Re: [PATCH] Make /proc/slabinfo 0400

[Jesper Juhl <jj@chaosbits.net>]

On Sat, 5 Mar 2011, Matt Mackall wrote:

> 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@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?
> > > 
> > > 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?
> 
No. And now I actually feel slightly insulted.


> 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.
> 
If we annoy users I agree we shouldn't. If we don't annoy users (and don't 
impact performance in any relevant way) then even trivial obstacles that 
stop just a few exploits are worth it IMHO.


-- 
Jesper Juhl <jj@chaosbits.net>            http://www.chaosbits.net/
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