flow_dissector: do not dissect l4 ports for fragments

From: 배석진 <[email protected]>

Only first fragment has the sport/dport information, not the following ones.

If we want consistent hash for all fragments, we need to ignore ports even for first fragment.

This bug is visible for IPv6 traffic, if incoming fragments do not have a flow label, since skb_get_hash() will give different results for first fragment and following ones.

It is also visible if any routing rule wants dissection and sport or dport.

See commit 5e5d6fed3741 ("ipv6: route: dissect flow in input path if fib rules need it") for details.

[edumazet] rewrote the changelog completely.

---
 net/core/flow_dissector.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 676f3ad629f95625422aa55f0f54157001ac477c..588f475019d47c9d6bae8883acebab48aaf63b48 100644
@@ -1166,8 +1166,8 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 		break;
 	}
 
-	if (dissector_uses_key(flow_dissector,
-			       FLOW_DISSECTOR_KEY_PORTS)) {
+	if (dissector_uses_key(flow_dissector, FLOW_DISSECTOR_KEY_PORTS) &&
+	    !(key_control->flags & FLOW_DIS_IS_FRAGMENT)) {
 		key_ports = skb_flow_dissector_target(flow_dissector,
 						      FLOW_DISSECTOR_KEY_PORTS,
 						      target_container);

2.19.1.930.g4563a0d9d0-goog

From: Eric Dumazet <[email protected]>
Date: Fri, 9 Nov 2018 16:53:06 -0800

Applied and queued up for -stable.