possible deadlock in lapb_data_received

Status: upstream: reported C repro on 2026/10/19 19:20
Subsystems: x25
Labels: prio:low
[Documentation on labels]
Reported-by: syzbot+059a2795ad4cff2a2695@syzkaller.appspotmail.com
First crash: 6d12h, last: 3d02h
▶ ▼ ✨ AI Jobs (2)
ID Workflow Result Correct Bug Created Started Finished Revision
c2514497 repro-c Reproduced: ✅ ❓ possible deadlock in lapb_data_received 2026/10/18 20:11 2026/10/18 20:11 2026/10/18 22:16 432c0077
468ba449 assessment-security DenialOfService: ✅ Exploitable: ❌ FilesystemTrigger: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ possible deadlock in lapb_data_received 2026/10/17 00:49 2026/10/17 00:49 2026/10/17 01:25 432c0077
▶ ▼ Discussions (1)
Title Replies (including bot) Last reply
[syzbot] [x25?] possible deadlock in lapb_data_received 2 (5) 2026/10/20 04:58
▶ ▼ Last patch testing requests (3)
Created Duration User Patch Repo Result
2026/10/20 04:29 20m hdanton@sina.com patch git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 70dd4ee72eb2 OK job log log
2026/10/20 00:50 6m hdanton@sina.com patch git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git 2c7935600733 error job log
2026/10/19 04:54 29m retest repro git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git 2c7935600733c2168b81d3f0f31247ed20d54be0 report job log log
▶ ▼ Cause bisection attempts (1)
Created Duration User Patch Repo Result
2026/10/19 05:49 0m bisect git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git error

Sample crash report:
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz-executor220/6011 is trying to acquire lock:
ffffffff913da7b8 (lapb_list_lock){++..}-{3:3}, at: lapb_devtostruct net/lapb/lapb_iface.c:102 [inline]
ffffffff913da7b8 (lapb_list_lock){++..}-{3:3}, at: lapb_data_received+0x2f/0x1c0 net/lapb/lapb_iface.c:394

but task is already holding lock:
ffff88802af94f00 (&lapbeth->up_lock){+...}-{3:3}, at: spin_lock_bh include/linux/spinlock.h:353 [inline]
ffff88802af94f00 (&lapbeth->up_lock){+...}-{3:3}, at: lapbeth_rcv+0x24e/0x640 drivers/net/wan/lapbether.c:131

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&lapbeth->up_lock){+...}-{3:3}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:182 [inline]
       _raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:205
       spin_lock_bh include/linux/spinlock.h:353 [inline]
       lapbeth_xmit+0x32/0x260 drivers/net/wan/lapbether.c:191
       __netdev_start_xmit include/linux/netdevice.h:5432 [inline]
       netdev_start_xmit include/linux/netdevice.h:5441 [inline]
       xmit_one net/core/dev.c:3953 [inline]
       dev_hard_start_xmit+0x2cd/0x830 net/core/dev.c:3969
       __dev_queue_xmit+0x14d1/0x3920 net/core/dev.c:4958
       dev_queue_xmit include/linux/netdevice.h:3464 [inline]
       tx+0x6b/0x190 drivers/block/aoe/aoenet.c:62
       kthread+0x1ed/0x420 drivers/block/aoe/aoecmd.c:1241
       kthread+0x38b/0x480 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 (_xmit_X25#2){+...}-{3:3}:
       __raw_spin_lock include/linux/spinlock_api_smp.h:190 [inline]
       _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:173
       spin_lock include/linux/spinlock.h:347 [inline]
       __netif_tx_lock include/linux/netdevice.h:4841 [inline]
       __dev_queue_xmit+0xe9d/0x3920 net/core/dev.c:4952
       neigh_output include/net/neighbour.h:560 [inline]
       ip_finish_output2+0xc4b/0x10b0 net/ipv4/ip_output.c:236
       NF_HOOK_COND include/linux/netfilter.h:314 [inline]
       ip_output+0x29f/0x450 net/ipv4/ip_output.c:437
       iptunnel_xmit+0x626/0xd10 net/ipv4/ip_tunnel_core.c:97
       udp_tunnel_xmit_skb+0x29b/0x3e0 net/ipv4/udp_tunnel_core.c:187
       vxlan_xmit_one+0x21f6/0x2eb0 drivers/net/vxlan/vxlan_core.c:2571
       vxlan_xmit+0x1e19/0x33b0 drivers/net/vxlan/vxlan_core.c:2860
       __netdev_start_xmit include/linux/netdevice.h:5432 [inline]
       netdev_start_xmit include/linux/netdevice.h:5441 [inline]
       xmit_one net/core/dev.c:3953 [inline]
       dev_hard_start_xmit+0x2cd/0x830 net/core/dev.c:3969
       __dev_queue_xmit+0x14d1/0x3920 net/core/dev.c:4958
       lapb_data_transmit+0x90/0xb0 net/lapb/lapb_iface.c:447
       lapb_transmit_buffer+0x163/0x200 net/lapb/lapb_out.c:149
       lapb_establish_data_link+0x89/0xe0 net/lapb/lapb_out.c:-1
       lapb_device_event+0x4eb/0x680 net/lapb/lapb_iface.c:-1
       notifier_call_chain+0x1a5/0x3d0 kernel/notifier.c:85
       call_netdevice_notifiers_extack net/core/dev.c:2313 [inline]
       call_netdevice_notifiers net/core/dev.c:2327 [inline]
       __dev_notify_flags+0x31e/0x510 net/core/dev.c:9896
       netif_change_flags+0xde/0x1b0 net/core/dev.c:9925
       dev_change_flags+0x128/0x260 net/core/dev_api.c:68
       devinet_ioctl+0x966/0x1c50 net/ipv4/devinet.c:1199
       inet_ioctl+0x42a/0x560 net/ipv4/af_inet.c:1010
       sock_do_ioctl+0x101/0x320 net/socket.c:1325
       sock_ioctl+0x57a/0x7e0 net/socket.c:1446
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
       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

