linux-kernel - Re: [PATCH v3 3/3] kasan: initialise array in kasan

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Message-ID: <877dy5t0fj.fsf@dja-thinkpad.axtens.net>
Date:   Sat, 25 Apr 2020 00:37:04 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v3 3/3] kasan: initialise array in kasan_memcmp test

Dmitry Vyukov <dvyukov@...gle.com> writes:

> On Thu, Apr 23, 2020 at 5:45 PM Daniel Axtens <dja@...ens.net> wrote:
>>
>> memcmp may bail out before accessing all the memory if the buffers
>> contain differing bytes. kasan_memcmp calls memcmp with a stack array.
>> Stack variables are not necessarily initialised (in the absence of a
>> compiler plugin, at least). Sometimes this causes the memcpy to bail
>> early thus fail to trigger kasan.
>>
>> Make sure the array initialised to zero in the code.
>>
>> No other test is dependent on the contents of an array on the stack.
>>
>> Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
>> Cc: Alexander Potapenko <glider@...gle.com>
>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>> Signed-off-by: Daniel Axtens <dja@...ens.net>
>> Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
>> ---
>>  lib/test_kasan.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
>> index 939f395a5392..7700097842c8 100644
>> --- a/lib/test_kasan.c
>> +++ b/lib/test_kasan.c
>> @@ -638,7 +638,7 @@ static noinline void __init kasan_memcmp(void)
>>  {
>>         char *ptr;
>>         size_t size = 24;
>> -       int arr[9];
>> +       int arr[9] = {};
>>
>>         pr_info("out-of-bounds in memcmp\n");
>>         ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
>
> My version of this function contains the following below:
>
> memset(arr, 0, sizeof(arr));
>
> What am I missing?

Ah! It turns out I accidentally removed the memset in patch 1. No idea
why I did that. I'll fix up patch 1 to not remove the memset and drop
this patch.

Daniel