[v6.6] possible deadlock in nr_rt_device_down

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a5df3a702b2c Linux 6.6.96
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=1775ebd4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2632deddafa957e8
dashboard link: https://syzkaller.appspot.com/bug?extid=72b241c4d6f250dc39cb
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9ba5b19f9f4d/disk-a5df3a70.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2cd779015729/vmlinux-a5df3a70.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b56fc39e5cb8/bzImage-a5df3a70.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+72b241...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.6.96-syzkaller #0 Not tainted
------------------------------------------------------
syz.7.5202/23398 is trying to acquire lock:
ffffffff8e1189f8 (nr_node_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffffffff8e1189f8 (nr_node_list_lock){+...}-{2:2}, at: nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517

but task is already holding lock:
ffffffff8e118998 (nr_neigh_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffffffff8e118998 (nr_neigh_list_lock){+...}-{2:2}, at: nr_rt_device_down+0x28/0x720 net/netrom/nr_route.c:514

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (nr_neigh_list_lock){+...}-{2:2}:
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]
nr_remove_neigh net/netrom/nr_route.c:307 [inline]
nr_dec_obs net/netrom/nr_route.c:472 [inline]
nr_rt_ioctl+0x391/0xd40 net/netrom/nr_route.c:692
sock_do_ioctl+0xd7/0x2f0 net/socket.c:1222
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #1 (&nr_node->node_lock){+...}-{2:2}:
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]
nr_node_lock include/net/netrom.h:152 [inline]
nr_dec_obs net/netrom/nr_route.c:459 [inline]
nr_rt_ioctl+0x194/0xd40 net/netrom/nr_route.c:692
sock_do_ioctl+0xd7/0x2f0 net/socket.c:1222
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #0 (nr_node_list_lock){+...}-{2:2}:
check_prev_add kernel/locking/lockdep.c:3134 [inline]
check_prevs_add kernel/locking/lockdep.c:3253 [inline]
validate_chain kernel/locking/lockdep.c:3869 [inline]
__lock_acquire+0x2ddb/0x7c80 kernel/locking/lockdep.c:5137
lock_acquire+0x197/0x410 kernel/locking/lockdep.c:5754
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]
nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517
nr_device_event+0x137/0x150 net/netrom/af_netrom.c:126
notifier_call_chain+0x197/0x390 kernel/notifier.c:93
call_netdevice_notifiers_extack net/core/dev.c:2064 [inline]
call_netdevice_notifiers net/core/dev.c:2078 [inline]
__dev_notify_flags+0x18e/0x2e0 net/core/dev.c:-1
dev_change_flags+0xe8/0x1a0 net/core/dev.c:8750
dev_ifsioc+0x6a7/0xe20 net/core/dev_ioctl.c:529
dev_ioctl+0x7e2/0x1170 net/core/dev_ioctl.c:786
sock_do_ioctl+0x226/0x2f0 net/socket.c:1236
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

other info that might help us debug this:

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

Possible unsafe locking scenario:

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

*** DEADLOCK ***

2 locks held by syz.7.5202/23398:
#0: ffffffff8dfbb748 (rtnl_mutex){+.+.}-{3:3}, at: dev_ioctl+0x7d2/0x1170 net/core/dev_ioctl.c:785
#1: ffffffff8e118998 (nr_neigh_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#1: ffffffff8e118998 (nr_neigh_list_lock){+...}-{2:2}, at: nr_rt_device_down+0x28/0x720 net/netrom/nr_route.c:514

