ipv6:introduce function to find route for redirect

RFC 4861 says that the IP source address of the Redirect is the
same as the current first-hop router for the specified ICMP
Destination Address, so the gateway should be taken into
consideration when we find the route for redirect.

There was once a check in commit
a6279458 ("NDISC: Search over
all possible rules on receipt of redirect.") and the check
went away in commit b94f1c09
("ipv6: Use icmpv6_notify() to propagate redirect, instead of
rt6_redirect()").

The bug is only "exploitable" on layer-2 because the source
address of the redirect is checked to be a valid link-local
address but it makes spoofing a lot easier in the same L2
domain nonetheless.

Thanks very much for Hannes's help.
Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ah6.c

@@ -628,7 +628,7 @@ static void ah6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return;
 
 	if (type == NDISC_REDIRECT)
-		ip6_redirect(skb, net, 0, 0);
+		ip6_redirect(skb, net, skb->dev->ifindex, 0);
 	else
 		ip6_update_pmtu(skb, net, info, 0, 0);
 	xfrm_state_put(x);

net/ipv6/esp6.c

@@ -447,7 +447,7 @@ static void esp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return;
 
 	if (type == NDISC_REDIRECT)
-		ip6_redirect(skb, net, 0, 0);
+		ip6_redirect(skb, net, skb->dev->ifindex, 0);
 	else
 		ip6_update_pmtu(skb, net, info, 0, 0);
 	xfrm_state_put(x);

net/ipv6/icmp.c

@@ -92,7 +92,7 @@ static void icmpv6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	if (type == ICMPV6_PKT_TOOBIG)
 		ip6_update_pmtu(skb, net, info, 0, 0);
 	else if (type == NDISC_REDIRECT)
-		ip6_redirect(skb, net, 0, 0);
+		ip6_redirect(skb, net, skb->dev->ifindex, 0);
 
 	if (!(type & ICMPV6_INFOMSG_MASK))
 		if (icmp6->icmp6_type == ICMPV6_ECHO_REQUEST)

net/ipv6/ipcomp6.c

@@ -76,7 +76,7 @@ static void ipcomp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return;
 
 	if (type == NDISC_REDIRECT)
-		ip6_redirect(skb, net, 0, 0);
+		ip6_redirect(skb, net, skb->dev->ifindex, 0);
 	else
 		ip6_update_pmtu(skb, net, info, 0, 0);
 	xfrm_state_put(x);

net/ipv6/ndisc.c

@@ -1367,7 +1367,8 @@ static void ndisc_redirect_rcv(struct sk_buff *skb)
 		return;
 
 	if (!ndopts.nd_opts_rh) {
-		ip6_redirect_no_header(skb, dev_net(skb->dev), 0, 0);
+		ip6_redirect_no_header(skb, dev_net(skb->dev),
+					skb->dev->ifindex, 0);
 		return;
 	}
 

net/ipv6/route.c

@@ -1156,6 +1156,77 @@ void ip6_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, __be32 mtu)
 }
 EXPORT_SYMBOL_GPL(ip6_sk_update_pmtu);
 
+/* Handle redirects */
+struct ip6rd_flowi {
+	struct flowi6 fl6;
+	struct in6_addr gateway;
+};
+
+static struct rt6_info *__ip6_route_redirect(struct net *net,
+					     struct fib6_table *table,
+					     struct flowi6 *fl6,
+					     int flags)
+{
+	struct ip6rd_flowi *rdfl = (struct ip6rd_flowi *)fl6;
+	struct rt6_info *rt;
+	struct fib6_node *fn;
+
+	/* Get the "current" route for this destination and
+	 * check if the redirect has come from approriate router.
+	 *
+	 * RFC 4861 specifies that redirects should only be
+	 * accepted if they come from the nexthop to the target.
+	 * Due to the way the routes are chosen, this notion
+	 * is a bit fuzzy and one might need to check all possible
+	 * routes.
+	 */
+
+	read_lock_bh(&table->tb6_lock);
+	fn = fib6_lookup(&table->tb6_root, &fl6->daddr, &fl6->saddr);
+restart:
+	for (rt = fn->leaf; rt; rt = rt->dst.rt6_next) {
+		if (rt6_check_expired(rt))
+			continue;
+		if (rt->dst.error)
+			break;
+		if (!(rt->rt6i_flags & RTF_GATEWAY))
+			continue;
+		if (fl6->flowi6_oif != rt->dst.dev->ifindex)
+			continue;
+		if (!ipv6_addr_equal(&rdfl->gateway, &rt->rt6i_gateway))
+			continue;
+		break;
+	}
+
+	if (!rt)
+		rt = net->ipv6.ip6_null_entry;
+	else if (rt->dst.error) {
+		rt = net->ipv6.ip6_null_entry;
+		goto out;
+	}
+	BACKTRACK(net, &fl6->saddr);
+out:
+	dst_hold(&rt->dst);
+
+	read_unlock_bh(&table->tb6_lock);
+
+	return rt;
+};
+
+static struct dst_entry *ip6_route_redirect(struct net *net,
+					const struct flowi6 *fl6,
+					const struct in6_addr *gateway)
+{
+	int flags = RT6_LOOKUP_F_HAS_SADDR;
+	struct ip6rd_flowi rdfl;
+
+	rdfl.fl6 = *fl6;
+	rdfl.gateway = *gateway;
+
+	return fib6_rule_lookup(net, &rdfl.fl6,
+				flags, __ip6_route_redirect);
+}
+
 void ip6_redirect(struct sk_buff *skb, struct net *net, int oif, u32 mark)
 {
 	const struct ipv6hdr *iph = (struct ipv6hdr *) skb->data;
@@ -1170,8 +1241,7 @@ void ip6_redirect(struct sk_buff *skb, struct net *net, int oif, u32 mark)
 	fl6.saddr = iph->saddr;
 	fl6.flowlabel = ip6_flowinfo(iph);
 
-	dst = ip6_route_output(net, NULL, &fl6);
-	if (!dst->error)
+	dst = ip6_route_redirect(net, &fl6, &ipv6_hdr(skb)->saddr);
 	rt6_do_redirect(dst, NULL, skb);
 	dst_release(dst);
 }
@@ -1192,8 +1262,7 @@ void ip6_redirect_no_header(struct sk_buff *skb, struct net *net, int oif,
 	fl6.daddr = msg->dest;
 	fl6.saddr = iph->daddr;
 
-	dst = ip6_route_output(net, NULL, &fl6);
-	if (!dst->error)
+	dst = ip6_route_redirect(net, &fl6, &iph->saddr);
 	rt6_do_redirect(dst, NULL, skb);
 	dst_release(dst);
 }