Re: [PATCH v3] mm: Add SLUB free list pointer obfuscation

[Christoph Lameter <cl@linux.com>]

On Thu, 6 Jul 2017, Kees Cook wrote:

> On Thu, Jul 6, 2017 at 6:43 AM, Christoph Lameter <cl@linux.com> wrote:
> > On Wed, 5 Jul 2017, Kees Cook wrote:
> >
> >> @@ -3536,6 +3565,9 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags)
> >>  {
> >>       s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor);
> >>       s->reserved = 0;
> >> +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> >> +     s->random = get_random_long();
> >> +#endif
> >>
> >>       if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
> >>               s->reserved = sizeof(struct rcu_head);
> >>
> >
> > So if an attacker knows the internal structure of data then he can simply
> > dereference page->kmem_cache->random to decode the freepointer.
>
> That requires a series of arbitrary reads. This is protecting against
> attacks that use an adjacent slab object write overflow to write the
> freelist pointer. This internal structure is very reliable, and has
> been the basis of freelist attacks against the kernel for a decade.

These reads are not arbitrary. You can usually calculate the page struct
address easily from the address and then do a couple of loads to get
there.

Ok so you get rid of the old attacks because we did not have that
hardening in effect when they designed their approaches?

> It is a probabilistic defense, but then so is the stack protector.
> This is a similar defense; while not perfect it makes the class of
> attack much more difficult to mount.

Na I am not convinced of the "much more difficult". Maybe they will just
have to upgrade their approaches to fetch the proper values to decode.

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