tipc: ignore STATE_MSG on wrong link session

The initial session number when a link is created is based on a random
value, taken from struct tipc_net->random. It is then incremented for
each link reset to avoid mixing protocol messages from different link
sessions.

However, when a bearer is reset all its links are deleted, and will
later be re-created using the same random value as the first time.
This means that if the link never went down between creation and
deletion we will still sometimes have two subsequent sessions with
the same session number. In virtual environments with potentially
long transmission times this has turned out to be a real problem.

We now fix this by randomizing the session number each time a link
is created.

With a session number size of 16 bits this gives a risk of session
collision of 1/64k. To reduce this further, we also introduce a sanity
check on the very first STATE message arriving at a link. If this has
an acknowledge value differing from 0, which is logically impossible,
we ignore the message. The final risk for session collision is hence
reduced to 1/4G, which should be sufficient.
Signed-off-by: LUU Duc Canh <canh.d.luu@dektech.com.au>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/link.c

@@ -1516,6 +1516,9 @@ bool tipc_link_validate_msg(struct tipc_link *l, struct tipc_msg *hdr)
 			return false;
 		if (session != curr_session)
 			return false;
+		/* Extra sanity check */
+		if (!link_is_up(l) && msg_ack(hdr))
+			return false;
 		if (!(l->peer_caps & TIPC_LINK_PROTO_SEQNO))
 			return true;
 		/* Accept only STATE with new sequence number */

net/tipc/node.c

@@ -913,6 +913,7 @@ void tipc_node_check_dest(struct net *net, u32 addr,
 	bool reset = true;
 	char *if_name;
 	unsigned long intv;
+	u16 session;
 
 	*dupl_addr = false;
 	*respond = false;
@@ -999,9 +1000,10 @@ void tipc_node_check_dest(struct net *net, u32 addr,
 			goto exit;
 
 		if_name = strchr(b->name, ':') + 1;
+		get_random_bytes(&session, sizeof(u16));
 		if (!tipc_link_create(net, if_name, b->identity, b->tolerance,
 				      b->net_plane, b->mtu, b->priority,
-				      b->window, mod(tipc_net(net)->random),
+				      b->window, session,
 				      tipc_own_addr(net), addr, peer_id,
 				      n->capabilities,
 				      tipc_bc_sndlink(n->net), n->bc_entry.link,
@@ -1625,7 +1627,6 @@ static bool tipc_node_check_state(struct tipc_node *n, struct sk_buff *skb,
 			tipc_link_create_dummy_tnl_msg(l, xmitq);
 			n->failover_sent = true;
 		}
-
 		/* If pkts arrive out of order, use lowest calculated syncpt */
 		if (less(syncpt, n->sync_point))
 			n->sync_point = syncpt;