Re: [PATCH 2/2] random: spread out jitter callback to different CPUs

From: Jason A. Donenfeld
Date: Thu Oct 06 2022 - 08:26:16 EST

On Thu, Oct 06, 2022 at 08:46:27AM +0200, Sebastian Andrzej Siewior wrote:
> On 2022-10-05 23:08:19 [+0200], Jason A. Donenfeld wrote:
> > Hi Sebastian,
> Hi Jason,
> > On Wed, Oct 05, 2022 at 07:26:42PM +0200, Sebastian Andrzej Siewior wrote:
> > > That del_timer_sync() at the end is what you want. If the timer is
> > > pending (as in enqueued in the timer wheel) then it will be removed
> > > before it is invoked. If the timer's callback is invoked then it will
> > > spin until the callback is done.
> >
> > del_timer_sync() is not guaranteed to succeed with add_timer_on() being
> > used in conjunction with timer_pending() though. That's why I've
> > abandoned this.
> But why? The timer is added to a timer-base on a different CPU. Should
> work.

So it's easier to talk about, I'll number a few lines:

1 while (conditions) {
2 if (!timer_pending(&stack.timer))
3 add_timer_on(&stack.timer, some_next_cpu);
4 }
5 del_timer_sync(&stack.timer);

Then, steps to cause UaF:

a) add_timer_on() on line 3 is called from CPU 1 and pends the timer on
CPU 2.

b) Just before the timer callback runs, not after, timer_pending() is
made false, so the condition on line 2 holds true again.

c) add_timer_on() on line 3 is called from CPU 1 and pends the timer on
CPU 3.

d) The conditions on line 1 are made false, and the loop breaks.

e) del_timer_sync() on line 5 is called, and its `base->running_timer !=
timer` check is false, because of step (c).

f) `stack.timer` gets freed / goes out of scope.

g) The callback scheduled from step (b) runs, and we have a UaF.

That's, anyway, what I understand Sultan to have pointed out to me. In
looking at this closely, though, to write this email, I noticed that
add_timer_on() does set TIMER_MIGRATING, which lock_timer_base() spins
on. So actually, maybe this scenario should be accounted for? Sultan, do
you care to comment here?