ipv4: fix a race in update_or_create_fnhe()

nh_exceptions is effectively used under rcu, but lacks proper
barriers. Between kzalloc() and setting of nh->nh_exceptions(),
we need a proper memory barrier.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 4895c771c7f00 ("ipv4: Add FIB nexthop exceptions.")
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index b10cd43a..5b6efb3 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -157,9 +157,12 @@
 
 static void free_nh_exceptions(struct fib_nh *nh)
 {
-	struct fnhe_hash_bucket *hash = nh->nh_exceptions;
+	struct fnhe_hash_bucket *hash;
 	int i;
 
+	hash = rcu_dereference_protected(nh->nh_exceptions, 1);
+	if (!hash)
+		return;
 	for (i = 0; i < FNHE_HASH_SIZE; i++) {
 		struct fib_nh_exception *fnhe;
 
@@ -205,8 +208,7 @@
 	change_nexthops(fi) {
 		if (nexthop_nh->nh_dev)
 			dev_put(nexthop_nh->nh_dev);
-		if (nexthop_nh->nh_exceptions)
-			free_nh_exceptions(nexthop_nh);
+		free_nh_exceptions(nexthop_nh);
 		rt_fibinfo_free_cpus(nexthop_nh->nh_pcpu_rth_output);
 		rt_fibinfo_free(&nexthop_nh->nh_rth_input);
 	} endfor_nexthops(fi);