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

[Kees Cook]

On Wed, Oct 13, 2021 at 05:00:05PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> 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.

Is this with W=1 ? I had explicitly disabled the read overflows for
"phase 1" of the overflow restriction tightening...

(And what do you think of using OPTIMIZER_HIDE_VAR() instead[1]?

-Kees

[1] https://lore.kernel.org/linux-hardening/20211006181544.1670992-1-keescook@xxxxxxxxxxxx/T/#u

> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  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);
>  }
>  
> -- 
> 2.29.2
> 

-- 
Kees Cook