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

[Stephane Eranian]

On Thu, May 21, 2015 at 7:03 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.
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