Re: [RFC][PATCH 0/3] Speed up SLUB poisoning + disable checks

[Kees Cook]

On Wed, Feb 3, 2016 at 10:46 AM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 01/25/2016 11:03 PM, Joonsoo Kim wrote:
>>
>> On Mon, Jan 25, 2016 at 05:15:10PM -0800, Laura Abbott wrote:
>>>
>>> Hi,
>>>
>>> Based on the discussion from the series to add slab sanitization
>>> (lkml.kernel.org/g/<1450755641-7856-1-git-send-email-laura@xxxxxxxxxxxx>)
>>> the existing SLAB_POISON mechanism already covers similar behavior.
>>> The performance of SLAB_POISON isn't very good. With hackbench -g 20 -l
>>> 1000
>>> on QEMU with one cpu:
>>
>>
>> I doesn't follow up that discussion, but, I think that reusing
>> SLAB_POISON for slab sanitization needs more changes. I assume that
>> completeness and performance is matter for slab sanitization.
>>
>> 1) SLAB_POISON isn't applied to specific kmem_cache which has
>> constructor or SLAB_DESTROY_BY_RCU flag. For debug, it's not necessary
>> to be applied, but, for slab sanitization, it is better to apply it to
>> all caches.
>
>
> The grsecurity patches get around this by calling the constructor again
> after poisoning. It could be worth investigating doing that as well
> although my focus was on the cases without the constructor.
>>
>>
>> 2) SLAB_POISON makes object size bigger so natural alignment will be
>> broken. For example, kmalloc(256) cache's size is 256 in normal
>> case but it would be 264 when SLAB_POISON is enabled. This causes
>> memory waste.
>
>
> The grsecurity patches also bump the size up to put the free pointer
> outside the object. For sanitization purposes it is cleaner to have
> no pointers in the object after free
>
>>
>> In fact, I'd prefer not reusing SLAB_POISON. It would make thing
>> simpler. But, it's up to Christoph.
>>
>> Thanks.
>>
>
> It basically looks like trying to poison on the fast path at all
> will have a negative impact even with the feature is turned off.
> Christoph has indicated this is not acceptable so we are forced
> to limit it to the slow path only if we want runtime enablement.

Is it possible to have both? i.e fast path via CONFIG, and slow path
via runtime options?

> If we're limited to the slow path only, we might as well work
> with SLAB_POISON to make it faster. We can reevaluate if it turns
> out the poisoning isn't fast enough to be useful.

And since I'm new to this area, I know of fast/slow path in the
syscall sense. What happens in the allocation/free fast/slow path that
makes it fast or slow?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security