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Date:   Thu, 14 Oct 2021 19:40:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>,
        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 October 14, 2021 1:12:54 AM PDT, Vincenzo Frascino <vincenzo.frascino@....com> wrote:
>
>
>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>

How about just explicitly making the size invisible to the compiler?

I did this for similar issues in the same source:

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


-Kees

>
>> ---
>>  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);
>>  }
>>  
>> 
>

-- 
Kees Cook

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