[PATCH] ip_forward: Drop frames with attached skb->sk

[Sebastian Poehn <sebastian.poehn@gmail.com>]

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.

Signed-off-by: Sebastian Poehn <sebastian.poehn@gmail.com>
---
diff --git a/net/ipv4/ip_forward.c b/net/ipv4/ip_forward.c
index 939992c..2fc3b3e 100644
--- a/net/ipv4/ip_forward.c
+++ b/net/ipv4/ip_forward.c
@@ -82,6 +82,10 @@ int ip_forward(struct sk_buff *skb)
 	if (skb->pkt_type != PACKET_HOST)
 		goto drop;
 
+	/* this should happen neither */
+	if (unlikely(skb->sk))
+		goto drop;
+
 	if (skb_warn_if_lro(skb))
 		goto drop;
--