netfilter: Fix handling simultaneous open in TCP conntrack

Dominique Martinet reported a TCP hang problem when simultaneous open was used.
The problem is that the tcp_conntracks state table is not smart enough
to handle the case. The state table could be fixed by introducing a new state,
but that would require more lines of code compared to this patch, due to the
required backward compatibility with ctnetlink.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Reported-by: Dominique Martinet <asmadeus@codewreck.org>
Tested-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_conntrack_tcp.h

@@ -46,6 +46,9 @@ enum tcp_conntrack {
 /* Marks possibility for expected RFC5961 challenge ACK */
 #define IP_CT_EXP_CHALLENGE_ACK 		0x40
 
+/* Simultaneous open initialized */
+#define IP_CT_TCP_SIMULTANEOUS_OPEN		0x80
+
 struct nf_ct_tcp_flags {
 	__u8 flags;
 	__u8 mask;