[kernel-hardening] Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX

Laura Abbott
2016-01-21T03:35:06Z

On 1/13/16 7:49 PM, Laura Abbott wrote:
> On 1/8/16 6:07 AM, Christoph Lameter wrote:
>> On Thu, 7 Jan 2016, Laura Abbott wrote:
>>
>>> The slub_debug=P not only poisons it enables other consistency checks on the
>>> slab as well, assuming my understanding of what check_object does is correct.
>>> My hope was to have the poison part only and none of the consistency checks in
>>> an attempt to mitigate performance issues. I misunderstood when the checks
>>> actually run and how SLUB_DEBUG was used.
>>
>> Ok I see that there pointer check is done without checking the
>> corresponding debug flag. Patch attached thar fixes it.
>>
>>> Another option would be to have a flag like SLAB_NO_SANITY_CHECK.
>>> sanitization enablement would just be that and SLAB_POISON
>>> in the debug options. The disadvantage to this approach would be losing
>>> the sanitization for ->ctor caches (the grsecurity version works around this
>>> by re-initializing with ->ctor, I haven't heard any feedback if this actually
>>> acceptable) and not having some of the fast paths enabled
>>> (assuming I'm understanding the code path correctly.) which would also
>>> be a performance penalty
>>
>> I think we simply need to fix the missing check there. There is already a
>> flag SLAB_DEBUG_FREE for the pointer checks.
>>
>>
>
> The patch improves performance but the overall performance of these full
> sanitization patches is still significantly better than slub_debug=P. I'll
> put some effort into seeing if I can figure out where the slow down is
> coming from.
>

There are quite a few other checks which need to be skipped over as well,
but I don't think skipping those are going to be sufficient to give an
acceptable performance; a quick 'hackbench -g 20 -l 1000' shows at least
a 3.5 second difference between just skipping all the checks+slab_debug=P
and this series.

The SLAB_DEBUG flags force everything to skip the CPU caches which is
causing the slow down. I experimented with allowing the debugging to
happen with CPU caches but I'm not convinced it's possible to do the
checking on the fast path in a consistent manner without adding
locking. Is it worth refactoring the debugging to be able to be used
on cpu caches or should I take the approach here of having the clear
be separate from free_debug_processing?

Thanks,
Laura