Re: [PATCH] Make /proc/slabinfo 0400

Pekka Enberg
2011-03-04T20:56:17Z

On Fri, 2011-03-04 at 14:31 -0600, Matt Mackall wrote:
>> On Fri, 2011-03-04 at 22:02 +0200, Pekka Enberg wrote:
>> > On Fri, Mar 4, 2011 at 8:14 PM, Matt Mackall <mpm@selenic.com> wrote:
>> > >> Of course, as you say, '/proc/meminfo' still does give you the trigger
>> > >> for "oh, now somebody actually allocated a new page". That's totally
>> > >> independent of slabinfo, though (and knowing the number of active
>> > >> slabs would neither help nor hurt somebody who uses meminfo - you
>> > >> might as well allocate new sockets in a loop, and use _only_ meminfo
>> > >> to see when that allocated a new page).
>> > >
>> > > I think lying to the user is much worse than changing the permissions.
>> > > The cost of the resulting confusion is WAY higher.
>> >
>> > Yeah, maybe. I've attached a proof of concept patch that attempts to
>> > randomize object layout in individual slabs. I'm don't completely
>> > understand the attack vector so I don't make any claims if the patch
>> > helps or not.
>>
>> In general, the attack relies on getting an object A (vulnerable to
>> overrun) immediately beneath an object B (that can be exploited when
>> overrun).
>>
>> I'm not sure how much randomization helps, though. Allocate 1000 objects
>> of type B, deallocate the 800th, then allocate an object of type A. It's
>> almost certainly next to a B.

On Fri, Mar 4, 2011 at 10:42 PM, Dan Rosenberg <drosenberg@vsecurity.com> wrote:
> On second thought, this does pose a problem.  Even if you don't know how
> full the most recent slab is or where free vs. used chunks are within
> it, if you can guarantee that you filled an entire previous slab with
> your objects and then free and reallocate one of them, then you can
> still win.

Guys, I still don't get it, sorry.

Why can you still win? With my patch, reallocation shouldn't matter;
the freelist randomization ought to make it less likely for *any* two
allocated objects to be adjacent.

Pekka