[syzbot] [net?] WARNING in tcf_skbmod_act

Hello,

syzbot found the following issue on:

HEAD commit:    66fb95a52111 Merge tag 'caps-pr-20260820' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=135f6179580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a70bec3546180b6
dashboard link: https://syzkaller.appspot.com/bug?extid=1d56f14f95c0480cfdc9
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/6da0d1c4d90d/disk-66fb95a5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/34b5360f76fd/vmlinux-66fb95a5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/bea190471e9f/bzImage-66fb95a5.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

------------[ cut here ]------------
!skb_transport_header_was_set(skb)
WARNING: ./include/linux/skbuff.h:3243 at skb_network_header_len include/linux/skbuff.h:3243 [inline], CPU#0: syz.0.2516/14949
WARNING: ./include/linux/skbuff.h:3243 at tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55, CPU#0: syz.0.2516/14949
Modules linked in:
CPU: 0 UID: 0 PID: 14949 Comm: syz.0.2516 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:skb_network_header_len include/linux/skbuff.h:3243 [inline]
RIP: 0010:tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55
Code: 48 c1 e8 03 42 80 3c 20 00 74 08 48 89 df e8 6f 05 60 f8 48 8b 03 65 ff 40 10 b8 02 00 00 00 e9 66 ff ff ff e8 99 5b f0 f7 90 <0f> 0b 90 e9 a0 f5 ff ff e8 8b 5b f0 f7 90 0f 0b 90 e9 49 f6 ff ff
RSP: 0018:ffffc90006c571c8 EFLAGS: 00010293
RAX: ffffffff89d6cdf7 RBX: 1ffff1100fa67d56 RCX: ffff888029678000
RDX: 0000000000000000 RSI: 000000000000ffff RDI: 000000000000ffff
RBP: 0000000000000010 R08: ffff888029678000 R09: 0000000000000002
R10: 000000000000dd86 R11: 0000000000000000 R12: 000000000000ffff
R13: ffff88807d33eab6 R14: 000000000000000e R15: ffff88807d33eaba
FS:  00007fecc9f156c0(0000) GS:ffff888124cfd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fecc9f14ff8 CR3: 00000000265a6000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 tc_act include/net/tc_wrapper.h:131 [inline]
 tcf_action_exec+0x185/0x8d0 net/sched/act_api.c:1150
 tcf_exts_exec include/net/pkt_cls.h:361 [inline]
 basic_classify+0x1b2/0x2d0 net/sched/cls_basic.c:54
 tc_classify include/net/tc_wrapper.h:198 [inline]
 __tcf_classify net/sched/cls_api.c:1779 [inline]
 tcf_classify+0x43d/0x1160 net/sched/cls_api.c:1875
 tc_run+0x42d/0x750 net/core/dev.c:4463
 sch_handle_ingress net/core/dev.c:4538 [inline]
 __netif_receive_skb_core+0x1443/0x30c0 net/core/dev.c:6115
 __netif_receive_skb_one_core net/core/dev.c:6262 [inline]
 __netif_receive_skb net/core/dev.c:6377 [inline]
 netif_receive_skb_internal net/core/dev.c:6463 [inline]
 netif_receive_skb+0x1fe/0xbf0 net/core/dev.c:6522
 tun_rx_batched+0x1de/0x790 drivers/net/tun.c:1571
 tun_get_user+0x2be0/0x44d0 drivers/net/tun.c:2045
 tun_chr_write_iter+0x113/0x200 drivers/net/tun.c:2091
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x612/0xba0 fs/read_write.c:687
 ksys_write+0x150/0x270 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fecc8f5e90e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007fecc9f14fb8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fecc9f156c0 RCX: 00007fecc8f5e90e
RDX: 0000000000000097 RSI: 0000200000000080 RDI: 00000000000000c8
RBP: 00007fecc9035024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fecc9226038 R14: 00007fecc9225fa0 R15: 00007fecc934fa48
 </TASK>


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