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

[Kees Cook]

On Tue, Apr 25, 2017 at 4:26 AM, PaX Team <pageexec@xxxxxxxxxxx> wrote:
> On 25 Apr 2017 at 0:01, Peter Zijlstra wrote:
>> 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.

Right, having a deterministic protection (checking for overflow) is
best since it stops all exploits using that path. Hoping that an
attacker is unlucky and hits a notification after they've already
landed their corruption is not a very useful defense. It certainly has
a non-zero value, but stopping overflow 100% is better. Especially
when we can do it with no meaningful change in performance which lets
us actually do the atomic_t -> refcount_t conversion everywhere.

-Kees

-- 
Kees Cook
Pixel Security