Re: [PATCH v8 2/3] mm, kasan: align free_meta_offset on sizeof(void*)

[Alexander Potapenko]

On Mon, Aug 1, 2016 at 5:11 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 08/01/2016 05:56 PM, Alexander Potapenko wrote:
>> On Mon, Aug 1, 2016 at 4:55 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>>>
>>>
>>> On 07/28/2016 06:31 PM, Alexander Potapenko wrote:
>>>> When free_meta_offset is not zero, it is usually aligned on 4 bytes,
>>>> because the size of preceding kasan_alloc_meta is aligned on 4 bytes.
>>>> As a result, accesses to kasan_free_meta fields may be misaligned.
>>>>
>>>> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
>>>> ---
>>>>  mm/kasan/kasan.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
>>>> index 6845f92..0379551 100644
>>>> --- a/mm/kasan/kasan.c
>>>> +++ b/mm/kasan/kasan.c
>>>> @@ -390,7 +390,8 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
>>>>       /* Add free meta. */
>>>>       if (cache->flags & SLAB_DESTROY_BY_RCU || cache->ctor ||
>>>>           cache->object_size < sizeof(struct kasan_free_meta)) {
>>>> -             cache->kasan_info.free_meta_offset = *size;
>>>> +             cache->kasan_info.free_meta_offset =
>>>> +                     ALIGN(*size, sizeof(void *));
>>>
>>> This cannot work.
>> Well, it does, at least on my tests.
>
> JFYI. You aligned only meta offset, but didn't change the size, so after the '*size += sizeof(struct kasan_free_meta);'
> *size may point into the middle of free_meta struct.
> Plus, alignment wasn't taken into account in kasan_metadata_size().
That's what I do in PATCH 3/3
(https://marc.info/?l=linux-mm&m=146971994204507&w=2)


-- 
Alexander Potapenko
Software Engineer

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