[syzbot] [nbd?] possible deadlock in nbd_disconnect_and_put

[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+3add0454d5a2619b8e80@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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