netfilter: conntrack: don't refresh sctp entries in closed state

Vivek Thrivikraman reported:
An SCTP server application which is accessed continuously by client
application.
When the session disconnects the client retries to establish a connection.
After restart of SCTP server application the session is not established
because of stale conntrack entry with connection state CLOSED as below.

(removing this entry manually established new connection):

sctp 9 CLOSED src=10.141.189.233 [..] [ASSURED]

Just skip timeout update of closed entries, we don't want them to
stay around forever.

Reported-and-tested-by: Vivek Thrivikraman <[e-mail]>
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1579
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_proto_sctp.c

···
 			pr_debug("Setting vtag %x for dir %d\n",
 				 ih->init_tag, !dir);
 			ct->proto.sctp.vtag[!dir] = ih->init_tag;
+
+			/* don't renew timeout on init retransmit so
+			 * port reuse by client or NAT middlebox cannot
+			 * keep entry alive indefinitely (incl. nat info).
+			 */
+			if (new_state == SCTP_CONNTRACK_CLOSED &&
+			    old_state == SCTP_CONNTRACK_CLOSED &&
+			    nf_ct_is_confirmed(ct))
+				ignore = true;
 		}
 
 		ct->proto.sctp.state = new_state;