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Date:   Tue, 14 May 2019 17:46:10 -0700
From:   Wei Wang <>
To:     David Miller <>,
Cc:     Martin KaFai Lau <>, Wei Wang <>,
        Mikael Magnusson <>,
        David Ahern <>,
        Eric Dumazet <>
Subject: [PATCH net] ipv6: fix src addr routing with the exception table

From: Wei Wang <>

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 <>
Bisected-by: David Ahern <>
Signed-off-by: Wei Wang <>
Acked-by: Eric Dumazet <>
 net/ipv6/route.c | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 23a20d62daac..c36900a07a78 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1574,23 +1574,36 @@ static struct rt6_info *rt6_find_cached_rt(const struct fib6_result *res,
 	struct rt6_exception *rt6_ex;
 	struct rt6_info *ret = NULL;
-	bucket = rcu_dereference(res->f6i->rt6i_exception_bucket);
 	/* 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;
+	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;
+	/* Use fib6_src as src_key and redo lookup */
+	if (!ret && src_key == saddr) {
+		src_key = &res->f6i->fib6_src.addr;
+		goto find_ex;
+	}
 	return ret;
@@ -2683,12 +2696,22 @@ u32 ip6_mtu_from_fib6(const struct fib6_result *res,
 	if (f6i->fib6_src.plen)
 		src_key = saddr;
 	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);
+	/* Similar logic as in rt6_find_cached_rt().
+	 * We need to use f6i->fib6_src to redo lookup in exception
+	 * table if saddr did not yield any result.
+	 */
+	else if (src_key == saddr) {
+		src_key = &f6i->fib6_src.addr;
+		goto find_ex;
+	}
 	if (likely(!mtu)) {
 		struct net_device *dev = nh->fib_nh_dev;

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