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

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Date:	Tue, 22 Dec 2015 19:37:40 +0100
From:	Mathias Krause <minipli@...glemail.com>
To:	Laura Abbott <laura@...bott.name>
Cc:	kernel-hardening@...ts.openwall.com,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for
 slab sanitization

On 22 December 2015 at 18:51, Laura Abbott <laura@...bott.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.

>> 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.

Regards,
Mathias
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