linux-kernel - Re: [RFC][PATCH 1/7] mm/slab_common.c: Add common support for slab saniziation

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Message-ID: <CAGXu5jLvS0jzi07QegCHoBoCc3wFhbcMOjCpmbe3KC2oJO9jPQ@mail.gmail.com>
Date:	Tue, 5 Jan 2016 16:17:53 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Laura Abbott <laura@...bott.name>,
	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 <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Mathias Krause <minipli@...glemail.com>
Subject: Re: [RFC][PATCH 1/7] mm/slab_common.c: Add common support for slab saniziation

On Tue, Dec 22, 2015 at 12:48 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
> On 22.12.2015 4:40, Laura Abbott wrote:
>> Each of the different allocators (SLAB/SLUB/SLOB) handles
>> clearing of objects differently depending on configuration.
>> Add common infrastructure for selecting sanitization levels
>> (off, slow path only, partial, full) and marking caches as
>> appropriate.
>>
>> All credit for the original work should be given to Brad Spengler and
>> the PaX Team.
>>
>> Signed-off-by: Laura Abbott <laura@...bott.name>
>>
>> +#ifdef CONFIG_SLAB_MEMORY_SANITIZE
>> +#ifdef CONFIG_X86_64
>> +#define SLAB_MEMORY_SANITIZE_VALUE       '\xfe'
>> +#else
>> +#define SLAB_MEMORY_SANITIZE_VALUE       '\xff'
>> +#endif
>> +enum slab_sanitize_mode {
>> +     /* No sanitization */
>> +     SLAB_SANITIZE_OFF = 0,
>> +
>> +     /* Partial sanitization happens only on the slow path */
>> +     SLAB_SANITIZE_PARTIAL_SLOWPATH = 1,
>
> Can you explain more about this variant? I wonder who might find it useful
> except someone getting a false sense of security, but cheaper.
> It sounds like wanting the cake and eat it too :)
> I would be surprised if such IMHO half-solution existed in the original
> PAX_MEMORY_SANITIZE too?
>
> Or is there something that guarantees that the objects freed on hotpath won't
> stay there for long so the danger of leak is low? (And what about
> use-after-free?) It depends on further slab activity, no? (I'm not that familiar
> with SLUB, but I would expect the hotpath there being similar to SLAB freeing
> the object on per-cpu array_cache. But, it seems the PARTIAL_SLOWPATH is not
> implemented for SLAB, so there might be some fundamental difference I'm missing.)

Perhaps the partial sanitize could be a separate patch so it's
features were more logically separated?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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