[v6.1] possible deadlock in nr_remove_neigh (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 3db754f56897 Linux 6.1.152
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=1700cc7c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=68aa5a3af1cb953a
dashboard link: https://syzkaller.appspot.com/bug?extid=0b99f66c1fd2235ec545
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
userspace arch: arm64

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9b92862bd95a/disk-3db754f5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f232864105d7/vmlinux-3db754f5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/30a3ed81e30f/Image-3db754f5.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0b99f6...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.69/4647 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:
ffff0000d3065070 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff0000d3065070 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
ffff0000d3065070 (&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.3.69/4647:
#0: ffff0000d3065070 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#0: ffff0000d3065070 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
#0: ffff0000d3065070 (&nr_node->node_lock){+...}-{2:2}, at: nr_add_node+0xd60/0x2120 net/netrom/nr_route.c:214

stack backtrace:
CPU: 0 PID: 4647 Comm: syz.3.69 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
netlink: 'syz.3.69': attribute type 2 has an invalid length.
netlink: 'syz.3.69': attribute type 1 has an invalid length.
netlink: 212408 bytes leftover after parsing attributes in process `syz.3.69'.


---
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[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit: 3db754f56897 Linux 6.1.152
git tree: linux-6.1.y

console output: https://syzkaller.appspot.com/x/log.txt?x=12c8a534580000

kernel config: https://syzkaller.appspot.com/x/.config?x=68aa5a3af1cb953a
dashboard link: https://syzkaller.appspot.com/bug?extid=0b99f66c1fd2235ec545
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
userspace arch: arm64

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11dc4712580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=165240e2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9b92862bd95a/disk-3db754f5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f232864105d7/vmlinux-3db754f5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/30a3ed81e30f/Image-3db754f5.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0b99f6...@syzkaller.appspotmail.com

======================================================
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

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

---
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