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

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

[Marco Elver]

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.

Reported-by: syzbot+142c9018f5962db69c7e@syzkaller.appspotmail.com
Signed-off-by: Marco Elver <elver@google.com>
---
 kernel/events/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6fee4a7e88d7..fe88d6eea3c2 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4609,7 +4609,9 @@ find_get_context(struct pmu *pmu, struct task_struct *task,
 		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
 		ctx = &cpuctx->ctx;
 		get_ctx(ctx);
+		raw_spin_lock_irqsave(&ctx->lock, flags);
 		++ctx->pin_count;
+		raw_spin_unlock_irqrestore(&ctx->lock, flags);
 
 		return ctx;
 	}
-- 
2.31.1.818.g46aad6cb9e-goog

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@syzkaller.appspotmail.com
> Signed-off-by: Marco Elver <elver@google.com>

Thanks!