refcount_t: underflow; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 1286 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: 1286 Comm: syz-executor.1 Not tainted 4.14.261-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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:ffff888096ee75e8 EFLAGS: 00010282
RAX: 0000000000000026 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 000000000000d18a RSI: ffffffff81442c40 RDI: ffffed1012ddceb3
RBP: ffff88809f91d858 R08: 0000000000000026 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
R13: 0000000000000000 R14: 0000000000000000 R15: ffff88809f91d868
 refcount_dec_and_mutex_lock+0x1a/0x60 lib/refcount.c:309
 nbd_put drivers/block/nbd.c:228 [inline]
 nbd_config_put+0x506/0x810 drivers/block/nbd.c:1172
 nbd_genl_connect+0xf6c/0x1400 drivers/block/nbd.c:1894
 genl_family_rcv_msg+0x572/0xb20 net/netlink/genetlink.c:600
 genl_rcv_msg+0xaf/0x140 net/netlink/genetlink.c:625
 netlink_rcv_skb+0x125/0x390 net/netlink/af_netlink.c:2446
 genl_rcv+0x24/0x40 net/netlink/genetlink.c:636
 netlink_unicast_kernel net/netlink/af_netlink.c:1294 [inline]
 netlink_unicast+0x437/0x610 net/netlink/af_netlink.c:1320
 netlink_sendmsg+0x648/0xbc0 net/netlink/af_netlink.c:1891
 sock_sendmsg_nosec net/socket.c:646 [inline]
 sock_sendmsg+0xb5/0x100 net/socket.c:656
 ___sys_sendmsg+0x6c8/0x800 net/socket.c:2062
 __sys_sendmsg+0xa3/0x120 net/socket.c:2096
 SYSC_sendmsg net/socket.c:2107 [inline]
 SyS_sendmsg+0x27/0x40 net/socket.c:2103
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x46/0xbb
RIP: 0033:0x7f68975bde99
RSP: 002b:00007f6895f33168 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f68976d0f60 RCX: 00007f68975bde99
RDX: 0000000000040840 RSI: 0000000020000540 RDI: 0000000000000005
RBP: 00007f6897617ff1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffe16d04aff R14: 00007f6895f33300 R15: 0000000000022000
Kernel Offset: disabled
Rebooting in 86400 seconds..