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

    * 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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