sctp: start t5 timer only when peer rwnd is 0 and local state is SHUTDOWN_PENDING

commit 8a0d19c5 upstream.

when A sends a data to B, then A close() and enter into SHUTDOWN_PENDING
state, if B neither claim his rwnd is 0 nor send SACK for this data, A
will keep retransmitting this data until t5 timeout, Max.Retrans times
can't work anymore, which is bad.

if B's rwnd is not 0, it should send abort after Max.Retrans times, only
when B's rwnd == 0 and A's retransmitting beyonds Max.Retrans times, A
will start t5 timer, which is also commit f8d96052 ("sctp: Enforce
retransmission limit during shutdown") means, but it lacks the condition
peer rwnd == 0.

so fix it by adding a bit (zero_window_announced) in peer to record if
the last rwnd is 0. If it was, zero_window_announced will be set. and use
this bit to decide if start t5 timer when local.state is SHUTDOWN_PENDING.

Fixes: commit f8d96052 ("sctp: Enforce retransmission limit during shutdown")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: change sack_needed to bitfield as done earlier upstream]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/net/sctp/structs.h

@@ -1586,7 +1586,8 @@ struct sctp_association {
 		 *             : order.  When DATA chunks are out of order,
 		 *             : SACK's are not delayed (see Section 6).
 		 */
-		__u8    sack_needed;     /* Do we need to sack the peer? */
+		__u8    sack_needed:1,     /* Do we need to sack the peer? */
+			zero_window_announced:1;
 		__u32	sack_cnt;
 
 		/* These are capabilities which our peer advertised.  */

net/sctp/outqueue.c

@@ -1265,6 +1265,7 @@ int sctp_outq_sack(struct sctp_outq *q, struct sctp_sackhdr *sack)
 	 */
 
 	sack_a_rwnd = ntohl(sack->a_rwnd);
+	asoc->peer.zero_window_announced = !sack_a_rwnd;
 	outstanding = q->outstanding_bytes;
 
 	if (outstanding < sack_a_rwnd)

net/sctp/sm_statefuns.c

@@ -5302,7 +5302,8 @@ sctp_disposition_t sctp_sf_do_6_3_3_rtx(const struct sctp_endpoint *ep,
 	SCTP_INC_STATS(SCTP_MIB_T3_RTX_EXPIREDS);
 
 	if (asoc->overall_error_count >= asoc->max_retrans) {
-		if (asoc->state == SCTP_STATE_SHUTDOWN_PENDING) {
+		if (asoc->peer.zero_window_announced &&
+		    asoc->state == SCTP_STATE_SHUTDOWN_PENDING) {
 			/*
 			 * We are here likely because the receiver had its rwnd
 			 * closed for a while and we have not been able to