ipv4: Fix route lookups when handling ICMP redirects and PMTU updates

The PMTU update and ICMP redirect helper functions initialise their fl4
variable with either __build_flow_key() or build_sk_flow_key(). These
initialisation functions always set ->flowi4_scope with
RT_SCOPE_UNIVERSE and might set the ECN bits of ->flowi4_tos. This is
not a problem when the route lookup is later done via
ip_route_output_key_hash(), which properly clears the ECN bits from
->flowi4_tos and initialises ->flowi4_scope based on the RTO_ONLINK
flag. However, some helpers call fib_lookup() directly, without
sanitising the tos and scope fields, so the route lookup can fail and,
as a result, the ICMP redirect or PMTU update aren't taken into
account.

Fix this by extracting the ->flowi4_tos and ->flowi4_scope sanitisation
code into ip_rt_fix_tos(), then use this function in handlers that call
fib_lookup() directly.

Note 1: We can't sanitise ->flowi4_tos and ->flowi4_scope in a central
place (like __build_flow_key() or flowi4_init_output()), because
ip_route_output_key_hash() expects non-sanitised values. When called
with sanitised values, it can erroneously overwrite RT_SCOPE_LINK with
RT_SCOPE_UNIVERSE in ->flowi4_scope. Therefore we have to be careful to
sanitise the values only for those paths that don't call
ip_route_output_key_hash().

Note 2: The problem is mostly about sanitising ->flowi4_tos. Having
->flowi4_scope initialised with RT_SCOPE_UNIVERSE instead of
RT_SCOPE_LINK probably wasn't really a problem: sockets with the
SOCK_LOCALROUTE flag set (those that'd result in RTO_ONLINK being set)
normally shouldn't receive ICMP redirects or PMTU updates.

Fixes: 4895c771 ("ipv4: Add FIB nexthop exceptions.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/route.c

@@ -499,6 +499,15 @@ void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)
 }
 EXPORT_SYMBOL(__ip_select_ident);
 
+static void ip_rt_fix_tos(struct flowi4 *fl4)
+{
+	__u8 tos = RT_FL_TOS(fl4);
+
+	fl4->flowi4_tos = tos & IPTOS_RT_MASK;
+	fl4->flowi4_scope = tos & RTO_ONLINK ?
+			    RT_SCOPE_LINK : RT_SCOPE_UNIVERSE;
+}
+
 static void __build_flow_key(const struct net *net, struct flowi4 *fl4,
 			     const struct sock *sk,
 			     const struct iphdr *iph,
@@ -824,6 +833,7 @@ static void ip_do_redirect(struct dst_entry *dst, struct sock *sk, struct sk_buf
 	rt = (struct rtable *) dst;
 
 	__build_flow_key(net, &fl4, sk, iph, oif, tos, prot, mark, 0);
+	ip_rt_fix_tos(&fl4);
 	__ip_do_redirect(rt, skb, &fl4, true);
 }
 
@@ -1048,6 +1058,7 @@ static void ip_rt_update_pmtu(struct dst_entry *dst, struct sock *sk,
 	struct flowi4 fl4;
 
 	ip_rt_build_flow_key(&fl4, sk, skb);
+	ip_rt_fix_tos(&fl4);
 
 	/* Don't make lookup fail for bridged encapsulations */
 	if (skb && netif_is_any_bridge_port(skb->dev))
@@ -1122,6 +1133,8 @@ void ipv4_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, u32 mtu)
 			goto out;
 
 		new = true;
+	} else {
+		ip_rt_fix_tos(&fl4);
 	}
 
 	__ip_rt_update_pmtu((struct rtable *)xfrm_dst_path(&rt->dst), &fl4, mtu);
@@ -2603,7 +2616,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 struct rtable *ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
 					const struct sk_buff *skb)
 {
-	__u8 tos = RT_FL_TOS(fl4);
 	struct fib_result res = {
 		.type		= RTN_UNSPEC,
 		.fi		= NULL,
@@ -2613,9 +2625,7 @@ struct rtable *ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
 	struct rtable *rth;
 
 	fl4->flowi4_iif = LOOPBACK_IFINDEX;
-	fl4->flowi4_tos = tos & IPTOS_RT_MASK;
-	fl4->flowi4_scope = ((tos & RTO_ONLINK) ?
-			 RT_SCOPE_LINK : RT_SCOPE_UNIVERSE);
+	ip_rt_fix_tos(fl4);
 
 	rcu_read_lock();
 	rth = ip_route_output_key_hash_rcu(net, fl4, &res, skb);