-> #1 (&lapb->lock){+.-.}-{3:3}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:182 [inline]
       _raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:205
       spin_lock_bh include/linux/spinlock.h:353 [inline]
       lapb_unregister+0x1e1/0x450 net/lapb/lapb_iface.c:187
       lapbeth_close+0x56/0xb0 drivers/net/wan/lapbether.c:350
       __dev_close_many+0x366/0x6e0 net/core/dev.c:1798
       __dev_close net/core/dev.c:1811 [inline]
       __dev_change_flags+0x3f5/0x820 net/core/dev.c:9853
       netif_change_flags+0x7c/0x1b0 net/core/dev.c:9920
       dev_change_flags+0x128/0x260 net/core/dev_api.c:68
       devinet_ioctl+0x966/0x1c50 net/ipv4/devinet.c:1199
       inet_ioctl+0x42a/0x560 net/ipv4/af_inet.c:1010
       sock_do_ioctl+0x101/0x320 net/socket.c:1325
       sock_ioctl+0x57a/0x7e0 net/socket.c:1446
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
       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

-> #0 (lapb_list_lock){++..}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3209 [inline]
       check_prevs_add kernel/locking/lockdep.c:3328 [inline]
       validate_chain kernel/locking/lockdep.c:3952 [inline]
       __lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
       lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
       __raw_read_lock_bh include/linux/rwlock_api_smp.h:192 [inline]
       _raw_read_lock_bh+0x3e/0x50 kernel/locking/spinlock.c:287
       lapb_devtostruct net/lapb/lapb_iface.c:102 [inline]
       lapb_data_received+0x2f/0x1c0 net/lapb/lapb_iface.c:394
       lapbeth_rcv+0x34d/0x640 drivers/net/wan/lapbether.c:142
       __netif_receive_skb_one_core net/core/dev.c:6297 [inline]
       __netif_receive_skb net/core/dev.c:6410 [inline]
       netif_receive_skb_internal net/core/dev.c:6496 [inline]
       netif_receive_skb+0x4af/0xbf0 net/core/dev.c:6555
       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

other info that might help us debug this:

Chain exists of:
  lapb_list_lock --> _xmit_X25#2 --> &lapbeth->up_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&lapbeth->up_lock);
                               lock(_xmit_X25#2);
                               lock(&lapbeth->up_lock);
  rlock(lapb_list_lock);

 *** DEADLOCK ***

locks held by syz-executor220/6011: 4, last CPU#0:
 #0: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
 #0: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
 #0: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: tun_get_user+0x27b0/0x44d0 drivers/net/tun.c:1986
 #1: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
 #1: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
 #1: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: netif_receive_skb_internal net/core/dev.c:6483 [inline]
 #1: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: netif_receive_skb+0x102/0xbf0 net/core/dev.c:6555
 #2: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
 #2: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
 #2: ffffffff8fd5ce60 (rcu_read_lock){....}-{1:3}, at: lapbeth_rcv+0x12f/0x640 drivers/net/wan/lapbether.c:127
 #3: ffff88802af94f00 (&lapbeth->up_lock){+...}-{3:3}, at: spin_lock_bh include/linux/spinlock.h:353 [inline]
 #3: ffff88802af94f00 (&lapbeth->up_lock){+...}-{3:3}, at: lapbeth_rcv+0x24e/0x640 drivers/net/wan/lapbether.c:131

stack backtrace:
CPU: 0 UID: 0 PID: 6011 Comm: syz-executor220 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2087
 check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2219
 check_prev_add kernel/locking/lockdep.c:3209 [inline]
 check_prevs_add kernel/locking/lockdep.c:3328 [inline]
 validate_chain kernel/locking/lockdep.c:3952 [inline]
 __lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
 lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
 __raw_read_lock_bh include/linux/rwlock_api_smp.h:192 [inline]
 _raw_read_lock_bh+0x3e/0x50 kernel/locking/spinlock.c:287
 lapb_devtostruct net/lapb/lapb_iface.c:102 [inline]
 lapb_data_received+0x2f/0x1c0 net/lapb/lapb_iface.c:394
 lapbeth_rcv+0x34d/0x640 drivers/net/wan/lapbether.c:142
 __netif_receive_skb_one_core net/core/dev.c:6297 [inline]
 __netif_receive_skb net/core/dev.c:6410 [inline]
 netif_receive_skb_internal net/core/dev.c:6496 [inline]
 netif_receive_skb+0x4af/0xbf0 net/core/dev.c:6555
 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: 0033:0x7f523ead1137
Code: 48 89 fa 4c 89 df e8 98 1d 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:00007ffcbef91f00 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00005555890c4400 RCX: 00007f523ead1137
RDX: 0000000000000010 RSI: 00007ffcbef91fb0 RDI: 0000000000000006
RBP: 0000000000000006 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 000000000000000a
R13: 00007ffcbef91fb0 R14: 00007ffcbef91fa0 R15: 000000000000002c
 </TASK>

Crashes (2):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/10/19 05:24 git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git 2c7935600733 3f2522c6 .config console log report C ci-upstream-bpf-next-kasan-gce possible deadlock in lapb_data_received
2026/10/15 19:17 bpf-next a5e144985d68 3f2522c6 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-bpf-next-kasan-gce possible deadlock in lapb_data_received
* Struck through repros no longer work on HEAD.