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

Kees Cook
2017-04-24T20:41:05Z

On Mon, Apr 24, 2017 at 8:15 AM, PaX Team <pageexec@freemail.hu> wrote:
> On 24 Apr 2017 at 15:33, Peter Zijlstra wrote:
>> On Mon, Apr 24, 2017 at 03:08:20PM +0200, PaX Team wrote:
>> > On 24 Apr 2017 at 13:15, Peter Zijlstra wrote:
>> > that was exactly my point: all this applies to you as well. so let me ask
>> > the 3rd time: what is your "argument for correctness" for a 0 refcount
>> > value check? how does it prevent exploitation?
>>
>> I think I've explained that before; per reference count rules 0 means
>> freed (or about to be freed when we talk RCU).
>
> you only said the same thing, what 0 means. you (still) didn't explain how
> checking for it prevents exploitation.
>
>> The whole pattern:
>>
>> if (dec_and_test(&obj->ref))
>> kfree(obj);
>>
>> expresses this etc.. Other reference counts also do this. No references
>> means its getting freed.
>>
>> Can you agree with this?
>
> sure, so far so good.
>
>> If so; any attempt to increase the reference count while its (being)
>> freed() is a use-after-free.
>
> why would ever be there such an attempt? a freed object with intact memory
> content is as useful for an attacker as a live one, that is, not at all.

I think we're way off in the weeds here. The "cannot inc from 0" check
is about general sanity checks on refcounts. It should never happen,
and if it does, there's a bug. However, what the refcount hardening
protection is trying to do is protect again the exploitable condition:
overflow. Inc-from-0 isn't an exploitable condition since in theory
the memory suddenly becomes correctly managed again. We're just
discussing different things.

The point is to have very fast refcount_t that protects against
overflow so the mm, net, and block subsystems aren't worried about
making the atomic_t -> refcount_t changes there.

-Kees

--
Kees Cook
Pixel Security