Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization

Laura Abbott
2015-12-22T19:18:34Z

On 12/22/15 10:37 AM, Mathias Krause wrote:
> On 22 December 2015 at 18:51, Laura Abbott <laura@labbott.name> wrote:
>>> [snip]
>>>
>>> Related to this, have you checked that the sanitization doesn't
>>> interfere with the various slab handling schemes, namely RCU related
>>> specialties? Not all caches are marked SLAB_DESTROY_BY_RCU, some use
>>> call_rcu() instead, implicitly relying on the semantics RCU'ed slabs
>>> permit, namely allowing a "use-after-free" access to be legitimate
>>> within the RCU grace period. Scrubbing the object during that period
>>> would break that assumption.
>>
>>
>> I haven't looked into that. I was working off the assumption that
>> if the regular SLAB debug poisoning worked so would the sanitization.
>> The regular debug poisoning only checks for SLAB_DESTROY_BY_RCU so
>> how does that work then?
>
> Maybe it doesn't? ;)
>
> How many systems, do you think, are running with enabled DEBUG_SLAB /
> SLUB_DEBUG in production? Not so many, I'd guess. And the ones running
> into issues probably just disable DEBUG_SLAB / SLUB_DEBUG.
>
> Btw, SLUB not only looks for SLAB_DESTROY_BY_RCU but also excludes
> "call_rcu slabs" via other mechanisms. As SLUB is the default SLAB
> allocator for quite some time now, even with enabled SLUB_DEBUG one
> wouldn't be able to trigger RCU related sanitization issues.
>

I've seen SLUB_DEBUG used in stress testing situations but you're
right about production and giving up if there are issues. I'll take
a closer look at this.

>>> Speaking of RCU, do you have a plan to support RCU'ed slabs as well?
>>>
>>
>> My only plan was to get the base support in. I didn't have a plan to
>> support RCU slabs but that's certainly something to be done in the
>> future.
>
> "Base support", in my opinion, includes covering the buddy allocator
> as well. Otherwise this feature is incomplete.

Point taken. I'll look at the buddy allocator post-holidays.

It was also pointed out I should be giving you full credit for this
feature originally. I apologize for not doing that. Thanks for
doing the original work and taking the time to review this series.

Thanks,
Laura