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

Kees Cook
2017-04-25T16:40:04Z

On Tue, Apr 25, 2017 at 4:26 AM, PaX Team <pageexec@freemail.hu> wrote:
> INT_MAX threads would be needed when the leaking path is locked so
> that it can only be exercised once and you'll need to get normal
> (balanced) paths preempted just after the increment. if the leaking
> path is lockless (can be exercised in parallel without bounds) then
> 2 threads are enough where the one triggering the signed overflow
> would have to be preempted while the other one does INT_MAX increments
> and trigger the UAF. this is where the other mechanisms i talked about
> in the past become relevant: preemption or interrupts can be disabled
> or negative refcount values can be detected and acted upon (your blind
> copy-pasting effort passed upon this latter opportunity by not
> specializing the 'jo' into 'js' for the refcount case).

Well, it's not "blind" -- I'm trying to bring the code as-is to
upstream for discussion/examination with as little functional
differences as possible so it's easier to compare apples to apples.
(Which already resulted in more eyes looking at the code to find a bug
-- thanks Jann!) But yes, jo -> js hugely increases the coverage. I'll
make that change for v2.

Thanks!

-Kees

--
Kees Cook
Pixel Security