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@xxxxxxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>