netfilter: conntrack: add new sysctl to disable RST check

This patch adds a new sysctl tcp_ignore_invalid_rst to disable marking
out of segments RSTs as INVALID.
Signed-off-by: Ali Abdallah <aabdallah@suse.de>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Documentation/networking/nf_conntrack-sysctl.rst

@@ -110,6 +110,12 @@ nf_conntrack_tcp_be_liberal - BOOLEAN
 	Be conservative in what you do, be liberal in what you accept from others.
 	If it's non-zero, we mark only out of window RST segments as INVALID.
 
+nf_conntrack_tcp_ignore_invalid_rst - BOOLEAN
+	- 0 - disabled (default)
+	- 1 - enabled
+
+	If it's 1, we don't mark out of window RST segments as INVALID.
+
 nf_conntrack_tcp_loose - BOOLEAN
 	- 0 - disabled
 	- not 0 - enabled (default)

include/net/netns/conntrack.h

@@ -27,6 +27,7 @@ struct nf_tcp_net {
 	u8 tcp_loose;
 	u8 tcp_be_liberal;
 	u8 tcp_max_retrans;
+	u8 tcp_ignore_invalid_rst;
 #if IS_ENABLED(CONFIG_NF_FLOW_TABLE)
 	unsigned int offload_timeout;
 	unsigned int offload_pickup;

net/netfilter/nf_conntrack_proto_tcp.c

@@ -1068,7 +1068,8 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
 			if (seq == 0 && !nf_conntrack_tcp_established(ct))
 				break;
 
-			if (before(seq, ct->proto.tcp.seen[!dir].td_maxack)) {
+			if (before(seq, ct->proto.tcp.seen[!dir].td_maxack) &&
+			    !tn->tcp_ignore_invalid_rst) {
 				/* Invalid RST  */
 				spin_unlock_bh(&ct->lock);
 				nf_ct_l4proto_log_invalid(skb, ct, state, "invalid rst");
@@ -1466,6 +1467,9 @@ void nf_conntrack_tcp_init_net(struct net *net)
 	 */
 	tn->tcp_be_liberal = 0;
 
+	/* If it's non-zero, we turn off RST sequence number check */
+	tn->tcp_ignore_invalid_rst = 0;
+
 	/* Max number of the retransmitted packets without receiving an (acceptable)
 	 * ACK from the destination. If this number is reached, a shorter timer
 	 * will be started.

net/netfilter/nf_conntrack_standalone.c

@@ -579,6 +579,7 @@ enum nf_ct_sysctl_index {
 #endif
 	NF_SYSCTL_CT_PROTO_TCP_LOOSE,
 	NF_SYSCTL_CT_PROTO_TCP_LIBERAL,
+	NF_SYSCTL_CT_PROTO_TCP_IGNORE_INVALID_RST,
 	NF_SYSCTL_CT_PROTO_TCP_MAX_RETRANS,
 	NF_SYSCTL_CT_PROTO_TIMEOUT_UDP,
 	NF_SYSCTL_CT_PROTO_TIMEOUT_UDP_STREAM,
@@ -798,6 +799,14 @@ static struct ctl_table nf_ct_sysctl_table[] = {
 		.extra1 	= SYSCTL_ZERO,
 		.extra2 	= SYSCTL_ONE,
 	},
+	[NF_SYSCTL_CT_PROTO_TCP_IGNORE_INVALID_RST] = {
+		.procname	= "nf_conntrack_tcp_ignore_invalid_rst",
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 	[NF_SYSCTL_CT_PROTO_TCP_MAX_RETRANS] = {
 		.procname	= "nf_conntrack_tcp_max_retrans",
 		.maxlen		= sizeof(u8),
@@ -1004,6 +1013,7 @@ static void nf_conntrack_standalone_init_tcp_sysctl(struct net *net,
 	XASSIGN(LOOSE, &tn->tcp_loose);
 	XASSIGN(LIBERAL, &tn->tcp_be_liberal);
 	XASSIGN(MAX_RETRANS, &tn->tcp_max_retrans);
+	XASSIGN(IGNORE_INVALID_RST, &tn->tcp_ignore_invalid_rst);
 #undef XASSIGN
 
 #if IS_ENABLED(CONFIG_NF_FLOW_TABLE)