[NETFILTER] ctnetlink: allow userspace to change TCP state

This patch adds the ability of changing the state a TCP connection. I know
that this must be used with care but it's required to provide a complete
conntrack creation via conntrack_netlink. So I'll document this aspect on
the upcoming docs.

Signed-off-by: Pablo Neira Ayuso <[e-mail]>
Signed-off-by: Harald Welte <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netfilter_ipv4/ip_conntrack_protocol.h

···
 	int (*to_nfattr)(struct sk_buff *skb, struct nfattr *nfa,
 			 const struct ip_conntrack *ct);
 
+	/* convert nfnetlink attributes to protoinfo */
+	int (*from_nfattr)(struct nfattr *tb[], struct ip_conntrack *ct);
+
 	int (*tuple_to_nfattr)(struct sk_buff *skb,
 			       const struct ip_conntrack_tuple *t);
 	int (*nfattr_to_tuple)(struct nfattr *tb[],

net/ipv4/netfilter/ip_conntrack_proto_tcp.c

···
 	read_unlock_bh(&tcp_lock);
 	return -1;
 }
+
+static int nfattr_to_tcp(struct nfattr *cda[], struct ip_conntrack *ct)
+{
+	struct nfattr *attr = cda[CTA_PROTOINFO_TCP-1];
+	struct nfattr *tb[CTA_PROTOINFO_TCP_MAX];
+
+        if (nfattr_parse_nested(tb, CTA_PROTOINFO_TCP_MAX, attr) < 0)
+                goto nfattr_failure;
+
+	if (!tb[CTA_PROTOINFO_TCP_STATE-1])
+		return -EINVAL;
+
+	write_lock_bh(&tcp_lock);
+	ct->proto.tcp.state = 
+		*(u_int8_t *)NFA_DATA(tb[CTA_PROTOINFO_TCP_STATE-1]);
+	write_unlock_bh(&tcp_lock);
+
+	return 0;
+
+nfattr_failure:
+	return -1;
+}
 #endif
 
 static unsigned int get_conntrack_index(const struct tcphdr *tcph)
···
 #if defined(CONFIG_IP_NF_CONNTRACK_NETLINK) || \
     defined(CONFIG_IP_NF_CONNTRACK_NETLINK_MODULE)
 	.to_nfattr		= tcp_to_nfattr,
+	.from_nfattr		= nfattr_to_tcp,
 	.tuple_to_nfattr	= ip_ct_port_tuple_to_nfattr,
 	.nfattr_to_tuple	= ip_ct_port_nfattr_to_tuple,
 #endif