stack backtrace:
CPU: 1 PID: 23398 Comm: syz.7.5202 Not tainted 6.6.96-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
<TASK>
dump_stack_lvl+0x16c/0x230 lib/dump_stack.c:106
check_noncircular+0x2bd/0x3c0 kernel/locking/lockdep.c:2187
check_prev_add kernel/locking/lockdep.c:3134 [inline]
check_prevs_add kernel/locking/lockdep.c:3253 [inline]
validate_chain kernel/locking/lockdep.c:3869 [inline]
__lock_acquire+0x2ddb/0x7c80 kernel/locking/lockdep.c:5137
lock_acquire+0x197/0x410 kernel/locking/lockdep.c:5754
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]
nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517
nr_device_event+0x137/0x150 net/netrom/af_netrom.c:126
notifier_call_chain+0x197/0x390 kernel/notifier.c:93
call_netdevice_notifiers_extack net/core/dev.c:2064 [inline]
call_netdevice_notifiers net/core/dev.c:2078 [inline]
__dev_notify_flags+0x18e/0x2e0 net/core/dev.c:-1
dev_change_flags+0xe8/0x1a0 net/core/dev.c:8750
dev_ifsioc+0x6a7/0xe20 net/core/dev_ioctl.c:529
dev_ioctl+0x7e2/0x1170 net/core/dev_ioctl.c:786
sock_do_ioctl+0x226/0x2f0 net/socket.c:1236
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fa10558e929
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fa1053ff038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fa1057b5fa0 RCX: 00007fa10558e929
RDX: 0000200000000000 RSI: 0000000000008914 RDI: 0000000000000007
RBP: 00007fa105610b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fa1057b5fa0 R15: 00007ffc32e3fc08
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 60a9e718726f Linux 6.6.106
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=16e01f62580000
kernel config: https://syzkaller.appspot.com/x/.config?x=12606d4b8832c7e4
dashboard link: https://syzkaller.appspot.com/bug?extid=72b241c4d6f250dc39cb
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12f3ae42580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/eca27e056a5a/disk-60a9e718.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bc64d4eeb7f6/vmlinux-60a9e718.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a345041561ac/bzImage-60a9e718.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+72b241...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected

syzkaller #0 Not tainted
------------------------------------------------------
syz.1.2292/10680 is trying to acquire lock:
ffff8880205d9c70 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff8880205d9c70 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
ffff8880205d9c70 (&nr_node->node_lock){+...}-{2:2}, at: nr_rt_device_down+0x12a/0x720 net/netrom/nr_route.c:519

but task is already holding lock:

ffffffff8e119c78 (nr_node_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffffffff8e119c78 (nr_node_list_lock){+...}-{2:2}, at: nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (nr_node_list_lock){+...}-{2:2}:

__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]
nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517
nr_device_event+0x137/0x150 net/netrom/af_netrom.c:126
notifier_call_chain+0x197/0x390 kernel/notifier.c:93
call_netdevice_notifiers_extack net/core/dev.c:2064 [inline]
call_netdevice_notifiers net/core/dev.c:2078 [inline]
__dev_notify_flags+0x18e/0x2e0 net/core/dev.c:-1

dev_change_flags+0xe8/0x1a0 net/core/dev.c:8762

dev_ifsioc+0x6a7/0xe20 net/core/dev_ioctl.c:529
dev_ioctl+0x7e2/0x1170 net/core/dev_ioctl.c:786
sock_do_ioctl+0x226/0x2f0 net/socket.c:1236
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #1 (nr_neigh_list_lock){+...}-{2:2}:

__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 kernel/locking/spinlock.c:178
spin_lock_bh include/linux/spinlock.h:356 [inline]

nr_remove_neigh+0x25/0xe0 net/netrom/nr_route.c:307
nr_add_node+0x1d2d/0x24b0 net/netrom/nr_route.c:249
nr_rt_ioctl+0xc0f/0xd40 net/netrom/nr_route.c:651

sock_do_ioctl+0xd7/0x2f0 net/socket.c:1222
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #0 (&nr_node->node_lock){+...}-{2:2}:

check_prev_add kernel/locking/lockdep.c:3134 [inline]
check_prevs_add kernel/locking/lockdep.c:3253 [inline]
validate_chain kernel/locking/lockdep.c:3869 [inline]
__lock_acquire+0x2ddb/0x7c80 kernel/locking/lockdep.c:5137
lock_acquire+0x197/0x410 kernel/locking/lockdep.c:5754
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 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+0x12a/0x720 net/netrom/nr_route.c:519

