[PATCH net v2 0/2] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[PATCH net v2 0/2] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[Ren Wei]

From: Zihan Xi <xizh2024@lzu.edu.cn>

Hi Linux kernel maintainers,

We found and validated an issue in net/sched/act_ct.c. The bug is
reachable when configuring TC with act_ct on a netdev (requires
CAP_NET_ADMIN). We have tested it, and the fix should not affect
other functionality.

We provide bug details, a PoC, and a crash log below.

v2 adds a tc-testing (TDC) selftest case in patch 2, per maintainer
feedback.

---- details below ----

Bug details:

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 in
include/net/sch_generic.h. Fragmented traffic through act_ct
therefore loses qdisc metadata such as pkt_segs.

Later qdisc dequeue paths call qdisc_bstats_update() ->
qdisc_pkt_segs(). For a non-GSO skb, clobbered pkt_segs == 0 trips
DEBUG_NET_WARN_ON_ONCE() in qdisc_pkt_segs(). With panic_on_warn=1
the kernel panics.

Unlike ovs_ct_handle_fragments() in net/openvswitch/conntrack.c, the
act_ct caller only restored mru after defrag, not the full control
buffer. The attached patch saves and restores struct tc_skb_cb around
nf_ct_handle_fragments(), matching the OVS pattern.

Reproducer:

Run as root in the guest (QEMU bullseye image, eth0):

    chmod +x ./poc.sh
    ./poc.sh eth0 10.0.2.2 100

The script installs a root prio qdisc, clsact egress with "action ct",
then sends oversized UDP datagrams with PMTUD disabled to force IPv4
fragmentation through the act_ct defrag path.

We run the PoC in a 2 vCPU, 2 GB RAM x86 QEMU environment.

------BEGIN poc.sh------

#!/bin/sh
set -eu

IFACE="${1:-eth0}"
DST="${2:-10.0.2.2}"
COUNT="${3:-100}"

sysctl -w kernel.panic_on_warn=1 >/dev/null

tc qdisc del dev "$IFACE" clsact 2>/dev/null || true
tc qdisc del dev "$IFACE" root 2>/dev/null || true

tc qdisc add dev "$IFACE" root handle 1: prio
tc qdisc add dev "$IFACE" clsact
tc filter add dev "$IFACE" egress protocol ip pref 1 u32 \
	match u32 0 0 action ct zone 1 pipe

python3 - "$DST" "$COUNT" <<'PY'
import socket
import sys
import time

dst = sys.argv[1]
count = int(sys.argv[2])

IP_MTU_DISCOVER = 10
IP_PMTUDISC_DONT = 0

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DONT)
payload = b"A" * 4000

for _ in range(count):
    s.sendto(payload, (dst, 9))
    time.sleep(0.01)
PY

------END poc.sh------

----BEGIN crash log----

[  549.900801][T10210] Kernel panic - not syncing: kernel: panic_on_warn set ...
[  549.901406][T10210] CPU: 2 UID: 0 PID: 10210 Comm: python3 Not tainted 7.1.0-rc1 #2 PREEMPT(full)
[  549.902720][T10210] Call Trace:
[  549.903756][T10210]  ? qdisc_dequeue_head+0x287/0x370
[  549.904713][T10210]  check_panic_on_warn+0x61/0x80
[  549.905053][T10210]  __warn+0xe8/0x330
[  549.905345][T10210]  ? qdisc_dequeue_head+0x287/0x370
[  549.909442][T10210] RIP: 0010:qdisc_dequeue_head+0x287/0x370
[  549.914217][T10210]  prio_dequeue+0x40c/0x6a0
[  549.914539][T10210]  __qdisc_run+0x170/0x1b30
[  549.915561][T10210]  __dev_queue_xmit+0x25e6/0x3ac0
[  549.920352][T10210]  ip_do_fragment+0x1188/0x19a0
[  549.924214][T10210]  udp_send_skb+0x885/0x1270
[  549.924556][T10210]  udp_sendmsg+0x13f3/0x20a0

-----END crash log-----

Best regards,
Zihan Xi

Zihan Xi (2):
  net/sched: act_ct: preserve tc_skb_cb across defragmentation
  selftests/tc-testing: act_ct: add TDC test for skb cb preservation
    across defrag

 net/sched/act_ct.c                            |  7 ++--
 .../tc-testing/tc-tests/actions/ct.json       | 38 +++++++++++++++++++
 2 files changed, 42 insertions(+), 3 deletions(-)

