netfilter: conntrack: check SCTP_CID_SHUTDOWN_ACK for vtag setting in sctp_new

The annotation says in sctp_new(): "If it is a shutdown ack OOTB packet, we
expect a return shutdown complete, otherwise an ABORT Sec 8.4 (5) and (8)".
However, it does not check SCTP_CID_SHUTDOWN_ACK before setting vtag[REPLY]
in the conntrack entry(ct).

Because of that, if the ct in Router disappears for some reason in [1]
with the packet sequence like below:

   Client > Server: sctp (1) [INIT] [init tag: 3201533963]
   Server > Client: sctp (1) [INIT ACK] [init tag: 972498433]
   Client > Server: sctp (1) [COOKIE ECHO]
   Server > Client: sctp (1) [COOKIE ACK]
   Client > Server: sctp (1) [DATA] (B)(E) [TSN: 3075057809]
   Server > Client: sctp (1) [SACK] [cum ack 3075057809]
   Server > Client: sctp (1) [HB REQ]
   (the ct in Router disappears somehow)  <-------- [1]
   Client > Server: sctp (1) [HB ACK]
   Client > Server: sctp (1) [DATA] (B)(E) [TSN: 3075057810]
   Client > Server: sctp (1) [DATA] (B)(E) [TSN: 3075057810]
   Client > Server: sctp (1) [HB REQ]
   Client > Server: sctp (1) [DATA] (B)(E) [TSN: 3075057810]
   Client > Server: sctp (1) [HB REQ]
   Client > Server: sctp (1) [ABORT]

when processing HB ACK packet in Router it calls sctp_new() to initialize
the new ct with vtag[REPLY] set to HB_ACK packet's vtag.

Later when sending DATA from Client, all the SACKs from Server will get
dropped in Router, as the SACK packet's vtag does not match vtag[REPLY]
in the ct. The worst thing is the vtag in this ct will never get fixed
by the upcoming packets from Server.

This patch fixes it by checking SCTP_CID_SHUTDOWN_ACK before setting
vtag[REPLY] in the ct in sctp_new() as the annotation says. With this
fix, it will leave vtag[REPLY] in ct to 0 in the case above, and the
next HB REQ/ACK from Server is able to fix the vtag as its value is 0
in nf_conntrack_sctp_packet().

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: lxin

net/netfilter/nf_conntrack_proto_sctp.c

@@ -283,7 +283,7 @@ sctp_new(struct nf_conn *ct, const struct sk_buff *skb,
 			pr_debug("Setting vtag %x for secondary conntrack\n",
 				 sh->vtag);
 			ct->proto.sctp.vtag[IP_CT_DIR_ORIGINAL] = sh->vtag;
-		} else {
+		} else if (sch->type == SCTP_CID_SHUTDOWN_ACK) {
 		/* If it is a shutdown ack OOTB packet, we expect a return
 		   shutdown complete, otherwise an ABORT Sec 8.4 (5) and (8) */
 			pr_debug("Setting vtag %x for new conn OOTB\n",