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

[Peter Zijlstra <peterz@infradead.org>]

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.