[syzbot] [nbd?] possible deadlock in nbd_disconnect_and_put

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 635c467cc14e Add linux-next specific files for 20260213
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=136df15a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=61690c38d1398936
dashboard link: https://syzkaller.appspot.com/bug?extid=3add0454d5a2619b8e80
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/78b3d15ca8e6/disk-635c467c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a95f3d108ef4/vmlinux-635c467c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e58086838b24/bzImage-635c467c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3add04...@syzkaller.appspotmail.com

block nbd1: NBD_DISCONNECT
block nbd1: Disconnected due to user request.
block nbd1: shutting down sockets
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.1.1462/9805 is trying to acquire lock:
ffff8880271c7948 ((wq_completion)nbd1-recv){+.+.}-{0:0}, at: touch_wq_lockdep_map+0xb5/0x180 kernel/workqueue.c:3994

but task is already holding lock:
ffff888027014278 (&nbd->config_lock){+.+.}-{4:4}, at: nbd_disconnect_and_put+0x2f/0x2c0 drivers/block/nbd.c:2256

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&nbd->config_lock){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:614 [inline]
__mutex_lock+0x19f/0x1300 kernel/locking/mutex.c:776
refcount_dec_and_mutex_lock+0x30/0xa0 lib/refcount.c:118
nbd_config_put+0x2c/0x580 drivers/block/nbd.c:1434
recv_work+0x1cc1/0x1d90 drivers/block/nbd.c:1026
process_one_work+0x949/0x1650 kernel/workqueue.c:3279
process_scheduled_works kernel/workqueue.c:3362 [inline]
worker_thread+0xb46/0x1140 kernel/workqueue.c:3443
kthread+0x388/0x470 kernel/kthread.c:467
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 ((work_completion)(&args->work)#3){+.+.}-{0:0}:
process_one_work+0x895/0x1650 kernel/workqueue.c:3255
process_scheduled_works kernel/workqueue.c:3362 [inline]
worker_thread+0xb46/0x1140 kernel/workqueue.c:3443
kthread+0x388/0x470 kernel/kthread.c:467
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 ((wq_completion)nbd1-recv){+.+.}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
touch_wq_lockdep_map+0xcb/0x180 kernel/workqueue.c:3994
__flush_workqueue+0x14b/0x14f0 kernel/workqueue.c:4036
nbd_disconnect_and_put+0x9e/0x2c0 drivers/block/nbd.c:2264
nbd_genl_disconnect+0x4a9/0x590 drivers/block/nbd.c:2303
genl_family_rcv_msg_doit+0x22a/0x330 net/netlink/genetlink.c:1115
genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
genl_rcv_msg+0x61c/0x7a0 net/netlink/genetlink.c:1210
netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec+0x18f/0x1d0 net/socket.c:737
__sock_sendmsg net/socket.c:752 [inline]
____sys_sendmsg+0x589/0x8c0 net/socket.c:2610
___sys_sendmsg+0x2a5/0x360 net/socket.c:2664
__sys_sendmsg net/socket.c:2696 [inline]
__do_sys_sendmsg net/socket.c:2701 [inline]
__se_sys_sendmsg net/socket.c:2699 [inline]
__x64_sys_sendmsg+0x1bd/0x2a0 net/socket.c:2699
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
(wq_completion)nbd1-recv --> (work_completion)(&args->work)#3 --> &nbd->config_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&nbd->config_lock);
lock((work_completion)(&args->work)#3);
lock(&nbd->config_lock);
lock((wq_completion)nbd1-recv);

*** DEADLOCK ***

3 locks held by syz.1.1462/9805:
#0: ffffffff8fe5ba70 (cb_lock){++++}-{4:4}, at: genl_rcv+0x19/0x40 net/netlink/genetlink.c:1218
#1: ffffffff8fe5b888 (genl_mutex){+.+.}-{4:4}, at: genl_lock net/netlink/genetlink.c:35 [inline]
#1: ffffffff8fe5b888 (genl_mutex){+.+.}-{4:4}, at: genl_op_lock net/netlink/genetlink.c:60 [inline]
#1: ffffffff8fe5b888 (genl_mutex){+.+.}-{4:4}, at: genl_rcv_msg+0x10b/0x7a0 net/netlink/genetlink.c:1209
#2: ffff888027014278 (&nbd->config_lock){+.+.}-{4:4}, at: nbd_disconnect_and_put+0x2f/0x2c0 drivers/block/nbd.c:2256

stack backtrace:
CPU: 0 UID: 0 PID: 9805 Comm: syz.1.1462 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
touch_wq_lockdep_map+0xcb/0x180 kernel/workqueue.c:3994
__flush_workqueue+0x14b/0x14f0 kernel/workqueue.c:4036
nbd_disconnect_and_put+0x9e/0x2c0 drivers/block/nbd.c:2264
nbd_genl_disconnect+0x4a9/0x590 drivers/block/nbd.c:2303
genl_family_rcv_msg_doit+0x22a/0x330 net/netlink/genetlink.c:1115
genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
genl_rcv_msg+0x61c/0x7a0 net/netlink/genetlink.c:1210
netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
sock_sendmsg_nosec+0x18f/0x1d0 net/socket.c:737
__sock_sendmsg net/socket.c:752 [inline]
____sys_sendmsg+0x589/0x8c0 net/socket.c:2610
___sys_sendmsg+0x2a5/0x360 net/socket.c:2664
__sys_sendmsg net/socket.c:2696 [inline]
__do_sys_sendmsg net/socket.c:2701 [inline]
__se_sys_sendmsg net/socket.c:2699 [inline]
__x64_sys_sendmsg+0x1bd/0x2a0 net/socket.c:2699
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f496959bf79
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f496a4cd028 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f4969816090 RCX: 00007f496959bf79
RDX: 00000000000000c0 RSI: 0000200000000280 RDI: 0000000000000004
RBP: 00007f49696327e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4969816128 R14: 00007f4969816090 R15: 00007ffd35895958
</TASK>


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