ip6_tunnel: disable caching when the traffic class is inherited

[ Upstream commit b5c2d495 ]

If an ip6 tunnel is configured to inherit the traffic class from
the inner header, the dst_cache must be disabled or it will foul
the policy routing.

The issue is apprently there since at leat Linux-2.6.12-rc2.
Reported-by: Liam McBirnie <liam.mcbirnie@boeing.com>
Cc: Liam McBirnie <liam.mcbirnie@boeing.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/ipv6/ip6_tunnel.c

@@ -894,13 +894,22 @@ static int ip6_tnl_xmit2(struct sk_buff *skb,
 	struct ipv6_tel_txoption opt;
 	struct dst_entry *dst = NULL, *ndst = NULL;
 	struct net_device *tdev;
+	bool use_cache = false;
 	int mtu;
 	unsigned int max_headroom = sizeof(struct ipv6hdr);
 	u8 proto;
 	int err = -1;
 	int pkt_len;
 
-	if (!fl6->flowi6_mark)
+	if (!(t->parms.flags &
+		     (IP6_TNL_F_USE_ORIG_TCLASS | IP6_TNL_F_USE_ORIG_FWMARK))) {
+		/* enable the cache only only if the routing decision does
+		 * not depend on the current inner header value
+		 */
+		use_cache = true;
+	}
+
+	if (use_cache)
 		dst = ip6_tnl_dst_check(t);
 	if (!dst) {
 		ndst = ip6_route_output(net, NULL, fl6);
@@ -959,7 +968,7 @@ static int ip6_tnl_xmit2(struct sk_buff *skb,
 		skb = new_skb;
 	}
 	skb_dst_drop(skb);
-	if (fl6->flowi6_mark) {
+	if (!use_cache) {
 		skb_dst_set(skb, dst);
 		ndst = NULL;
 	} else {