Re: [PATCH v13 19/25] kasan: add hooks implementation for tag-based mode

[Vincenzo Frascino]

On 12/12/18 3:04 PM, Andrey Konovalov wrote:
> On Tue, Dec 11, 2018 at 5:22 PM Vincenzo Frascino
> <vincenzo.frascino@xxxxxxx> wrote:
>>
>> Hi Andrey,
>>
>> On 06/12/2018 12:24, Andrey Konovalov wrote:
>>> This commit adds tag-based KASAN specific hooks implementation and
>>> adjusts common generic and tag-based KASAN ones.
>>>
>>> 1. When a new slab cache is created, tag-based KASAN rounds up the size of
>>>    the objects in this cache to KASAN_SHADOW_SCALE_SIZE (== 16).
>>>
>>> 2. On each kmalloc tag-based KASAN generates a random tag, sets the shadow
>>>    memory, that corresponds to this object to this tag, and embeds this
>>>    tag value into the top byte of the returned pointer.
>>>
>>> 3. On each kfree tag-based KASAN poisons the shadow memory with a random
>>>    tag to allow detection of use-after-free bugs.
>>>
>>> The rest of the logic of the hook implementation is very much similar to
>>> the one provided by generic KASAN. Tag-based KASAN saves allocation and
>>> free stack metadata to the slab object the same way generic KASAN does.
>>>
>>> Reviewed-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>>> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>>> ---
>>>  mm/kasan/common.c | 116 ++++++++++++++++++++++++++++++++++++++--------
>>>  mm/kasan/kasan.h  |   8 ++++
>>>  mm/kasan/tags.c   |  48 +++++++++++++++++++
>>>  3 files changed, 153 insertions(+), 19 deletions(-)
>>>
>>
>>
>> [...]
>>
>>> @@ -265,6 +290,8 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>>>               return;
>>>       }
>>>
>>> +     cache->align = round_up(cache->align, KASAN_SHADOW_SCALE_SIZE);
>>> +
>>
>> Did you consider to set ARCH_SLAB_MINALIGN instead of this round up?
> 
> I didn't know about this macro. Looks like we can use it to do the
> same thing. Do you think it's a better solution to redefine
> ARCH_SLAB_MINALIGN to KASAN_SHADOW_SCALE_SIZE for arm64 when tag-based
> KASAN is enabled instead of adjusting cache->align in
> kasan_cache_create?
>

Yes, I think it is better because in this way we do not need to add extra code
to do the rounding.

Curiosity, did you try your patches with SLUB red zoning enabled?
Since the area used for the Redzone is just after the payload, aligning the
object_size independently from the allocator could have side effects, at least
if I understand well how the mechanism works.

Setting ARCH_SLAB_MINALIGN should avoid this as well.

What do you think?

>>
>> --
>> Regards,
>> Vincenzo
>>
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-- 
Regards,
Vincenzo