Subject: Re: [PATCH] Make /proc/slabinfo 0400
From: Matt Mackall <mpm@selenic.com>
To: Pekka Enberg <penberg@kernel.org>
Cc: Dan Rosenberg <drosenberg@vsecurity.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave@linux.vnet.ibm.com>, Theodore Tso <tytso@mit.edu>,
        cl@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
        Andrew Morton <akpm@linux-foundation.org>
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Date: Fri, 04 Mar 2011 15:12:55 -0600
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On Fri, 2011-03-04 at 22:58 +0200, Pekka Enberg wrote:
> On Fri, Mar 4, 2011 at 10:37 PM, Dan Rosenberg <drosenberg@vsecurity.com> wrote:
> > This patch makes these techniques more difficult by making it hard to
> > know whether the last attacker-allocated object resides before a free or
> > allocated object.  Especially with vulnerabilities that only allow one
> > attempt at exploitation before recovery is needed to avoid trashing too
> > much heap state and causing a crash, this could go a long way.  I'd
> > still argue in favor of removing the ability to know how many objects
> > are used in a given slab, since randomizing objects doesn't help if you
> > know every object is allocated.
> 
> So if the attacker knows every object is allocated, how does that help
> if we're randomizing the initial freelist?

First note that all of these attacks are probabilistic. 

Now, with a randomized free list, if I create 1000 objects of type B,
then, on average, the partially-filled page the next allocation comes
from will be half-full of B objects. Thus, the next object will have a
50% chance of being in the right spot for an exploit. 

Now if I delete the 800th B object, it's probably on a slab that's
otherwise full of B objects since we fill partial slabs before creating
new ones. If my next allocation comes from that slab, it will thus get a
spot that's almost guaranteed to be in the right spot.

Similarly, if I create 1000 objects and then delete every tenth one,
I've now got a swiss cheese heap where just about every hole is
well-positioned. 

-- 
Mathematics is the supreme nostalgia of our time.


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