[PATCH] trace/trace_stat: fix a race condition in register_stat_tracer

[Luis Henriques]

A race condition exits (at least theoretically) if a tracer_stat
registration is interrupted after verifying it hasn't been registered yet
but before being added to the all_stat_sessions list.

This patch fixes this race by making both operations atomic (protected by
the all_stat_sessions_mutex mutex).

Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 kernel/trace/trace_stat.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/kernel/trace/trace_stat.c b/kernel/trace/trace_stat.c
index 7af67360b330..cb70ac12cfe4 100644
--- a/kernel/trace/trace_stat.c
+++ b/kernel/trace/trace_stat.c
@@ -310,16 +310,6 @@ int register_stat_tracer(struct tracer_stat *trace)
 	if (!trace->stat_start || !trace->stat_next || !trace->stat_show)
 		return -EINVAL;
 
-	/* Already registered? */
-	mutex_lock(&all_stat_sessions_mutex);
-	list_for_each_entry(node, &all_stat_sessions, session_list) {
-		if (node->ts == trace) {
-			mutex_unlock(&all_stat_sessions_mutex);
-			return -EINVAL;
-		}
-	}
-	mutex_unlock(&all_stat_sessions_mutex);
-
 	/* Init the session */
 	session = kzalloc(sizeof(*session), GFP_KERNEL);
 	if (!session)
@@ -337,10 +327,21 @@ int register_stat_tracer(struct tracer_stat *trace)
 
 	/* Register */
 	mutex_lock(&all_stat_sessions_mutex);
+	/* Already registered? */
+	list_for_each_entry(node, &all_stat_sessions, session_list) {
+		if (node->ts == trace) {
+			ret = -EINVAL;
+			goto out;
+		}
+	}
 	list_add_tail(&session->session_list, &all_stat_sessions);
+
+out:
 	mutex_unlock(&all_stat_sessions_mutex);
+	if (ret)
+		destroy_session(session);
 
-	return 0;
+	return ret;
 }
 
 void unregister_stat_tracer(struct tracer_stat *trace)
-- 
1.9.1

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