kernel: fix WED offloaded flow timeout

Add a pending patch to fix the WED flow timeout.

OpenWrt has recently migrated many platforms to kernel 6.18. On the
MediaTek platform, which supports hardware network offloading, WiFi
connections accelerated via the WED path were observed to drop after
roughly 300 seconds.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: LGA1150

target/linux/generic/backport-6.18/704-v7.2-netfilter-flowtable-fix-offloaded-ct-timeout-never-b.patch

@@ -0,0 +1,76 @@
+From 53b3e60edb674b442b2b3bbdba484667b0f47a5d Mon Sep 17 00:00:00 2001
+From: Adrian Bente <adibente@gmail.com>
+Date: Thu, 28 May 2026 10:08:51 +0300
+Subject: [PATCH] netfilter: flowtable: fix offloaded ct timeout never being extended
+
+OpenWrt has recently migrated many platforms to kernel 6.18. On the
+MediaTek platform, which supports hardware network offloading, WiFi
+connections accelerated via the WED path were observed to drop after
+roughly 300 seconds.
+
+After several debugging sessions, assisted by the Claude LLM, the
+problem was narrowed down as follows:
+
+nf_flow_table_extend_ct_timeout() extends ct->timeout for offloaded
+flows using:
+
+	cmpxchg(&ct->timeout, expires, new_timeout);
+
+'expires' comes from nf_ct_expires(ct) and is a relative value, while
+ct->timeout holds an absolute timestamp. The two are never equal, so
+the cmpxchg always fails and the timeout is never extended.
+
+This goes unnoticed for most flows, but a long-lived hardware (WED)
+offloaded flow on MediaTek MT7986 eventually has ct->timeout decay to
+zero, the conntrack entry is reaped and the connection breaks.
+
+Open-code the relative value from a single READ_ONCE(ct->timeout)
+snapshot and compare against that same absolute snapshot in the
+cmpxchg, so the timeout extension actually takes effect while the
+datapath remains authoritative if it updates ct->timeout concurrently.
+
+Fixes: 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn timeout extension logic")
+Cc: stable@vger.kernel.org
+Suggested-by: Florian Westphal <fw@strlen.de>
+Signed-off-by: Adrian Bente <adibente@gmail.com>
+Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
+---
+ net/netfilter/nf_flow_table_core.c | 13 +++++++++----
+ 1 file changed, 9 insertions(+), 4 deletions(-)
+
+--- a/net/netfilter/nf_flow_table_core.c
++++ b/net/netfilter/nf_flow_table_core.c
+@@ -500,8 +500,13 @@ static u32 nf_flow_table_tcp_timeout(con
+  */
+ static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
+ {
+-	static const u32 min_timeout = 5 * 60 * HZ;
+-	u32 expires = nf_ct_expires(ct);
++	static const s32 min_timeout = 5 * 60 * HZ;
++	u32 ct_timeout = READ_ONCE(ct->timeout);
++	s32 expires;
++
++	expires = ct_timeout - nfct_time_stamp;
++	if (expires <= 0) /* already expired */
++		return;
+ 
+ 	/* normal case: large enough timeout, nothing to do. */
+ 	if (likely(expires >= min_timeout))
+@@ -519,7 +524,7 @@ static void nf_flow_table_extend_ct_time
+ 	if (nf_ct_is_confirmed(ct) &&
+ 	    test_bit(IPS_OFFLOAD_BIT, &ct->status)) {
+ 		u8 l4proto = nf_ct_protonum(ct);
+-		u32 new_timeout = true;
++		u32 new_timeout = 1;
+ 
+ 		switch (l4proto) {
+ 		case IPPROTO_UDP:
+@@ -544,7 +549,7 @@ static void nf_flow_table_extend_ct_time
+ 		 */
+ 		if (new_timeout) {
+ 			new_timeout += nfct_time_stamp;
+-			cmpxchg(&ct->timeout, expires, new_timeout);
++			cmpxchg(&ct->timeout, ct_timeout, new_timeout);
+ 		}
+ 	}
+