rxrpc: Fix incoming call setup race

An incoming call can race with rxrpc socket destruction, leading to a
leaked call.  This may result in an oops when the call timer eventually
expires:

   BUG: kernel NULL pointer dereference, address: 0000000000000874
   RIP: 0010:_raw_spin_lock_irqsave+0x2a/0x50
   Call Trace:
    <IRQ>
    try_to_wake_up+0x59/0x550
    ? __local_bh_enable_ip+0x37/0x80
    ? rxrpc_poke_call+0x52/0x110 [rxrpc]
    ? rxrpc_poke_call+0x110/0x110 [rxrpc]
    ? rxrpc_poke_call+0x110/0x110 [rxrpc]
    call_timer_fn+0x24/0x120

with a warning in the kernel log looking something like:

   rxrpc: Call 00000000ba5e571a still in use (1,SvAwtACK,1061d,0)!

incurred during rmmod of rxrpc.  The 1061d is the call flags:

   RECVMSG_READ_ALL, RX_HEARD, BEGAN_RX_TIMER, RX_LAST, EXPOSED,
   IS_SERVICE, RELEASED

but no DISCONNECTED flag (0x800), so it's an incoming (service) call and
it's still connected.

The race appears to be that:

 (1) rxrpc_new_incoming_call() consults the service struct, checks sk_state
     and allocates a call - then pauses, possibly for an interrupt.

 (2) rxrpc_release_sock() sets RXRPC_CLOSE, nulls the service pointer,
     discards the prealloc and releases all calls attached to the socket.

 (3) rxrpc_new_incoming_call() resumes, launching the new call, including
     its timer and attaching it to the socket.

Fix this by read-locking local->services_lock to access the AF_RXRPC socket
providing the service rather than RCU in rxrpc_new_incoming_call().
There's no real need to use RCU here as local->services_lock is only
write-locked by the socket side in two places: when binding and when
shutting down.

Fixes: 5e6ef4f101 ("rxrpc: Make the I/O thread take over the call and local processor work")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org

net/rxrpc/af_rxrpc.c

@@ -155,10 +155,10 @@ static int rxrpc_bind(struct socket *sock, struct sockaddr *saddr, int len)
 
 		if (service_id) {
 			write_lock(&local->services_lock);
-			if (rcu_access_pointer(local->service))
+			if (local->service)
 				goto service_in_use;
 			rx->local = local;
-			rcu_assign_pointer(local->service, rx);
+			local->service = rx;
 			write_unlock(&local->services_lock);
 
 			rx->sk.sk_state = RXRPC_SERVER_BOUND;
@@ -875,9 +875,9 @@ static int rxrpc_release_sock(struct sock *sk)
 
 	sk->sk_state = RXRPC_CLOSE;
 
-	if (rx->local && rcu_access_pointer(rx->local->service) == rx) {
+	if (rx->local && rx->local->service == rx) {
 		write_lock(&rx->local->services_lock);
-		rcu_assign_pointer(rx->local->service, NULL);
+		rx->local->service = NULL;
 		write_unlock(&rx->local->services_lock);
 	}
 

net/rxrpc/ar-internal.h

@@ -283,7 +283,7 @@ struct rxrpc_local {
 	struct socket		*socket;	/* my UDP socket */
 	struct task_struct	*io_thread;
 	struct completion	io_thread_ready; /* Indication that the I/O thread started */
-	struct rxrpc_sock __rcu	*service;	/* Service(s) listening on this endpoint */
+	struct rxrpc_sock	*service;	/* Service(s) listening on this endpoint */
 	struct rw_semaphore	defrag_sem;	/* control re-enablement of IP DF bit */
 	struct sk_buff_head	rx_queue;	/* Received packets */
 	struct list_head	conn_attend_q;	/* Conns requiring immediate attention */

net/rxrpc/call_accept.c

@@ -343,13 +343,13 @@ bool rxrpc_new_incoming_call(struct rxrpc_local *local,
 	if (sp->hdr.type != RXRPC_PACKET_TYPE_DATA)
 		return rxrpc_protocol_error(skb, rxrpc_eproto_no_service_call);
 
-	rcu_read_lock();
+	read_lock(&local->services_lock);
 
 	/* Weed out packets to services we're not offering.  Packets that would
 	 * begin a call are explicitly rejected and the rest are just
 	 * discarded.
 	 */
-	rx = rcu_dereference(local->service);
+	rx = local->service;
 	if (!rx || (sp->hdr.serviceId != rx->srx.srx_service &&
 		    sp->hdr.serviceId != rx->second_service)
 	    ) {
@@ -399,7 +399,7 @@ bool rxrpc_new_incoming_call(struct rxrpc_local *local,
 	spin_unlock(&conn->state_lock);
 
 	spin_unlock(&rx->incoming_lock);
-	rcu_read_unlock();
+	read_unlock(&local->services_lock);
 
 	if (hlist_unhashed(&call->error_link)) {
 		spin_lock(&call->peer->lock);
@@ -413,20 +413,20 @@ bool rxrpc_new_incoming_call(struct rxrpc_local *local,
 	return true;
 
 unsupported_service:
-	rcu_read_unlock();
+	read_unlock(&local->services_lock);
 	return rxrpc_direct_abort(skb, rxrpc_abort_service_not_offered,
 				  RX_INVALID_OPERATION, -EOPNOTSUPP);
 unsupported_security:
-	rcu_read_unlock();
+	read_unlock(&local->services_lock);
 	return rxrpc_direct_abort(skb, rxrpc_abort_service_not_offered,
 				  RX_INVALID_OPERATION, -EKEYREJECTED);
 no_call:
 	spin_unlock(&rx->incoming_lock);
-	rcu_read_unlock();
+	read_unlock(&local->services_lock);
 	_leave(" = f [%u]", skb->mark);
 	return false;
 discard:
-	rcu_read_unlock();
+	read_unlock(&local->services_lock);
 	return true;
 }
 

net/rxrpc/security.c

@@ -178,9 +178,9 @@ struct key *rxrpc_look_up_server_security(struct rxrpc_connection *conn,
 		sprintf(kdesc, "%u:%u",
 			sp->hdr.serviceId, sp->hdr.securityIndex);
 
-	rcu_read_lock();
+	read_lock(&conn->local->services_lock);
 
-	rx = rcu_dereference(conn->local->service);
+	rx = conn->local->service;
 	if (!rx)
 		goto out;
 
@@ -202,6 +202,6 @@ struct key *rxrpc_look_up_server_security(struct rxrpc_connection *conn,
 	}
 
 out:
-	rcu_read_unlock();
+	read_unlock(&conn->local->services_lock);
 	return key;
 }