refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 1 PID: 13528 at lib/refcount.c:153 refcount_inc /lib/refcount.c:153 [inline]
WARNING: CPU: 1 PID: 13528 at lib/refcount.c:153 refcount_inc.cold+0x18/0x1f /lib/refcount.c:151
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 13528 Comm: syz-executor.0 Not tainted 4.14.133 #28
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+0x138/0x19c /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:960
RIP: 0010:refcount_inc /lib/refcount.c:153 [inline]
RIP: 0010:refcount_inc.cold+0x18/0x1f /lib/refcount.c:151
RSP: 0018:ffff8880544efb70 EFLAGS: 00010286
RAX: 000000000000002b RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000009086 RSI: ffffffff814afc95 RDI: ffffed100a89df64
RBP: ffff8880544efb78 R08: 000000000000002b R09: ffff8880558b8bc8
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88808b997100
R13: 0000000000000002 R14: ffff88809be87a60 R15: ffff88809be87a40
 sock_hold /./include/net/sock.h:619 [inline]
 l2tp_session_add_to_tunnel /net/l2tp/l2tp_core.c:374 [inline]
 l2tp_session_create+0xb49/0x1600 /net/l2tp/l2tp_core.c:1846
 pppol2tp_connect+0x11bf/0x18b0 /net/l2tp/l2tp_ppp.c:721
 SYSC_connect+0x1f6/0x2d0 /net/socket.c:1655
 SyS_connect+0x24/0x30 /net/socket.c:1636
 do_syscall_64+0x1e8/0x640 /arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x459819
RSP: 002b:00007f05b46bac78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459819
RDX: 0000000000000026 RSI: 0000000020000180 RDI: 0000000000000006
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f05b46bb6d4
R13: 00000000004bfd8b R14: 00000000004d1948 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..