memcmp(const void *s1, const void *s2, size_t n)
> +{
> +	return memcmp(s1, s2, n);
> +}
> +
> +
>  static void kasan_memchr(struct kunit *test)
>  {
>  	char *ptr;
> @@ -870,7 +885,7 @@ static void kasan_memchr(struct kunit *test)
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  
>  	KUNIT_EXPECT_KASAN_FAIL(test,
> -		kasan_ptr_result = memchr(ptr, '1', size + 1));
> +		kasan_ptr_result = __kasan_memchr(ptr, '1', size + 1));
>  
>  	kfree(ptr);
>  }
> @@ -895,7 +910,7 @@ static void kasan_memcmp(struct kunit *test)
>  	memset(arr, 0, sizeof(arr));
>  
>  	KUNIT_EXPECT_KASAN_FAIL(test,
> -		kasan_int_result = memcmp(ptr, arr, size+1));
> +		kasan_int_result = __kasan_memcmp(ptr, arr, size+1));
>  	kfree(ptr);
>  }
>  
> 

-- 
Regards,
Vincenzo