Re: [PATCH] Make /proc/slabinfo 0400

Dan Rosenberg
2011-03-06T00:57:39Z

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?
> Same with making /proc/slabinfo 0400, if it just makes things a little
> harder in a few cases, why not do it? It's not like a admin who needs
> /proc/slabinfo to have other permissions can't arrange for that.
>
> Having been employed as a systems administrator for many years and having
> seen many a box cracked, my oppinion is that every little bit helps. The
> kernel is currently not a hard target and everything we can do to harden
> it is a good thing (within reason of course).
>
> Why not just do both randomization and 0400 as a start? We can always
> harden further later.

I agree that there's no harm in these patches, and they might make it
(only) slightly harder in some cases, so yes, we might as well. I just
don't want to trick anyone into a false sense of security by thinking
that these measures by themselves are doing anything especially
substantial to prevent heap exploits. But as you say, it's a start.

Another hardening measure that's been mentioned before is validating the
address of each to-be-returned pointer during allocation, to avoid
attacks that rely on corrupting free list pointers (i.e. compare against
TASK_SIZE). But then we're talking about introducing additional
overhead into every single kmalloc() call.

-Dan