possible deadlock in nr_remove_neigh (2)

Status: upstream: reported C repro on 2025/09/18 00:16
Bug presence: origin:upstream
Labels: missing-backport
[Documentation on labels]
Reported-by: syzbot+0b99f66c1fd2235ec545@syzkaller.appspotmail.com
First crash: 346d, last: 3d10h
Fix bisection: failed (error log, bisect log)
  
▶ ▼ Bug presence (3)
Date Name Commit Repro Result
2026/08/27 linux-6.1.y (ToT) 05208478041c C [report] possible deadlock in nr_remove_neigh
2025/09/21 upstream (ToT) f975f08c2e89 C [report] possible deadlock in nr_remove_neigh
2026/08/26 upstream (ToT) b035a8be20dd C Didn't crash
▶ ▼ Similar bugs (4)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-5.15 possible deadlock in nr_remove_neigh origin:upstream 4 syz, C error 25 94d 585d 0/3 upstream: reported C repro on 2025/01/22 07:32
linux-6.6 possible deadlock in nr_remove_neigh 4 1 256d 256d 0/2 auto-obsoleted due to no activity on 2026/03/27 08:30
upstream possible deadlock in nr_remove_neigh (2) hams 4 syz, C unreliable error 1122 128d 608d 0/29 upstream: reported C repro on 2024/12/30 02:36
linux-6.1 possible deadlock in nr_remove_neigh 4 13 481d 608d 0/3 auto-obsoleted due to no activity on 2025/08/13 20:50
▶ ▼ Fix bisection attempts (4)
Created Duration User Patch Repo Result
2026/01/30 13:12 0m bisect fix linux-6.1.y error job log
2025/12/31 09:04 2h21m bisect fix linux-6.1.y OK (0) job log log
2025/11/29 15:59 3h08m bisect fix linux-6.1.y OK (0) job log log
2025/10/29 01:28 2h57m bisect fix linux-6.1.y OK (0) job log log

Sample crash report:
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.223/4768 is trying to acquire lock:
ffff800017876878 (nr_neigh_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff800017876878 (nr_neigh_list_lock){+...}-{2:2}, at: nr_remove_neigh+0x30/0xec net/netrom/nr_route.c:307

but task is already holding lock:
ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: nr_add_node+0xd60/0x2120 net/netrom/nr_route.c:214

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&nr_node->node_lock){+...}-{2:2}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
       _raw_spin_lock_bh+0x54/0x6c kernel/locking/spinlock.c:178
       spin_lock_bh include/linux/spinlock.h:356 [inline]
       nr_node_lock include/net/netrom.h:152 [inline]
       nr_rt_device_down+0x128/0x660 net/netrom/nr_route.c:519
       nr_device_event+0x138/0x164 net/netrom/af_netrom.c:126
       notifier_call_chain kernel/notifier.c:87 [inline]
       raw_notifier_call_chain+0xd4/0x164 kernel/notifier.c:455
       call_netdevice_notifiers_info net/core/dev.c:2001 [inline]
       call_netdevice_notifiers_extack net/core/dev.c:2039 [inline]
       call_netdevice_notifiers net/core/dev.c:2053 [inline]
       __dev_notify_flags+0x250/0x46c net/core/dev.c:-1
       dev_change_flags+0xc8/0x154 net/core/dev.c:8714
       dev_ifsioc+0x524/0xed0 net/core/dev_ioctl.c:327
       dev_ioctl+0x4c8/0xc8c net/core/dev_ioctl.c:588
       sock_do_ioctl+0x18c/0x240 net/socket.c:1218
       sock_ioctl+0x5c4/0x840 net/socket.c:1321
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:870 [inline]
       __se_sys_ioctl fs/ioctl.c:856 [inline]
       __arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:856
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 arch/arm64/kernel/entry-common.c:637
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #1 (nr_node_list_lock){+...}-{2:2}:
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
       _raw_spin_lock_bh+0x54/0x6c kernel/locking/spinlock.c:178
       spin_lock_bh include/linux/spinlock.h:356 [inline]
       nr_rt_device_down+0xb4/0x660 net/netrom/nr_route.c:517
       nr_device_event+0x138/0x164 net/netrom/af_netrom.c:126
       notifier_call_chain kernel/notifier.c:87 [inline]
       raw_notifier_call_chain+0xd4/0x164 kernel/notifier.c:455
       call_netdevice_notifiers_info net/core/dev.c:2001 [inline]
       call_netdevice_notifiers_extack net/core/dev.c:2039 [inline]
       call_netdevice_notifiers net/core/dev.c:2053 [inline]
       __dev_notify_flags+0x250/0x46c net/core/dev.c:-1
       dev_change_flags+0xc8/0x154 net/core/dev.c:8714
       dev_ifsioc+0x524/0xed0 net/core/dev_ioctl.c:327
       dev_ioctl+0x4c8/0xc8c net/core/dev_ioctl.c:588
       sock_do_ioctl+0x18c/0x240 net/socket.c:1218
       sock_ioctl+0x5c4/0x840 net/socket.c:1321
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:870 [inline]
       __se_sys_ioctl fs/ioctl.c:856 [inline]
       __arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:856
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 arch/arm64/kernel/entry-common.c:637
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