nr_device_event+0x137/0x150 net/netrom/af_netrom.c:126
notifier_call_chain+0x197/0x390 kernel/notifier.c:93
call_netdevice_notifiers_extack net/core/dev.c:2064 [inline]
call_netdevice_notifiers net/core/dev.c:2078 [inline]
__dev_notify_flags+0x18e/0x2e0 net/core/dev.c:-1

dev_change_flags+0xe8/0x1a0 net/core/dev.c:8762

dev_ifsioc+0x6a7/0xe20 net/core/dev_ioctl.c:529
dev_ioctl+0x7e2/0x1170 net/core/dev_ioctl.c:786
sock_do_ioctl+0x226/0x2f0 net/socket.c:1236
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

other info that might help us debug this:

Chain exists of:

&nr_node->node_lock --> nr_neigh_list_lock --> nr_node_list_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----

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

*** DEADLOCK ***

3 locks held by syz.1.2292/10680:
#0: ffffffff8dfbc908 (rtnl_mutex){+.+.}-{3:3}, at: dev_ioctl+0x7d2/0x1170 net/core/dev_ioctl.c:785
#1: ffffffff8e119c18 (nr_neigh_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#1: ffffffff8e119c18 (nr_neigh_list_lock){+...}-{2:2}, at: nr_rt_device_down+0x28/0x720 net/netrom/nr_route.c:514
#2: ffffffff8e119c78 (nr_node_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#2: ffffffff8e119c78 (nr_node_list_lock){+...}-{2:2}, at: nr_rt_device_down+0xa9/0x720 net/netrom/nr_route.c:517

stack backtrace:
CPU: 1 PID: 10680 Comm: syz.1.2292 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025

Call Trace:
<TASK>
dump_stack_lvl+0x16c/0x230 lib/dump_stack.c:106
check_noncircular+0x2bd/0x3c0 kernel/locking/lockdep.c:2187
check_prev_add kernel/locking/lockdep.c:3134 [inline]
check_prevs_add kernel/locking/lockdep.c:3253 [inline]
validate_chain kernel/locking/lockdep.c:3869 [inline]
__lock_acquire+0x2ddb/0x7c80 kernel/locking/lockdep.c:5137
lock_acquire+0x197/0x410 kernel/locking/lockdep.c:5754
__raw_spin_lock_bh include/linux/spinlock_api_smp.h:126 [inline]
_raw_spin_lock_bh+0x36/0x50 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+0x12a/0x720 net/netrom/nr_route.c:519

nr_device_event+0x137/0x150 net/netrom/af_netrom.c:126
notifier_call_chain+0x197/0x390 kernel/notifier.c:93
call_netdevice_notifiers_extack net/core/dev.c:2064 [inline]
call_netdevice_notifiers net/core/dev.c:2078 [inline]
__dev_notify_flags+0x18e/0x2e0 net/core/dev.c:-1

dev_change_flags+0xe8/0x1a0 net/core/dev.c:8762

dev_ifsioc+0x6a7/0xe20 net/core/dev_ioctl.c:529
dev_ioctl+0x7e2/0x1170 net/core/dev_ioctl.c:786
sock_do_ioctl+0x226/0x2f0 net/socket.c:1236
sock_ioctl+0x623/0x7a0 net/socket.c:1341
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

RIP: 0033:0x7f41a618eba9

Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48

RSP: 002b:00007f41a7084038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f41a63d5fa0 RCX: 00007f41a618eba9
RDX: 0000200000000000 RSI: 0000000000008914 RDI: 0000000000000006
RBP: 00007f41a6211e19 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000

R13: 00007f41a63d6038 R14: 00007f41a63d5fa0 R15: 00007ffc1e84e1d8
</TASK>


---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.