Re: [PATCH] x86/refcount: Implement fast refcount_t handling

["PaX Team" <pageexec@freemail.hu>]

On 25 Apr 2017 at 0:01, Peter Zijlstra wrote:

> On Mon, Apr 24, 2017 at 01:40:56PM -0700, Kees Cook wrote:
> > I think we're way off in the weeds here. The "cannot inc from 0" check
> > is about general sanity checks on refcounts.
> 
> I disagree, although sanity check are good too.

exactly, an attacker doesn't care how a premature free occurs due
to reaching a 0 refcount, afterwards it's memory corruption time for
both old and new references regardless.

> > However, what the refcount hardening protection is trying to do is
> > protect again the exploitable condition: overflow.
> 
> Sure..

underflow is also exploitable, it's just much harder to defend against
(there're no known practical solutions).

> > Inc-from-0 isn't an exploitable condition since in theory
> > the memory suddenly becomes correctly managed again.
> 
> It does not. It just got free'ed. Nothing will stop the free from
> happening (or already having happened).

now hold this thought...

> How is the below not useful fodder for an exploit? It might be a less
> common bug, and perhaps a bit more fiddly to make work, but afaict its
> still a full use-after-free and therefore useful.
> 
> ---
> 
> Thread-A                                        Thread-B
> 
> if(dec_and_test(&obj->ref)) { // true, ref==0
> 
>                                                 inc(&obj->ref) // ref: 0->1
> 
>         kfree(obj);
> }

... and tell me why an attacker would let Thread-B do that increment
(that you're trying to detect) *before* the underlying memory gets
reused and thus the 0 changed to something else? hint: he'll do everything
in his power to prevent that, either by winning the race or if there's
no race (no refcount users outside his control), he'll win every time.
IOW, checking for 0 is pointless and you kinda proved it yourself now.