ip_forward: Drop frames with attached skb->sk

Initial discussion was:
[FYI] xfrm: Don't lookup sk_policy for timewait sockets

Forwarded frames should not have a socket attached. Especially tw sockets will lead to panics later-on in the stack.

This was observed with TPROXY assigning a tw socket and broken policy routing (misconfigured). As a result frame enters forwarding path instead of input. We cannot solve this in TPROXY as it cannot know that policy routing is broken.

v2:
Remove useless comment

---

index 939992c..3674484 100644
@@ -82,6 +82,9 @@ int ip_forward(struct sk_buff *skb)
 	if (skb->pkt_type != PACKET_HOST)
 		goto drop;
 
+	if (unlikely(skb->sk))
+		goto drop;
+
 	if (skb_warn_if_lro(skb))
 		goto drop;
 

--

From: Sebastian Poehn <[email protected]>
Date: Mon, 20 Apr 2015 09:19:20 +0200

Applied and queued up for -stable, thanks Sebastian.