refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 633 at lib/refcount.c:153 refcount_inc lib/refcount.c:153 [inline]
WARNING: CPU: 0 PID: 633 at lib/refcount.c:153 refcount_inc.cold+0x18/0x1f lib/refcount.c:151
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 633 Comm: kworker/u4:3 Not tainted 4.14.146 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: l2tp l2tp_tunnel_del_work
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x138/0x197 lib/dump_stack.c:53
 panic+0x1f2/0x426 kernel/panic.c:182
 __warn.cold+0x2f/0x36 kernel/panic.c:546
 report_bug+0x216/0x254 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:177 [inline]
 fixup_bug arch/x86/kernel/traps.c:172 [inline]
 do_error_trap+0x1bb/0x310 arch/x86/kernel/traps.c:295
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:314
 invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:963
RIP: 0010:refcount_inc lib/refcount.c:153 [inline]
RIP: 0010:refcount_inc.cold+0x18/0x1f lib/refcount.c:151
RSP: 0018:ffff8880a8e97c48 EFLAGS: 00010286
RAX: 000000000000002b RBX: ffff888098bb9480 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff866d10e0 RDI: ffffed10151d2f7f
RBP: ffff8880a8e97c50 R08: 000000000000002b R09: ffff8880a8dfea30
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888098bb9500
R13: ffff888093a6b780 R14: ffff8880a52b32e0 R15: ffff8880a52b32a0
 sock_hold include/net/sock.h:619 [inline]
 pppol2tp_session_sock_hold+0x48/0x60 net/l2tp/l2tp_ppp.c:263
 l2tp_tunnel_closeall+0x1ef/0x380 net/l2tp/l2tp_core.c:1332
 l2tp_tunnel_del_work+0x64/0x410 net/l2tp/l2tp_core.c:1380
 process_one_work+0x863/0x1600 kernel/workqueue.c:2114
 worker_thread+0x5d9/0x1050 kernel/workqueue.c:2248
 kthread+0x319/0x430 kernel/kthread.c:232
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404
Kernel Offset: disabled
Rebooting in 86400 seconds..