[syzbot] [netfilter?] WARNING in nf_flow_offload_ip_hook

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 075b74841bd0 Linux 7.2-rc6
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=13a8a649580000
kernel config: https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=76d4e3a055aec3b007ec
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14725db9580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12581c9e580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1dc04730d019/disk-075b7484.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d76da42271f4/vmlinux-075b7484.xz
kernel image: https://storage.googleapis.com/syzbot-assets/07b1cdbcbe45/bzImage-075b7484.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+76d4e3...@syzkaller.appspotmail.com

------------[ cut here ]------------
(skb->_skb_refdst & ~(1UL)) && !(skb->_skb_refdst & 1UL)
WARNING: ./include/linux/skbuff.h:1170 at skb_dst_check_unset include/linux/skbuff.h:1170 [inline], CPU#0: syz-executor103/5623
WARNING: ./include/linux/skbuff.h:1170 at skb_dst_set_noref include/linux/skbuff.h:1234 [inline], CPU#0: syz-executor103/5623
WARNING: ./include/linux/skbuff.h:1170 at nf_flow_offload_ip_hook+0xf6c/0x2b60 net/netfilter/nf_flow_table_ip.c:864, CPU#0: syz-executor103/5623
Modules linked in:
CPU: 0 UID: 0 PID: 5623 Comm: syz-executor103 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:skb_dst_check_unset include/linux/skbuff.h:1170 [inline]
RIP: 0010:skb_dst_set_noref include/linux/skbuff.h:1234 [inline]
RIP: 0010:nf_flow_offload_ip_hook+0xf6c/0x2b60 net/netfilter/nf_flow_table_ip.c:864
Code: 89 ee e8 87 4b 14 f8 48 83 fd 01 76 20 e8 dc 50 14 f8 83 e5 01 31 ff 48 89 ee e8 6f 4b 14 f8 48 85 ed 75 09 e8 c5 50 14 f8 90 <0f> 0b 90 e8 bc 50 14 f8 e8 e7 73 f7 f7 31 ff 89 c5 89 c6 e8 1c 4b
RSP: 0018:ffffc90000007798 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff89f61151
RDX: ffff88807cee8000 RSI: ffffffff89f6115b RDI: ffff88807cee8000
RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88806f5d0558
R13: ffff88807a526900 R14: ffff8880291401c0 R15: ffff88807a526900
FS: 00005555816fc400(0000) GS:ffff888123dd8000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000006a1b90 CR3: 000000007029a000 CR4: 00000000003526f0
Call Trace:
<IRQ>
nf_hook_entry_hookfn include/linux/netfilter.h:158 [inline]
nf_hook_slow+0xbf/0x220 net/netfilter/core.c:619
nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
nf_ingress net/core/dev.c:5975 [inline]
__netif_receive_skb_core.constprop.0+0x1689/0x3530 net/core/dev.c:6071
__netif_receive_skb_one_core+0xb0/0x1e0 net/core/dev.c:6210
__netif_receive_skb+0x1f/0x120 net/core/dev.c:6325
process_backlog+0x37a/0x1580 net/core/dev.c:6676
__napi_poll.constprop.0+0xaf/0x450 net/core/dev.c:7735
napi_poll net/core/dev.c:7798 [inline]
net_rx_action+0xa40/0xf20 net/core/dev.c:7955
handle_softirqs+0x1ea/0x9b0 kernel/softirq.c:622
do_softirq kernel/softirq.c:523 [inline]
do_softirq+0xac/0xe0 kernel/softirq.c:510
</IRQ>
<TASK>
__local_bh_enable_ip+0xf8/0x120 kernel/softirq.c:450
local_bh_enable include/linux/bottom_half.h:33 [inline]
tun_rx_batched.isra.0+0x407/0x750 drivers/net/tun.c:1512
tun_get_user+0x1e31/0x3c20 drivers/net/tun.c:1955
tun_chr_write_iter+0xdc/0x200 drivers/net/tun.c:2001
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x6ac/0x1050 fs/read_write.c:687
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd3f3236427
Code: 48 89 fa 4c 89 df e8 78 1e 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
RSP: 002b:00007ffce22a8bd0 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00005555816fc400 RCX: 00007fd3f3236427
RDX: 000000000000002a RSI: 0000200000000300 RDI: 00000000000000c8
RBP: 00007ffce22a8c60 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00007ffce22a8c30
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>


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[syzbot]

syzbot has bisected this issue to:

commit a890348adcc993f48d1ae38f1174dc8de4c3c5ac
Author: Stanislav Fomichev <s...@fomichev.me>
Date: Mon Aug 18 15:40:32 2025 +0000

net: Add skb_dst_check_unset

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=135ab649580000
start commit: 075b74841bd0 Linux 7.2-rc6
git tree: upstream
final oops: https://syzkaller.appspot.com/x/report.txt?x=10dab649580000
console output: https://syzkaller.appspot.com/x/log.txt?x=175ab649580000

kernel config: https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=76d4e3a055aec3b007ec

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14725db9580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12581c9e580000

Reported-by: syzbot+76d4e3...@syzkaller.appspotmail.com
Fixes: a890348adcc9 ("net: Add skb_dst_check_unset")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection