sctp: Add RCU protection to assoc->transport_addr_list

peer.transport_addr_list is currently only protected by sk_sock
which is inpractical to acquire for procfs dumping purposes.

This patch adds RCU protection allowing for the procfs readers to
enter RCU read-side critical sections.

Modification of the list continues to be serialized via sk_lock.

V2: Use list_del_rcu() in sctp_association_free() to be safe
Skip transports marked dead when dumping for procfs

Cc: Vlad Yasevich <[e-mail]>
Cc: Neil Horman <[e-mail]>
Signed-off-by: Thomas Graf <[e-mail]>
Acked-by: Vlad Yasevich <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sctp/structs.h

···
 
 	/* 64-bit random number sent with heartbeat. */
 	__u64 hb_nonce;
+
+	struct rcu_head rcu;
 };
 
 struct sctp_transport *sctp_transport_new(struct net *, const union sctp_addr *,

net/sctp/associola.c

···
 	/* Release the transport structures. */
 	list_for_each_safe(pos, temp, &asoc->peer.transport_addr_list) {
 		transport = list_entry(pos, struct sctp_transport, transports);
-		list_del(pos);
+		list_del_rcu(pos);
 		sctp_transport_free(transport);
 	}
 
···
 		sctp_assoc_update_retran_path(asoc);
 
 	/* Remove this peer from the list. */
-	list_del(&peer->transports);
+	list_del_rcu(&peer->transports);
 
 	/* Get the first transport of asoc. */
 	pos = asoc->peer.transport_addr_list.next;
···
 	peer->state = peer_state;
 
 	/* Attach the remote transport to our asoc.  */
-	list_add_tail(&peer->transports, &asoc->peer.transport_addr_list);
+	list_add_tail_rcu(&peer->transports, &asoc->peer.transport_addr_list);
 	asoc->peer.transport_count++;
 
 	/* If we do not yet have a primary path, set one.  */

net/sctp/proc.c

···
 	struct sctp_af *af;
 
 	primary = &assoc->peer.primary_addr;
-	list_for_each_entry(transport, &assoc->peer.transport_addr_list,
+	rcu_read_lock();
+	list_for_each_entry_rcu(transport, &assoc->peer.transport_addr_list,
 			transports) {
 		addr = &transport->ipaddr;
+		if (transport->dead)
+			continue;
+
 		af = sctp_get_af_specific(addr->sa.sa_family);
 		if (af->cmp_addr(addr, primary)) {
 			seq_printf(seq, "*");
 		}
 		af->seq_dump_addr(seq, addr);
 	}
+	rcu_read_unlock();
 }
 
 static void * sctp_eps_seq_start(struct seq_file *seq, loff_t *pos)
···
 	head = &sctp_assoc_hashtable[hash];
 	sctp_local_bh_disable();
 	read_lock(&head->lock);
+	rcu_read_lock();
 	sctp_for_each_hentry(epb, node, &head->chain) {
 		if (!net_eq(sock_net(epb->sk), seq_file_net(seq)))
 			continue;
 		assoc = sctp_assoc(epb);
-		list_for_each_entry(tsp, &assoc->peer.transport_addr_list,
+		list_for_each_entry_rcu(tsp, &assoc->peer.transport_addr_list,
 					transports) {
+			if (tsp->dead)
+				continue;
+
 			/*
 			 * The remote address (ADDR)
 			 */
···
 		}
 	}
 
+	rcu_read_unlock();
 	read_unlock(&head->lock);
 	sctp_local_bh_enable();
 

net/sctp/transport.c

···
 	sctp_transport_put(transport);
 }
 
-/* Destroy the transport data structure.
- * Assumes there are no more users of this structure.
- */
-static void sctp_transport_destroy(struct sctp_transport *transport)
+static void sctp_transport_destroy_rcu(struct rcu_head *head)
 {
-	SCTP_ASSERT(transport->dead, "Transport is not dead", return);
+	struct sctp_transport *transport;
 
+	transport = container_of(head, struct sctp_transport, rcu);
 	if (transport->asoc)
 		sctp_association_put(transport->asoc);
 
···
 	dst_release(transport->dst);
 	kfree(transport);
 	SCTP_DBG_OBJCNT_DEC(transport);
+}
+
+/* Destroy the transport data structure.
+ * Assumes there are no more users of this structure.
+ */
+static void sctp_transport_destroy(struct sctp_transport *transport)
+{
+	SCTP_ASSERT(transport->dead, "Transport is not dead", return);
+
+	call_rcu(&transport->rcu, sctp_transport_destroy_rcu);
 }
 
 /* Start T3_rtx timer if it is not already running and update the heartbeat