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

[Rik van Riel]

On Thu, 2017-07-06 at 10:55 -0500, Christoph Lameter wrote:
> On Thu, 6 Jul 2017, Kees Cook wrote:
> 
> > 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)
> > > > Â{
> > > > ÂÂÂÂÂÂs->flags = kmem_cache_flags(s->size, flags, s->name, s-
> > > > >ctor);
> > > > ÂÂÂÂÂÂs->reserved = 0;
> > > > +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> > > > +ÂÂÂÂÂs->random = get_random_long();
> > > > +#endif
> > > > 
> > > > ÂÂÂÂÂÂif (need_reserve_slab_rcu && (s->flags &
> > > > SLAB_TYPESAFE_BY_RCU))
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂs->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?

The hardening protects against situations where
people do not have arbitrary code execution and
memory read access in the kernel, with the goal
of protecting people from acquiring those abilities.

> > 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.

Easier said than done. Most of the time there is an
unpatched vulnerability outstanding, there is only
one known issue, before the kernel is updated by the
user, to a version that does not have that issue.

Bypassing kernel hardening typically requires the
use of multiple vulnerabilities, and the absence of
roadblocks (like hardening) that make a type of
vulnerability exploitable.

Between usercopy hardening, and these slub freelist
canaries (which is what they effectively are), several
classes of exploits are no longer usable.

-- 
All rights reversed

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