-- 
2.43.0

[PATCH net v2 1/2] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[Ren Wei]

From: Zihan Xi <xizh2024@lzu.edu.cn>

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>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zihan Xi <xizh2024@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
---
changes in v2:
  - Add TDC selftest in patch 2 per maintainer feedback
  - v1 Link: https://lore.kernel.org/all/20260611154939.2615919-1-n05ec@lzu.edu.cn/
 net/sched/act_ct.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 6158e13c98d3..ebd40daf05a6 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -845,10 +845,10 @@ static int tcf_ct_handle_fragments(struct net *net, struct sk_buff *skb,
 {
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct;
+	struct tc_skb_cb cb;
 	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;
 }
-- 
2.43.0

[PATCH net v2 2/2] selftests/tc-testing: act_ct: add TDC test for skb cb preservation across defrag

[Ren Wei]

From: Zihan Xi <xizh2024@lzu.edu.cn>

Add a tc-testing case that sends IPv4 fragments through act_ct on clsact
egress while a root prio qdisc is present on the transmit path.

The test verifies that packet processing and qdisc accounting continue
to work after conntrack defragmentation, covering tc_skb_cb preservation
across defragmentation.

Signed-off-by: Zihan Xi <xizh2024@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>

---
changes in v2:
  - Add tc-testing case 9c2a for skb cb preservation across defrag
  - v1 Link: https://lore.kernel.org/all/20260611154939.2615919-1-n05ec@lzu.edu.cn/
 .../tc-testing/tc-tests/actions/ct.json       | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/tools/testing/selftests/tc-testing/tc-tests/actions/ct.json b/tools/testing/selftests/tc-testing/tc-tests/actions/ct.json
index 33bb8f3ff8ed..da65f838bd52 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/actions/ct.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/actions/ct.json
@@ -664,5 +664,43 @@
         "teardown": [
             "$TC qdisc del dev $DEV1 ingress_block 21 clsact"
         ]
+    },
+    {
+        "id": "9c2a",
+        "name": "Act_ct preserves skb cb across defrag before prio dequeue",
+        "category": [
+            "actions",
+            "ct",
+            "scapy"
+        ],
+        "plugins": {
+            "requires": [
+                "nsPlugin",
+                "scapyPlugin"
+            ]
+        },
+        "setup": [
+            "$TC qdisc add dev $DUMMY root handle 1: prio",
+            "$TC qdisc add dev $DUMMY clsact",
+            "$TC qdisc add dev $DEV1 clsact",
+            "$TC filter add dev $DEV1 ingress protocol ip prio 1 matchall action mirred egress redirect dev $DUMMY"
+        ],
+        "cmdUnderTest": "$TC filter add dev $DUMMY egress protocol ip prio 1 matchall action ct zone 1 pipe",
+        "scapy": [
+            {
+                "iface": "$DEV0",
+                "count": 1,
+                "packet": "[Ether()/frag for frag in fragment(IP(src='10.0.0.10', dst='10.0.0.1', id=1)/UDP(sport=12345, dport=9)/Raw(b'A' * 4000), fragsize=1400)]"
+            }
+        ],
+        "expExitCode": "0",
+        "verifyCmd": "$TC -s qdisc show dev $DUMMY | grep -A 1 '^qdisc prio 1:'",
+        "matchPattern": "Sent [1-9][0-9]* bytes [1-9][0-9]* pkt",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $DEV1 clsact",
+            "$TC qdisc del dev $DUMMY clsact",
+            "$TC qdisc del dev $DUMMY root handle 1:"
+        ]
     }
 ]
-- 
2.43.0

Re: [PATCH net v2 0/2] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Sun, 14 Jun 2026 01:42:38 +0800 you wrote:
> From: Zihan Xi <xizh2024@lzu.edu.cn>
> 
> Hi Linux kernel maintainers,
> 
> We found and validated an issue in net/sched/act_ct.c. The bug is
> reachable when configuring TC with act_ct on a netdev (requires
> CAP_NET_ADMIN). We have tested it, and the fix should not affect
> other functionality.
> 
> [...]

Here is the summary with links:
  - [net,v2,1/2] net/sched: act_ct: preserve tc_skb_cb across defragmentation
    https://git.kernel.org/netdev/net/c/9092e15defbe
  - [net,v2,2/2] selftests/tc-testing: act_ct: add TDC test for skb cb preservation across defrag
    https://git.kernel.org/netdev/net/c/1fd8f80a199d

You are awesome, thank you!
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