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: 2b760fcf5cfb ("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>

1 files changed, 27 insertions, 24 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 27c0cc5d9d30..7a014ca877ed 100644
--- a/net/ipv6/route.c
+++ b/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,
-                                           struct in6_addr *saddr);
+                                           const struct in6_addr *daddr,
+                                           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,
-                                           struct in6_addr *saddr)
+                                           const struct in6_addr *daddr,
+                                           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
-         * a hash of only fib6_dst.
+         * However, the src addr used to create the hash
+         * 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;
-#ifdef CONFIG_IPV6_SUBTREES
-        if (f6i->fib6_src.plen)
-                src_key = saddr;
-#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)) {
+        rt = rt6_find_cached_rt(res, daddr, saddr);
+        if (unlikely(rt)) {
+                mtu = dst_metric_raw(&rt->dst, RTAX_MTU);
+        } else {
                 struct net_device *dev = nh->fib_nh_dev;
 
                 mtu = IPV6_MIN_MTU;