sctp: check duplicate node before inserting a new transport

sctp has changed to use rhlist for transport rhashtable since commit
7fda702f ("sctp: use new rhlist interface on sctp transport
rhashtable").

But rhltable_insert_key doesn't check the duplicate node when inserting
a node, unlike rhashtable_lookup_insert_key. It may cause duplicate
assoc/transport in rhashtable. like:

 client (addr A, B)                 server (addr X, Y)
    connect to X           INIT (1)
                        ------------>
    connect to Y           INIT (2)
                        ------------>
                         INIT_ACK (1)
                        <------------
                         INIT_ACK (2)
                        <------------

After sending INIT (2), one transport will be created and hashed into
rhashtable. But when receiving INIT_ACK (1) and processing the address
params, another transport will be created and hashed into rhashtable
with the same addr Y and EP as the last transport. This will confuse
the assoc/transport's lookup.

This patch is to fix it by returning err if any duplicate node exists
before inserting it.

Fixes: 7fda702f ("sctp: use new rhlist interface on sctp transport rhashtable")
Reported-by: NFabio M. Di Nitto <fdinitto@redhat.com>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -872,6 +872,8 @@ void sctp_transport_hashtable_destroy(void)
 
 int sctp_hash_transport(struct sctp_transport *t)
 {
+	struct sctp_transport *transport;
+	struct rhlist_head *tmp, *list;
 	struct sctp_hash_cmp_arg arg;
 	int err;
 
@@ -882,8 +884,19 @@ int sctp_hash_transport(struct sctp_transport *t)
 	arg.paddr = &t->ipaddr;
 	arg.lport = htons(t->asoc->base.bind_addr.port);
 
+	list = rhltable_lookup(&sctp_transport_hashtable, &arg,
+			       sctp_hash_params);
+
+	rhl_for_each_entry_rcu(transport, tmp, list, node)
+		if (transport->asoc->ep == t->asoc->ep) {
+			err = -EEXIST;
+			goto out;
+		}
+
 	err = rhltable_insert_key(&sctp_transport_hashtable, &arg,
 				  &t->node, sctp_hash_params);
+
+out:
 	if (err)
 		pr_err_once("insert transport fail, errno %d\n", err);