Re: [PATCH] perf: Fix data race between pin_count increment/decrement

[Peter Zijlstra]

On Thu, May 27, 2021 at 12:47:11PM +0200, Marco Elver wrote:
> KCSAN reports a data race between increment and decrement of pin_count:
> 
>   write to 0xffff888237c2d4e0 of 4 bytes by task 15740 on cpu 1:
>    find_get_context		kernel/events/core.c:4617
>    __do_sys_perf_event_open	kernel/events/core.c:12097 [inline]
>    __se_sys_perf_event_open	kernel/events/core.c:11933
>    ...
>   read to 0xffff888237c2d4e0 of 4 bytes by task 15743 on cpu 0:
>    perf_unpin_context		kernel/events/core.c:1525 [inline]
>    __do_sys_perf_event_open	kernel/events/core.c:12328 [inline]
>    __se_sys_perf_event_open	kernel/events/core.c:11933
>    ...
> 
> Because neither read-modify-write here is atomic, this can lead to one of the
> operations being lost, resulting in an inconsistent pin_count. Fix it by adding
> the missing locking in the CPU-event case.
> 

Indeed so!

Fixes: fe4b04fa31a6 ("perf: Cure task_oncpu_function_call() races")

> Reported-by: syzbot+142c9018f5962db69c7e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>

Thanks!