[v6.1] possible deadlock in nr_rt_ioctl (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cd9b81672742 Linux 6.1.161
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=153d425a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b1adc0bfde2d8a4a
dashboard link: https://syzkaller.appspot.com/bug?extid=139199693c41eb7b5515
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/d21df4411014/disk-cd9b8167.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/246aed0358fb/vmlinux-cd9b8167.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a6f546504ec2/Image-cd9b8167.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+139199...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.1.1653/9530 is trying to acquire lock:
ffff8000179b37b8 (nr_neigh_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff8000179b37b8 (nr_neigh_list_lock){+...}-{2:2}, at: nr_remove_neigh net/netrom/nr_route.c:307 [inline]
ffff8000179b37b8 (nr_neigh_list_lock){+...}-{2:2}, at: nr_dec_obs net/netrom/nr_route.c:472 [inline]
ffff8000179b37b8 (nr_neigh_list_lock){+...}-{2:2}, at: nr_rt_ioctl+0x32c/0xbb4 net/netrom/nr_route.c:692

but task is already holding lock:
ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_dec_obs net/netrom/nr_route.c:459 [inline]
ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_rt_ioctl+0x1c0/0xbb4 net/netrom/nr_route.c:692

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+0x128/0x658 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+0x1ec/0x480 net/core/dev.c:8678
dev_change_flags+0xc8/0x154 net/core/dev.c:8714
dev_ifsioc+0x138/0xc04 net/core/dev_ioctl.c:326
dev_ioctl+0x4b0/0xc74 net/core/dev_ioctl.c:572
sock_do_ioctl+0x198/0x258 net/socket.c:1215
sock_ioctl+0x238/0x824 net/socket.c:1320
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+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
el0_svc+0x58/0x128 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+0xb4/0x658 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+0x1ec/0x480 net/core/dev.c:8678
dev_change_flags+0xc8/0x154 net/core/dev.c:8714
dev_ifsioc+0x138/0xc04 net/core/dev_ioctl.c:326
dev_ioctl+0x4b0/0xc74 net/core/dev_ioctl.c:572
sock_do_ioctl+0x198/0x258 net/socket.c:1215
sock_ioctl+0x238/0x824 net/socket.c:1320
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+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
el0_svc+0x58/0x128 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+0x2880/0x6800 kernel/locking/lockdep.c:5049
lock_acquire+0x20c/0x63c 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 net/netrom/nr_route.c:307 [inline]
nr_dec_obs net/netrom/nr_route.c:472 [inline]
nr_rt_ioctl+0x32c/0xbb4 net/netrom/nr_route.c:692
nr_ioctl+0x19c/0x474 net/netrom/af_netrom.c:1254
sock_do_ioctl+0x100/0x258 net/socket.c:1201
sock_ioctl+0x238/0x824 net/socket.c:1320
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+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
el0_svc+0x58/0x128 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 ***

2 locks held by syz.1.1653/9530:
#0: ffff8000179b3818 (nr_node_list_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#0: ffff8000179b3818 (nr_node_list_lock){+...}-{2:2}, at: nr_dec_obs net/netrom/nr_route.c:457 [inline]
#0: ffff8000179b3818 (nr_node_list_lock){+...}-{2:2}, at: nr_rt_ioctl+0x124/0xbb4 net/netrom/nr_route.c:692
#1: ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: spin_lock_bh include/linux/spinlock.h:356 [inline]
#1: ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_node_lock include/net/netrom.h:152 [inline]
#1: ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_dec_obs net/netrom/nr_route.c:459 [inline]
#1: ffff0000dc6aa670 (&nr_node->node_lock){+...}-{2:2}, at: nr_rt_ioctl+0x1c0/0xbb4 net/netrom/nr_route.c:692

stack backtrace:
CPU: 0 PID: 9530 Comm: syz.1.1653 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/03/2025
Call trace:
dump_backtrace+0x1c0/0x1ec 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+0xf4/0x15c 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+0x264/0x2f8 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+0x2880/0x6800 kernel/locking/lockdep.c:5049
lock_acquire+0x20c/0x63c 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 net/netrom/nr_route.c:307 [inline]
nr_dec_obs net/netrom/nr_route.c:472 [inline]
nr_rt_ioctl+0x32c/0xbb4 net/netrom/nr_route.c:692
nr_ioctl+0x19c/0x474 net/netrom/af_netrom.c:1254
sock_do_ioctl+0x100/0x258 net/socket.c:1201
sock_ioctl+0x238/0x824 net/socket.c:1320
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+0x98/0x2b4 arch/arm64/kernel/syscall.c:52
el0_svc_common+0x138/0x258 arch/arm64/kernel/syscall.c:140
do_el0_svc+0x58/0x130 arch/arm64/kernel/syscall.c:204
el0_svc+0x58/0x128 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


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