tcp: add a missing nf_reset_ct() in 3WHS handling

When the third packet of 3WHS connection establishment
contains payload, it is added into socket receive queue
without the XFRM check and the drop of connection tracking
context.

This means that if the data is left unread in the socket
receive queue, conntrack module can not be unloaded.

As most applications usually reads the incoming data
immediately after accept(), bug has been hiding for
quite a long time.

Commit 68822bd ("net: generalize skb freeing
deferral to per-cpu lists") exposed this bug because
even if the application reads this data, the skb
with nfct state could stay in a per-cpu cache for
an arbitrary time, if said cpu no longer process RX softirqs.

Many thanks to Ilya Maximets for reporting this issue,
and for testing various patches:
https://lore.kernel.org/netdev/[email protected]/

Note that I also added a missing xfrm4_policy_check() call,
although this is probably not a big issue, as the SYN
packet should have been dropped earlier.

Fixes: b59c270 ("[NETFILTER]: Keep conntrack reference until IPsec policy checks are done")
Reported-by: Ilya Maximets <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Florian Westphal <[email protected]>
Cc: Pablo Neira Ayuso <[email protected]>
Cc: Steffen Klassert <[email protected]>
Tested-by: Ilya Maximets <[email protected]>
Reviewed-by: Ilya Maximets <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/ipv4/tcp_ipv4.c

@@ -1964,7 +1964,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 		struct sock *nsk;
 
 		sk = req->rsk_listener;
-		drop_reason = tcp_inbound_md5_hash(sk, skb,
+		if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
+			drop_reason = SKB_DROP_REASON_XFRM_POLICY;
+		else
+			drop_reason = tcp_inbound_md5_hash(sk, skb,
 						   &iph->saddr, &iph->daddr,
 						   AF_INET, dif, sdif);
 		if (unlikely(drop_reason)) {
@@ -2016,6 +2019,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 			}
 			goto discard_and_relse;
 		}
+		nf_reset_ct(skb);
 		if (nsk == sk) {
 			reqsk_put(req);
 			tcp_v4_restore_cb(skb);