ipv6: fix src addr routing with the exception table

When inserting route cache into the exception table, the key is
generated with both src_addr and dest_addr with src addr routing.
However, current logic always assumes the src_addr used to generate the
key is a /128 host address. This is not true in the following scenarios:
1. When the route is a gateway route or does not have next hop.
   (rt6_is_gw_or_nonexthop() == false)
2. When calling ip6_rt_cache_alloc(), saddr is passed in as NULL.
This means, when looking for a route cache in the exception table, we
have to do the lookup twice: first time with the passed in /128 host
address, second time with the src_addr stored in fib6_info.

This solves the pmtu discovery issue reported by Mikael Magnusson where
a route cache with a lower mtu info is created for a gateway route with
src addr. However, the lookup code is not able to find this route cache.

Fixes: 2b760fcf5c ("ipv6: hook up exception table to store dst cache")
Reported-by: Mikael Magnusson <mikael.kernel@lists.m7n.se>
Bisected-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Cc: Martin Lau <kafai@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -111,8 +111,8 @@ static int rt6_fill_node(struct net *net, struct sk_buff *skb,
 			 int iif, int type, u32 portid, u32 seq,
 			 unsigned int flags);
 static struct rt6_info *rt6_find_cached_rt(const struct fib6_result *res,
-					   struct in6_addr *daddr,
+					   const struct in6_addr *daddr,
-					   struct in6_addr *saddr);
+					   const struct in6_addr *saddr);
 
 #ifdef CONFIG_IPV6_ROUTE_INFO
 static struct fib6_info *rt6_add_route_info(struct net *net,
@@ -1573,31 +1573,44 @@ out:
  * Caller has to hold rcu_read_lock()
  */
 static struct rt6_info *rt6_find_cached_rt(const struct fib6_result *res,
-					   struct in6_addr *daddr,
+					   const struct in6_addr *daddr,
-					   struct in6_addr *saddr)
+					   const struct in6_addr *saddr)
 {
+	const struct in6_addr *src_key = NULL;
 	struct rt6_exception_bucket *bucket;
-	struct in6_addr *src_key = NULL;
 	struct rt6_exception *rt6_ex;
 	struct rt6_info *ret = NULL;
 
-	bucket = rcu_dereference(res->f6i->rt6i_exception_bucket);
-
 #ifdef CONFIG_IPV6_SUBTREES
 	/* fib6i_src.plen != 0 indicates f6i is in subtree
 	 * and exception table is indexed by a hash of
 	 * both fib6_dst and fib6_src.
-	 * Otherwise, the exception table is indexed by
+	 * However, the src addr used to create the hash
-	 * a hash of only fib6_dst.
+	 * might not be exactly the passed in saddr which
+	 * is a /128 addr from the flow.
+	 * So we need to use f6i->fib6_src to redo lookup
+	 * if the passed in saddr does not find anything.
+	 * (See the logic in ip6_rt_cache_alloc() on how
+	 * rt->rt6i_src is updated.)
 	 */
 	if (res->f6i->fib6_src.plen)
 		src_key = saddr;
+find_ex:
 #endif
+	bucket = rcu_dereference(res->f6i->rt6i_exception_bucket);
 	rt6_ex = __rt6_find_exception_rcu(&bucket, daddr, src_key);
 
 	if (rt6_ex && !rt6_check_expired(rt6_ex->rt6i))
 		ret = rt6_ex->rt6i;
 
+#ifdef CONFIG_IPV6_SUBTREES
+	/* Use fib6_src as src_key and redo lookup */
+	if (!ret && src_key && src_key != &res->f6i->fib6_src.addr) {
+		src_key = &res->f6i->fib6_src.addr;
+		goto find_ex;
+	}
+#endif
+
 	return ret;
 }
 
@@ -2672,12 +2685,10 @@ u32 ip6_mtu_from_fib6(const struct fib6_result *res,
 		      const struct in6_addr *daddr,
 		      const struct in6_addr *saddr)
 {
-	struct rt6_exception_bucket *bucket;
 	const struct fib6_nh *nh = res->nh;
 	struct fib6_info *f6i = res->f6i;
-	const struct in6_addr *src_key;
-	struct rt6_exception *rt6_ex;
 	struct inet6_dev *idev;
+	struct rt6_info *rt;
 	u32 mtu = 0;
 
 	if (unlikely(fib6_metric_locked(f6i, RTAX_MTU))) {
@@ -2686,18 +2697,10 @@ u32 ip6_mtu_from_fib6(const struct fib6_result *res,
 			goto out;
 	}
 
-	src_key = NULL;
+	rt = rt6_find_cached_rt(res, daddr, saddr);
-#ifdef CONFIG_IPV6_SUBTREES
+	if (unlikely(rt)) {
-	if (f6i->fib6_src.plen)
+		mtu = dst_metric_raw(&rt->dst, RTAX_MTU);
-		src_key = saddr;
+	} else {
-#endif
-
-	bucket = rcu_dereference(f6i->rt6i_exception_bucket);
-	rt6_ex = __rt6_find_exception_rcu(&bucket, daddr, src_key);
-	if (rt6_ex && !rt6_check_expired(rt6_ex->rt6i))
-		mtu = dst_metric_raw(&rt6_ex->rt6i->dst, RTAX_MTU);
-
-	if (likely(!mtu)) {
 		struct net_device *dev = nh->fib_nh_dev;
 
 		mtu = IPV6_MIN_MTU;