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

From: Kees Cook
Date: Thu Jul 06 2017 - 11:48:49 EST

On Thu, Jul 6, 2017 at 6:43 AM, Christoph Lameter <cl@xxxxxxxxx> 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;
>> + 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.

> Assuming someone is already targeting a freelist pointer (which indicates
> detailed knowledge of the internal structure) then I would think that
> someone like that will also figure out how to follow the pointer links to
> get to the random value.

The kind of hardening this provides is to frustrate the expansion of
an attacker's capabilities. Most attacks are a chain of exploits that
slowly builds up the ability to perform arbitrary writes. For example,
a slab object overflow isn't an arbitrary write on its own, but when
combined with heap allocation layout control and an adjacent free
object, this can be upgraded to an arbitrary write.

> Not seeing the point of all of this.

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.


Kees Cook
Pixel Security