perf/core: Fix sideband list-iteration vs. event ordering NULL pointer deference crash

Vegard Nossum reported that perf fuzzing generates a NULL
pointer dereference crash:

> Digging a bit deeper into this, it seems the event itself is getting
> created by perf_event_open() and it gets added to the pmu_event_list
> through:
>
> perf_event_open()
>  - perf_event_alloc()
>     - account_event()
>        - account_pmu_sb_event()
>           - attach_sb_event()
>
> so at this point the event is being attached but its ->ctx is still
> NULL. It seems like ->ctx is set just a bit later in
> perf_event_open(), though.
>
> But before that, __schedule() comes along and creates a stack trace
> similar to the one above:
>
> __schedule()
>  - __perf_event_task_sched_out()
>    - perf_iterate_sb()
>      - perf_iterate_sb_cpu()
>         - event_filter_match()
>           - perf_cgroup_match()
>             - __get_cpu_context()
>               - (dereference ctx which is NULL)
>
> So I guess the question is... should the event be attached (= put on
> the list) before ->ctx gets set? Or should the cgroup code check for a
> NULL ->ctx?

The latter seems like the simplest solution. Moving the list-add later
creates a bit of a mess.

Reported-by: Vegard Nossum <[email protected]>
Tested-by: Vegard Nossum <[email protected]>
Tested-by: Vince Weaver <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: David Carrillo-Cisneros <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Fixes: f2fb6bef9251 ("perf/core: Optimize side-band event delivery")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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