sit: Fail to create tunnel, if it already exists

When locating the tunnel, do not continue if it is found. Otherwise
a different tunnel with similar configuration would be returned and
parts could be overwritten.
Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -165,9 +165,14 @@ static struct ip_tunnel * ipip6_tunnel_locate(struct net *net,
 	struct sit_net *sitn = net_generic(net, sit_net_id);
 
 	for (tp = __ipip6_bucket(sitn, parms); (t = *tp) != NULL; tp = &t->next) {
-		if (local == t->parms.iph.saddr && remote == t->parms.iph.daddr)
+		if (local == t->parms.iph.saddr &&
+		    remote == t->parms.iph.daddr) {
+			if (create)
+				return NULL;
+			else
 				return t;
 		}
+	}
 	if (!create)
 		goto failed;