rxrpc: Reduce the rxrpc_local::services list to a pointer

Reduce the rxrpc_local::services list to just a pointer as we don't permit
multiple service endpoints to bind to a single transport endpoints (this is
excluded by rxrpc_lookup_local()).

The reason we don't allow this is that if you send a request to an AFS
filesystem service, it will try to talk back to your cache manager on the
port you sent from (this is how file change notifications are handled).  To
prevent someone from stealing your CM callbacks, we don't let AF_RXRPC
sockets share a UDP socket if at least one of them has a service bound.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/af_rxrpc.c

@@ -136,7 +136,8 @@ static int rxrpc_bind(struct socket *sock, struct sockaddr *saddr, int len)
 	struct sockaddr_rxrpc *srx = (struct sockaddr_rxrpc *)saddr;
 	struct sock *sk = sock->sk;
 	struct rxrpc_local *local;
-	struct rxrpc_sock *rx = rxrpc_sk(sk), *prx;
+	struct rxrpc_sock *rx = rxrpc_sk(sk);
+	u16 service_id = srx->srx_service;
 	int ret;
 
 	_enter("%p,%p,%d", rx, saddr, len);
@@ -160,15 +161,12 @@ static int rxrpc_bind(struct socket *sock, struct sockaddr *saddr, int len)
 		goto error_unlock;
 	}
 
-	if (rx->srx.srx_service) {
+	if (service_id) {
 		write_lock(&local->services_lock);
-		hlist_for_each_entry(prx, &local->services, listen_link) {
-			if (prx->srx.srx_service == rx->srx.srx_service)
-				goto service_in_use;
-		}
-
+		if (rcu_access_pointer(local->service))
+			goto service_in_use;
 		rx->local = local;
-		hlist_add_head_rcu(&rx->listen_link, &local->services);
+		rcu_assign_pointer(local->service, rx);
 		write_unlock(&local->services_lock);
 
 		rx->sk.sk_state = RXRPC_SERVER_BOUND;
@@ -599,7 +597,6 @@ static int rxrpc_create(struct net *net, struct socket *sock, int protocol,
 	rx->family = protocol;
 	rx->calls = RB_ROOT;
 
-	INIT_HLIST_NODE(&rx->listen_link);
 	spin_lock_init(&rx->incoming_lock);
 	INIT_LIST_HEAD(&rx->sock_calls);
 	INIT_LIST_HEAD(&rx->to_be_accepted);
@@ -681,11 +678,9 @@ static int rxrpc_release_sock(struct sock *sk)
 	sk->sk_state = RXRPC_CLOSE;
 	spin_unlock_bh(&sk->sk_receive_queue.lock);
 
-	ASSERTCMP(rx->listen_link.next, !=, LIST_POISON1);
-
-	if (!hlist_unhashed(&rx->listen_link)) {
+	if (rx->local && rx->local->service == rx) {
 		write_lock(&rx->local->services_lock);
-		hlist_del_rcu(&rx->listen_link);
+		rx->local->service = NULL;
 		write_unlock(&rx->local->services_lock);
 	}
 

net/rxrpc/ar-internal.h

@@ -93,7 +93,6 @@ struct rxrpc_sock {
 	rxrpc_notify_new_call_t	notify_new_call; /* Func to notify of new call */
 	rxrpc_discard_new_call_t discard_new_call; /* Func to discard a new call */
 	struct rxrpc_local	*local;		/* local endpoint */
-	struct hlist_node	listen_link;	/* link in the local endpoint's listen list */
 	struct rxrpc_backlog	*backlog;	/* Preallocation for services */
 	spinlock_t		incoming_lock;	/* Incoming call vs service shutdown lock */
 	struct list_head	sock_calls;	/* List of calls owned by this socket */
@@ -216,7 +215,7 @@ struct rxrpc_local {
 	struct list_head	link;
 	struct socket		*socket;	/* my UDP socket */
 	struct work_struct	processor;
-	struct hlist_head	services;	/* services listening on this endpoint */
+	struct rxrpc_sock __rcu	*service;	/* Service(s) listening on this endpoint */
 	struct rw_semaphore	defrag_sem;	/* control re-enablement of IP DF bit */
 	struct sk_buff_head	reject_queue;	/* packets awaiting rejection */
 	struct sk_buff_head	event_queue;	/* endpoint event packets awaiting processing */

net/rxrpc/call_accept.c

@@ -331,14 +331,14 @@ struct rxrpc_call *rxrpc_new_incoming_call(struct rxrpc_local *local,
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	struct rxrpc_sock *rx;
 	struct rxrpc_call *call;
+	u16 service_id = sp->hdr.serviceId;
 
 	_enter("");
 
 	/* Get the socket providing the service */
-	hlist_for_each_entry_rcu_bh(rx, &local->services, listen_link) {
-		if (rx->srx.srx_service == sp->hdr.serviceId)
-			goto found_service;
-	}
+	rx = rcu_dereference(local->service);
+	if (service_id == rx->srx.srx_service)
+		goto found_service;
 
 	trace_rxrpc_abort("INV", sp->hdr.cid, sp->hdr.callNumber, sp->hdr.seq,
 			  RX_INVALID_OPERATION, EOPNOTSUPP);

net/rxrpc/local_object.c

@@ -86,7 +86,6 @@ static struct rxrpc_local *rxrpc_alloc_local(const struct sockaddr_rxrpc *srx)
 		atomic_set(&local->usage, 1);
 		INIT_LIST_HEAD(&local->link);
 		INIT_WORK(&local->processor, rxrpc_local_processor);
-		INIT_HLIST_HEAD(&local->services);
 		init_rwsem(&local->defrag_sem);
 		skb_queue_head_init(&local->reject_queue);
 		skb_queue_head_init(&local->event_queue);
@@ -292,7 +291,7 @@ static void rxrpc_local_destroyer(struct rxrpc_local *local)
 	mutex_unlock(&rxrpc_local_mutex);
 
 	ASSERT(RB_EMPTY_ROOT(&local->client_conns));
-	ASSERT(hlist_empty(&local->services));
+	ASSERT(!local->service);
 
 	if (socket) {
 		local->socket = NULL;

net/rxrpc/security.c

@@ -131,10 +131,10 @@ int rxrpc_init_server_conn_security(struct rxrpc_connection *conn)
 
 	/* find the service */
 	read_lock(&local->services_lock);
-	hlist_for_each_entry(rx, &local->services, listen_link) {
-		if (rx->srx.srx_service == conn->params.service_id)
-			goto found_service;
-	}
+	rx = rcu_dereference_protected(local->service,
+				       lockdep_is_held(&local->services_lock));
+	if (rx && rx->srx.srx_service == conn->params.service_id)
+		goto found_service;
 
 	/* the service appears to have died */
 	read_unlock(&local->services_lock);