sctp: fix a potential OOB access in sctp_sched_set_sched()

The 'sched' index value must be checked before accessing an element
of the 'sctp_sched_ops' array. Otherwise, it can lead to OOB access.

Note that it's harmless since the 'sched' parameter is checked before
calling 'sctp_sched_set_sched'.

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Ilia.Gavrilov <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/stream_sched.c

@@ -148,18 +148,19 @@ static void sctp_sched_free_sched(struct sctp_stream *stream)
 int sctp_sched_set_sched(struct sctp_association *asoc,
 			 enum sctp_sched_type sched)
 {
-	struct sctp_sched_ops *n = sctp_sched_ops[sched];
 	struct sctp_sched_ops *old = asoc->outqueue.sched;
 	struct sctp_datamsg *msg = NULL;
+	struct sctp_sched_ops *n;
 	struct sctp_chunk *ch;
 	int i, ret = 0;
 
-	if (old == n)
-		return ret;
-
 	if (sched > SCTP_SS_MAX)
 		return -EINVAL;
 
+	n = sctp_sched_ops[sched];
+	if (old == n)
+		return ret;
+
 	if (old)
 		sctp_sched_free_sched(&asoc->stream);