linux-kernel - Re: [PATCH] Make /proc/slabinfo 0400

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Date:	Fri, 04 Mar 2011 15:12:55 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Dan Rosenberg <drosenberg@...curity.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Theodore Tso <tytso@....edu>, cl@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Make /proc/slabinfo 0400

On Fri, 2011-03-04 at 22:58 +0200, Pekka Enberg wrote:
> On Fri, Mar 4, 2011 at 10:37 PM, Dan Rosenberg <drosenberg@...curity.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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