refcount_t: underflow; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 4469 at lib/refcount.c:281 refcount_dec_not_one.cold+0x18/0x25 lib/refcount.c:281
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 4469 Comm: systemd-udevd Not tainted 4.14.291-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/22/2022
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x1b2/0x281 lib/dump_stack.c:58
 panic+0x1f9/0x42d kernel/panic.c:183
 __warn.cold+0x20/0x44 kernel/panic.c:547
 report_bug+0x208/0x250 lib/bug.c:183
 fixup_bug arch/x86/kernel/traps.c:177 [inline]
 fixup_bug arch/x86/kernel/traps.c:172 [inline]
 do_error_trap+0x195/0x2d0 arch/x86/kernel/traps.c:295
 invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:964
RIP: 0010:refcount_dec_not_one.cold+0x18/0x25 lib/refcount.c:281
RSP: 0018:ffff88807dd7fc38 EFLAGS: 00010286
RAX: 0000000000000026 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff878bc8e0 RDI: ffffed100fbaff7d
RBP: ffff88809c3f2e18 R08: 0000000000000026 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
R13: 0000000000000000 R14: ffffed100d977266 R15: ffff88809c3f2e28
 refcount_dec_and_mutex_lock+0x1a/0x60 lib/refcount.c:309
 nbd_put drivers/block/nbd.c:228 [inline]
 nbd_config_put+0x442/0x6c0 drivers/block/nbd.c:1172
 nbd_release+0xdf/0x150 drivers/block/nbd.c:1451
 __blkdev_put+0x5aa/0x800 fs/block_dev.c:1803
 blkdev_close+0x86/0xb0 fs/block_dev.c:1875
 __fput+0x25f/0x7a0 fs/file_table.c:210
 task_work_run+0x11f/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1ad/0x200 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:270 [inline]
 do_syscall_64+0x4a3/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x46/0xbb
RIP: 0033:0x7f51bbf15270
RSP: 002b:00007ffdae2c4d58 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000007 RCX: 00007f51bbf15270
RDX: 000000000aba9500 RSI: 0000000000000000 RDI: 0000000000000007
RBP: 00007f51bcdcf710 R08: 000055878c34ad30 R09: 0000000000000018
R10: 000055878c3498f8 R11: 0000000000000246 R12: 0000000000000000
R13: 000055878c338760 R14: 0000000000000003 R15: 000000000000000e
Kernel Offset: disabled
Rebooting in 86400 seconds..