Re: [PATCH] Make /proc/slabinfo 0400

Dan Rosenberg
2011-03-04T20:43:03Z

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 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.

-Dan

> --
> Mathematics is the supreme nostalgia of our time.