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

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Date:	Sat, 05 Mar 2011 19:09:41 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	Dan Rosenberg <drosenberg@...curity.com>,
	Pekka Enberg <penberg@...nel.org>,
	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 Sun, 2011-03-06 at 01:42 +0100, Jesper Juhl wrote:
> On Fri, 4 Mar 2011, Dan Rosenberg wrote:
> 
> > 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?
> > 
> > If you know you've got a slab completely full of your objects, then it
> > doesn't matter that they happened to be allocated in a random fashion -
> > they're still all allocated, and by freeing one of them and
> > reallocating, you'll still be next to your target.
> > 
> 
> But still, if randomizing allocations makes life just a little harder for 
> attackers in some scenarios, why not just do it?

Lemme guess, you work for the TSA?

As far as I can tell neither of the patches under discussion do anything
that couldn't be worked around by an exploit writer in the time it takes
to write this email. And the second attacker, of course, will have even
less trouble.

Putting trivial obstacles in the way of attackers accomplishes little
beyond annoying users.

-- 
Mathematics is the supreme nostalgia of our time.


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