ipv6: add key length check into rt6_select()

After rwlock is replaced with rcu and spinlock, fib6_lookup() could
potentially return an intermediate node if other thread is doing
fib6_del() on a route which is the only route on the node so that
fib6_repair_tree() will be called on this node and potentially assigns
fn->leaf to the its child's fn->leaf.

In order to detect this situation in rt6_select(), we have to check if
fn->fn_bit is consistent with the key length stored in the route. And
depending on if the fn is in the subtree or not, the key is either
rt->rt6i_dst or rt->rt6i_src.
If any inconsistency is found, that means the node no longer holds valid
routes in it. So net->ipv6.ip6_null_entry is returned.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -755,6 +755,7 @@ static struct rt6_info *rt6_select(struct net *net, struct fib6_node *fn,
 	struct rt6_info *leaf = fn->leaf;
 	struct rt6_info *match, *rt0;
 	bool do_rr = false;
+	int key_plen;
 
 	if (!leaf)
 		return net->ipv6.ip6_null_entry;
@@ -763,6 +764,19 @@ static struct rt6_info *rt6_select(struct net *net, struct fib6_node *fn,
 	if (!rt0)
 		fn->rr_ptr = rt0 = leaf;
 
+	/* Double check to make sure fn is not an intermediate node
+	 * and fn->leaf does not points to its child's leaf
+	 * (This might happen if all routes under fn are deleted from
+	 * the tree and fib6_repair_tree() is called on the node.)
+	 */
+	key_plen = rt0->rt6i_dst.plen;
+#ifdef CONFIG_IPV6_SUBTREES
+	if (rt0->rt6i_src.plen)
+		key_plen = rt0->rt6i_src.plen;
+#endif
+	if (fn->fn_bit != key_plen)
+		return net->ipv6.ip6_null_entry;
+
 	match = find_rr_leaf(fn, leaf, rt0, rt0->rt6i_metric, oif, strict,
 			     &do_rr);