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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        syzbot+142c9018f5962db69c7e@syzkaller.appspotmail.com,
        Marco Elver <elver@google.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Subject: [PATCH 4.9 34/42] perf: Fix data race between pin_count increment/decrement
Date:   Mon, 14 Jun 2021 12:27:25 +0200
Message-Id: <20210614102643.790442311@linuxfoundation.org>
In-Reply-To: <20210614102642.700712386@linuxfoundation.org>
References: <20210614102642.700712386@linuxfoundation.org>
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From: Marco Elver <elver@google.com>

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>
---
 kernel/events/core.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3782,7 +3782,9 @@ find_get_context(struct pmu *pmu, struct
 		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;
 	}