linux-kernel - Re: [RFC][PATCH] mm/slub.c: Allow poisoning to use the fast path

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Date:   Mon, 7 Aug 2017 15:23:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Christopher 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 <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [RFC][PATCH] mm/slub.c: Allow poisoning to use the fast path

On Mon, Aug 7, 2017 at 3:00 PM, Laura Abbott <labbott@...hat.com> wrote:
> On 08/07/2017 11:03 AM, Christopher Lameter wrote:
>> On Mon, 7 Aug 2017, Laura Abbott wrote:
>>
>>>> Ok I see that the objects are initialized with poisoning and redzoning but
>>>> I do not see that there is fastpath code to actually check the values
>>>> before the object is reinitialized. Is that intentional or am
>>>> I missing something?
>>>
>>> Yes, that's intentional here. I see the validation as a separate more
>>> expensive feature. I had a crude patch to do some checks for testing
>>> and I know Daniel Micay had an out of tree patch to do some checks
>>> as well.
>>
>> Ok then this patch does nothing? How does this help?
>
> The purpose of this patch is to ensure the poisoning can happen without
> too much penalty. Even if there aren't checks to abort/warn when there
> is a problem, there's still value in ensuring objects are always poisoned.

To clarify, this is desirable to kill exploitation of
exposure-after-free flaws and some classes of use-after-free flaws,
since the contents will have be wiped out after a free. (Verification
of poison is nice, but is expensive compared to the benefit against
these exploits -- and notably doesn't protect against the other
use-after-free attacks where the contents are changed after the next
allocation, which would have passed the poison verification.)

-Kees

-- 
Kees Cook
Pixel Security