audit: check if queue is full after prepare_to_wait_exclusive()

Commit 7ffb8e3 ("audit: we don't need to
__set_current_state(TASK_RUNNING)") accidentally moved queue full check
before add_wait_queue_exclusive() which introduced the following race:

    CPU1                           CPU2
  ========                       ========
  (in audit_log_start())         (in kauditd_thread())

  queue is full
                                 wake_up(&audit_backlog_wait)
                                 wait_event_freezable()
  add_wait_queue_exclusive()
  ...
  schedule_timeout()

Once this happens, both audit_log_start() and kauditd_thread() can cause
deadlock for up to backlog_wait_time waiting for each other. To prevent
the race, this patch adds queue full check after
prepare_to_wait_exclusive().

Fixes: 7ffb8e3 ("audit: we don't need to __set_current_state(TASK_RUNNING)")
Signed-off-by: Eiichi Tsukata <eiichi.tsukata@nutanix.com>

kernel/audit.c

@@ -644,8 +644,14 @@ static long wait_for_kauditd(long stime)
 
 	prepare_to_wait_exclusive(&audit_backlog_wait, &wait,
 				  TASK_UNINTERRUPTIBLE);
-	rtime = schedule_timeout(stime);
-	atomic_add(stime - rtime, &audit_backlog_wait_time_actual);
+
+	/* need to check if the queue is full again because kauditd might have
+	 * flushed the queue and went to sleep after prepare_to_wait_exclusive()
+	 */
+	if (audit_queue_full(&audit_queue)) {
+		rtime = schedule_timeout(stime);
+		atomic_add(stime - rtime, &audit_backlog_wait_time_actual);
+	}
 	finish_wait(&audit_backlog_wait, &wait);
 
 	return rtime;