sctp: fix panic when T2-shutdown timer expire on removed transport

If T2-shutdown timer is expired on a removed transport, kernel
panic will occur when we do failure management on that transport.
You can reproduce this use the following sequence:

  Endpoint A                           Endpoint B
  (ESTABLISHED)                        (ESTABLISHED)

                <-----------------      SHUTDOWN
                                        (SRC=X)
  ASCONF        ----------------->
  (Delete IP Address = X)
                <-----------------      ASCONF-ACK
                                        (Success Indication)
                <-----------------      SHUTDOWN
                                        (T2-shutdown timer expire)
This patch fixed the problem.
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/associola.c

@@ -567,6 +567,14 @@ void sctp_assoc_rm_peer(struct sctp_association *asoc,
 	if (asoc->init_last_sent_to == peer)
 		asoc->init_last_sent_to = NULL;
 
+	/* If we remove the transport an SHUTDOWN was last sent to, set it
+	 * to NULL. Combined with the update of the retran path above, this
+	 * will cause the next SHUTDOWN to be sent to the next available
+	 * transport, maintaining the cycle.
+	 */
+	if (asoc->shutdown_last_sent_to == peer)
+		asoc->shutdown_last_sent_to = NULL;
+
 	asoc->peer.transport_count--;
 
 	sctp_transport_free(peer);

net/sctp/sm_statefuns.c

@@ -5432,7 +5432,11 @@ sctp_disposition_t sctp_sf_t2_timer_expire(const struct sctp_endpoint *ep,
 	if (!reply)
 		goto nomem;
 
-	/* Do some failure management (Section 8.2). */
+	/* Do some failure management (Section 8.2).
+	 * If we remove the transport an SHUTDOWN was last sent to, don't
+	 * do failure management.
+	 */
+	if (asoc->shutdown_last_sent_to)
 		sctp_add_cmd_sf(commands, SCTP_CMD_STRIKE,
 				SCTP_TRANSPORT(asoc->shutdown_last_sent_to));