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

Laura Abbott
2016-01-08T01:23:01Z

On 1/7/16 8:26 AM, Christoph Lameter wrote:
> On Tue, 5 Jan 2016, Laura Abbott wrote:
>
>> It's not the poisoning per se that's incompatible, it's how the poisoning is
>> set up. At least for slub, the current poisoning is part of SLUB_DEBUG which
>> enables other consistency checks on the allocator. Trying to pull out just
>> the poisoning for use when SLUB_DEBUG isn't on would result in roughly what
>> would be here anyway. I looked at trying to reuse some of the existing
>> poisoning
>> and came to the conclusion it was less intrusive to the allocator to keep it
>> separate.
>
> SLUB_DEBUG does *not* enable any debugging features. It builds the logic
> for debugging into the kernel but does not activate it. CONFIG_SLUB_DEBUG
> is set for production kernels. The poisoning is build in by default into
> any recent linux kernel out there. You can enable poisoning selectively
> (and no other debug feature) by specifying slub_debug=P on the Linux
> kernel command line right now.
>
> There is a SLAB_POISON flag for each kmem_cache that can be set to
> *only* enable poisoning and nothing else from code.
>
>

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.

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

Thanks,
Laura