tipc: eliminate risk of finding to-be-deleted node instance

Although we have never seen it happen, we have identified the
following problematic scenario when nodes are stopped and deleted:

CPU0: CPU1:

tipc_node_xxx() //ref == 1
tipc_node_put() //ref -> 0
tipc_node_find() // node still in table
tipc_node_delete()
list_del_rcu(n. list)
tipc_node_get() //ref -> 1, bad
kfree_rcu()

tipc_node_put() //ref to 0 again.
kfree_rcu() // BOOM!

We fix this by introducing use of the conditional kref_get_if_not_zero()
instead of kref_get() in the function tipc_node_find(). This eliminates
any risk of post-mortem access.

Reported-by: Zhijiang Hu <[e-mail]>
Acked-by: Ying Xue <[e-mail]>
Signed-off-by: Jon Maloy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/node.c

···
  */
 static struct tipc_node *tipc_node_find(struct net *net, u32 addr)
 {
-	struct tipc_net *tn = net_generic(net, tipc_net_id);
+	struct tipc_net *tn = tipc_net(net);
 	struct tipc_node *node;
+	unsigned int thash = tipc_hashfn(addr);
 
 	if (unlikely(!in_own_cluster_exact(net, addr)))
 		return NULL;
 
 	rcu_read_lock();
-	hlist_for_each_entry_rcu(node, &tn->node_htable[tipc_hashfn(addr)],
-				 hash) {
-		if (node->addr == addr) {
-			tipc_node_get(node);
-			rcu_read_unlock();
-			return node;
-		}
+	hlist_for_each_entry_rcu(node, &tn->node_htable[thash], hash) {
+		if (node->addr != addr)
+			continue;
+		if (!kref_get_unless_zero(&node->kref))
+			node = NULL;
+		break;
 	}
 	rcu_read_unlock();
-	return NULL;
+	return node;
 }
 
 static void tipc_node_read_lock(struct tipc_node *n)