Revert "kernel: remove obsolete netfilter tcp window size check bypass patch"

This reverts commit c2331038b2.

target/linux/generic/pending-5.10/613-netfilter_optional_tcp_window_check.patch

@@ -0,0 +1,73 @@
+From: Felix Fietkau <nbd@nbd.name>
+Subject: netfilter: optional tcp window check
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+ net/netfilter/nf_conntrack_proto_tcp.c | 13 +++++++++++++
+ 1 file changed, 13 insertions(+)
+
+--- a/net/netfilter/nf_conntrack_proto_tcp.c
++++ b/net/netfilter/nf_conntrack_proto_tcp.c
+@@ -31,6 +31,9 @@
+ #include <net/netfilter/ipv4/nf_conntrack_ipv4.h>
+ #include <net/netfilter/ipv6/nf_conntrack_ipv6.h>
+ 
++/* Do not check the TCP window for incoming packets  */
++static int nf_ct_tcp_no_window_check __read_mostly = 1;
++
+ /* "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. */
+@@ -476,6 +479,9 @@ static bool tcp_in_window(const struct n
+ 	s32 receiver_offset;
+ 	bool res, in_recv_win;
+ 
++	if (nf_ct_tcp_no_window_check)
++		return true;
++
+ 	/*
+ 	 * Get the required data from the packet.
+ 	 */
+@@ -1139,7 +1145,7 @@ int nf_conntrack_tcp_packet(struct nf_co
+ 		 IP_CT_TCP_FLAG_DATA_UNACKNOWLEDGED &&
+ 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_UNACK])
+ 		timeout = timeouts[TCP_CONNTRACK_UNACK];
+-	else if (ct->proto.tcp.last_win == 0 &&
++	else if (!nf_ct_tcp_no_window_check && ct->proto.tcp.last_win == 0 &&
+ 		 timeouts[new_state] > timeouts[TCP_CONNTRACK_RETRANS])
+ 		timeout = timeouts[TCP_CONNTRACK_RETRANS];
+ 	else
+--- a/net/netfilter/nf_conntrack_standalone.c
++++ b/net/netfilter/nf_conntrack_standalone.c
+@@ -25,6 +25,9 @@
+ #include <net/netfilter/nf_conntrack_timestamp.h>
+ #include <linux/rculist_nulls.h>
+ 
++/* Do not check the TCP window for incoming packets  */
++static int nf_ct_tcp_no_window_check __read_mostly = 1;
++
+ static bool enable_hooks __read_mostly;
+ MODULE_PARM_DESC(enable_hooks, "Always enable conntrack hooks");
+ module_param(enable_hooks, bool, 0000);
+@@ -657,6 +660,7 @@ enum nf_ct_sysctl_index {
+ 	NF_SYSCTL_CT_PROTO_TIMEOUT_GRE_STREAM,
+ #endif
+ 
++	NF_SYSCTL_CT_PROTO_TCP_NO_WINDOW_CHECK,
+ 	__NF_SYSCTL_CT_LAST_SYSCTL,
+ };
+ 
+@@ -993,6 +997,13 @@ static struct ctl_table nf_ct_sysctl_tab
+ 		.proc_handler   = proc_dointvec_jiffies,
+ 	},
+ #endif
++	[NF_SYSCTL_CT_PROTO_TCP_NO_WINDOW_CHECK] = {
++		.procname       = "nf_conntrack_tcp_no_window_check",
++		.data           = &nf_ct_tcp_no_window_check,
++		.maxlen         = sizeof(unsigned int),
++		.mode           = 0644,
++		.proc_handler   = proc_dointvec,
++	},
+ 	{}
+ };
+