FAILED: patch "[PATCH] net/sched: act_ct: preserve tc_skb_cb across defragmentation" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 9092e15defbe6c7bc241c306093ca9d358a578e7
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026072019-provoking-acutely-210b@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9092e15defbe6c7bc241c306093ca9d358a578e7 Mon Sep 17 00:00:00 2001
From: Zihan Xi <xizh2024@lzu.edu.cn>
Date: Sun, 14 Jun 2026 01:42:39 +0800
Subject: [PATCH] net/sched: act_ct: preserve tc_skb_cb across defragmentation

tcf_ct_handle_fragments() calls nf_ct_handle_fragments() without saving
and restoring skb->cb. The defrag helper clears IPCB/IP6CB, which aliases
the tc_skb_cb/qdisc_skb_cb control buffer. Fragmented traffic through
act_ct therefore loses qdisc metadata such as pkt_segs and can trigger
WARN_ON_ONCE() in qdisc_pkt_segs() when panic_on_warn is enabled.

Save and restore the full tc_skb_cb around nf_ct_handle_fragments(),
matching the pattern used by ovs_ct_handle_fragments().

Fixes: ec624fe740b4 ("net/sched: Extend qdisc control block with tc control block")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Zihan Xi <xizh2024@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Link: https://patch.msgid.link/510c51217fd7aaf29c6dc298bab8d643fe229b1c.1781358692.git.xizh2024@lzu.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 6158e13c98d3..d47a82f9ac6c 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -844,11 +844,11 @@ static int tcf_ct_handle_fragments(struct net *net, struct sk_buff *skb,
 				   u8 family, u16 zone, bool *defrag)
 {
 	enum ip_conntrack_info ctinfo;
+	struct tc_skb_cb cb;
 	struct nf_conn *ct;
 	int err = 0;
 	bool frag;
 	u8 proto;
-	u16 mru;
 
 	/* Previously seen (loopback)? Ignore. */
 	ct = nf_ct_get(skb, &ctinfo);
@@ -862,12 +862,13 @@ static int tcf_ct_handle_fragments(struct net *net, struct sk_buff *skb,
 	if (err || !frag)
 		return err;
 
-	err = nf_ct_handle_fragments(net, skb, zone, family, &proto, &mru);
+	cb = *tc_skb_cb(skb);
+	err = nf_ct_handle_fragments(net, skb, zone, family, &proto, &cb.mru);
 	if (err)
 		return err;
 
 	*defrag = true;
-	tc_skb_cb(skb)->mru = mru;
+	*tc_skb_cb(skb) = cb;
 
 	return 0;
 }