Re: [PATCH 01/10] perf,x86: Fix event/group validation

[Ingo Molnar <mingo@kernel.org>]

* Stephane Eranian <eranian@google.com> wrote:

> On Thu, May 21, 2015 at 7:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, 2015-05-21 at 06:36 -0700, Stephane Eranian wrote:
> >> On Thu, May 21, 2015 at 6:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > On Thu, 2015-05-21 at 06:27 -0700, Stephane Eranian wrote:
> >> >> Or are you talking about a preemption while executing x86_schedule_events()?
> >> >
> >> > That.
> >> >
> >> > And we can of course cure that by an earlier patch I send; but I find it
> >> > a much simpler rule to just never allow modifying global state for
> >> > validation.
> >>
> >> I can see  validation being preempted, but not the context switch code path.
> >> Is that what you are talking about?
> >>
> >> You are saying validate_group() is in the middle of x86_schedule_events()
> >> using fake_cpuc, when it gets preempted. The context switch code when it loads
> >> the new thread's PMU state calls x86_schedule_events() which modifies the
> >> cpuc->event_list[]->hwc. But this is cpuc vs. fake_cpuc again. So yes, the calls
> >> nest but they do not touch the same state.
> >
> > They both touch event->hw->constraint.
> >
> >>  And when you eventually come back
> >> to validate_group() you are back to using the fake_cpuc. So I am still not clear
> >> on how the corruption can happen.
> >
> > validate_group()
> >   x86_schedule_events()
> >     event->hw.constraint = c; # store
> >
> >      <context switch>
> >        perf_task_event_sched_in()
> >          ...
> >            x86_schedule_events();
> >              event->hw.constraint = c2; # store
> >
> >              ...
> >
> >              put_event_constraints(event); # assume failure to schedule
> >                intel_put_event_constraints()
> >                  event->hw.constraint = NULL;
> >
> >       <context switch end>
> >
> >     c = event->hw.constraint; # read -> NULL
> >
> >     if (!test_bit(hwc->idx, c->idxmsk)) # <- *BOOM* NULL deref
> >
> >
> > This in particular is possible when the event in question is a cpu-wide
> > event and group-leader, where the validate_group() tries to add an event
> > to the group.
>
> Ok, I think I get it now. It is not related to fake_cpuc vs. cpuc, 
> it is related to the fact that the constraint is cached in the event 
> struct itself and that one is shared between validate_group() and 
> x86_schedule_events() because cpu_hw_event->event_list[] is an array 
> of pointers to events and not an array of events.

Btw., comments and the code structure should be greatly enhanced to 
make all that very clear and hard to mess up.

A month ago perf became fuzzing-proof, and now that's down the drain 
again...

Thanks,

	Ingo