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Date:   Thu, 27 May 2021 12:47:11 +0200
Message-Id: <20210527104711.2671610-1-elver@google.com>
Mime-Version: 1.0
Subject: [PATCH] perf: Fix data race between pin_count increment/decrement
From:   Marco Elver <elver@google.com>
To:     elver@google.com, peterz@infradead.org, mingo@redhat.com,
        acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, linux-perf-users@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     kasan-dev@googlegroups.com, dvyukov@google.com,
        syzbot+142c9018f5962db69c7e@syzkaller.appspotmail.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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