Messages in this thread | | | Date | Fri, 22 May 2015 08:49:55 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 01/10] perf,x86: Fix event/group validation |
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* 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
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