perf: Fix data race between pin_count increment/decrement

BugLink: https://bugs.launchpad.net/bugs/1933369
commit 6c605f8371159432ec61cbb1488dcf7ad24ad19a upstream.

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.

Fixes: fe4b04fa31a6 ("perf: Cure task_oncpu_function_call() races")
Reported-by: syzbot+142c9018f5962db69c7e@syzkaller.appspotmail.com
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210527104711.2671610-1-elver@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index efd10ef548f3802e6b278919b4f315e1000a2429..0c34702501afa702697bd9c1c37007fb8db559b9 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4254,7 +4254,9 @@ 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;
        }