From 828b6f0e26170938d617e99a17177453be4d77a3 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed, 27 Jan 2016 21:59:04 +0100
Subject: [PATCH] perf: Fix NULL deref

Dan reported:

  1229                  if (ctx->task == TASK_TOMBSTONE ||
  1230                      !atomic_inc_not_zero(&ctx->refcount)) {
  1231                          raw_spin_unlock(&ctx->lock);
  1232                          ctx = NULL;
                                ^^^^^^^^^^
ctx is NULL.

  1233                  }
  1234
  1235                  WARN_ON_ONCE(ctx->task != task);
                                     ^^^^^^^^^^^^^^^^^
The patch adds a NULL dereference.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 63b6da39bb38 ("perf: Fix perf_event_exit_task() race")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1d243fadfd12..fe97f95f204e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1230,9 +1230,9 @@ retry:
 		    !atomic_inc_not_zero(&ctx->refcount)) {
 			raw_spin_unlock(&ctx->lock);
 			ctx = NULL;
+		} else {
+			WARN_ON_ONCE(ctx->task != task);
 		}
-
-		WARN_ON_ONCE(ctx->task != task);
 	}
 	rcu_read_unlock();
 	if (!ctx)