-> #0 (nr_neigh_list_lock){+...}-{2:2}:
       check_prev_add kernel/locking/lockdep.c:3090 [inline]
       check_prevs_add kernel/locking/lockdep.c:3209 [inline]
       validate_chain kernel/locking/lockdep.c:3825 [inline]
       __lock_acquire+0x293c/0x6544 kernel/locking/lockdep.c:5049
       lock_acquire+0x20c/0x644 kernel/locking/lockdep.c:5662
       __raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
       _raw_spin_lock_bh+0x54/0x6c kernel/locking/spinlock.c:178
       spin_lock_bh include/linux/spinlock.h:356 [inline]
       nr_remove_neigh+0x30/0xec net/netrom/nr_route.c:307
       nr_add_node+0x197c/0x2120 net/netrom/nr_route.c:249
       nr_rt_ioctl+0x9f8/0xb80 net/netrom/nr_route.c:651
       nr_ioctl+0x1c4/0x4e4 net/netrom/af_netrom.c:1254
       sock_do_ioctl+0xf8/0x240 net/socket.c:1204
       sock_ioctl+0x5c4/0x840 net/socket.c:1321
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:870 [inline]
       __se_sys_ioctl fs/ioctl.c:856 [inline]
       __arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:856
       __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
       invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
       el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
       do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
       el0_svc+0x58/0x138 arch/arm64/kernel/entry-common.c:637
       el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
       el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

other info that might help us debug this:

Chain exists of:
  nr_neigh_list_lock --> nr_node_list_lock --> &nr_node->node_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&nr_node->node_lock);
                               lock(nr_node_list_lock);
                               lock(&nr_node->node_lock);
  lock(nr_neigh_list_lock);

 *** DEADLOCK ***

1 lock held by syz.3.223/4768:
 #0: ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
 #0: ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
 #0: ffff0000c407b670 (&nr_node->node_lock){+...}-{2:2}, at: nr_add_node+0xd60/0x2120 net/netrom/nr_route.c:214

stack backtrace:
CPU: 1 PID: 4768 Comm: syz.3.223 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/30/2025
Call trace:
 dump_backtrace+0x1c8/0x1f4 arch/arm64/kernel/stacktrace.c:158
 show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:165
 __dump_stack+0x30/0x40 lib/dump_stack.c:88
 dump_stack_lvl+0xf8/0x160 lib/dump_stack.c:106
 dump_stack+0x1c/0x5c lib/dump_stack.c:113
 print_circular_bug+0x148/0x1b0 kernel/locking/lockdep.c:2048
 check_noncircular+0x240/0x2d4 kernel/locking/lockdep.c:2170
 check_prev_add kernel/locking/lockdep.c:3090 [inline]
 check_prevs_add kernel/locking/lockdep.c:3209 [inline]
 validate_chain kernel/locking/lockdep.c:3825 [inline]
 __lock_acquire+0x293c/0x6544 kernel/locking/lockdep.c:5049
 lock_acquire+0x20c/0x644 kernel/locking/lockdep.c:5662
 __raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
 _raw_spin_lock_bh+0x54/0x6c kernel/locking/spinlock.c:178
 spin_lock_bh include/linux/spinlock.h:356 [inline]
 nr_remove_neigh+0x30/0xec net/netrom/nr_route.c:307
 nr_add_node+0x197c/0x2120 net/netrom/nr_route.c:249
 nr_rt_ioctl+0x9f8/0xb80 net/netrom/nr_route.c:651
 nr_ioctl+0x1c4/0x4e4 net/netrom/af_netrom.c:1254
 sock_do_ioctl+0xf8/0x240 net/socket.c:1204
 sock_ioctl+0x5c4/0x840 net/socket.c:1321
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl fs/ioctl.c:856 [inline]
 __arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:856
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2bc arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
 do_el0_svc+0x58/0x13c arch/arm64/kernel/syscall.c:204
 el0_svc+0x58/0x138 arch/arm64/kernel/entry-common.c:637
 el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:655
 el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:585

Crashes (10):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2025/09/18 12:44 linux-6.1.y 3db754f56897 e2beed91 .config console log report syz / log C [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2026/03/31 03:37 linux-6.1.y 1989cd3d56e2 d0af506e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2026/03/31 03:36 linux-6.1.y 1989cd3d56e2 d0af506e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2026/03/31 03:35 linux-6.1.y 1989cd3d56e2 d0af506e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2026/03/31 03:29 linux-6.1.y 1989cd3d56e2 d0af506e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2026/03/31 03:29 linux-6.1.y 1989cd3d56e2 d0af506e .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2025/09/20 01:42 linux-6.1.y 363a599da6d9 67c37560 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2025/09/18 02:24 linux-6.1.y 3db754f56897 e2beed91 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2025/09/18 02:07 linux-6.1.y 3db754f56897 e2beed91 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
2025/09/18 00:15 linux-6.1.y 3db754f56897 e2beed91 .config console log report info [disk image] [vmlinux] [kernel image] ci2-linux-6-1-kasan-arm64 possible deadlock in nr_remove_neigh
* Struck through repros no longer work on HEAD.