llc: Fix races between llc2 handler use and (un)registration

When registering the handlers, any state they rely on must be
completely initialised first. When unregistering, we must wait until
they are definitely no longer running. llc_rcv() must also avoid
reading the handler pointers again after checking for NULL.

Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/llc/llc_input.c

···
 void llc_add_pack(int type, void (*handler)(struct llc_sap *sap,
 					    struct sk_buff *skb))
 {
+	smp_wmb(); /* ensure initialisation is complete before it's called */
 	if (type == LLC_DEST_SAP || type == LLC_DEST_CONN)
 		llc_type_handlers[type - 1] = handler;
 }
···
 {
 	if (type == LLC_DEST_SAP || type == LLC_DEST_CONN)
 		llc_type_handlers[type - 1] = NULL;
+	synchronize_net();
 }
 
 void llc_set_station_handler(void (*handler)(struct sk_buff *skb))
 {
+	/* Ensure initialisation is complete before it's called */
+	if (handler)
+		smp_wmb();
+
 	llc_station_handler = handler;
+
+	if (!handler)
+		synchronize_net();
 }
 
 /**
···
 	int dest;
 	int (*rcv)(struct sk_buff *, struct net_device *,
 		   struct packet_type *, struct net_device *);
+	void (*sta_handler)(struct sk_buff *skb);
+	void (*sap_handler)(struct llc_sap *sap, struct sk_buff *skb);
 
 	if (!net_eq(dev_net(dev), &init_net))
 		goto drop;
···
 	 */
 	rcv = rcu_dereference(sap->rcv_func);
 	dest = llc_pdu_type(skb);
-	if (unlikely(!dest || !llc_type_handlers[dest - 1])) {
+	sap_handler = dest ? ACCESS_ONCE(llc_type_handlers[dest - 1]) : NULL;
+	if (unlikely(!sap_handler)) {
 		if (rcv)
 			rcv(skb, dev, pt, orig_dev);
 		else
···
 			if (cskb)
 				rcv(cskb, dev, pt, orig_dev);
 		}
-		llc_type_handlers[dest - 1](sap, skb);
+		sap_handler(sap, skb);
 	}
 	llc_sap_put(sap);
 out:
···
 	kfree_skb(skb);
 	goto out;
 handle_station:
-	if (!llc_station_handler)
+	sta_handler = ACCESS_ONCE(llc_station_handler);
+	if (!sta_handler)
 		goto drop;
-	llc_station_handler(skb);
+	sta_handler(skb);
 	goto out;
 }
 

net/llc/llc_station.c

···
 			(unsigned long)&llc_main_station);
 	llc_main_station.ack_timer.expires  = jiffies +
 						sysctl_llc_station_ack_timeout;
-	llc_set_station_handler(llc_station_rcv);
 	llc_main_station.maximum_retry	= 1;
 	llc_main_station.state		= LLC_STATION_STATE_UP;
+	llc_set_station_handler(llc_station_rcv);
 }
 
 void llc_station_exit(void)