syzbot

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refcount_t: increment on 0; use-after-free.
WARNING: CPU: 1 PID: 3697 at lib/refcount.c:153 refcount_inc+0x47/0x50 lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 3697 Comm: syzkaller479670 Not tainted 4.15.0-rc8+ #174
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+0x194/0x257 lib/dump_stack.c:53
 panic+0x1e4/0x41c kernel/panic.c:183
 __warn+0x1dc/0x200 kernel/panic.c:547
 report_bug+0x211/0x2d0 lib/bug.c:184
 fixup_bug.part.11+0x37/0x80 arch/x86/kernel/traps.c:178
 fixup_bug arch/x86/kernel/traps.c:247 [inline]
 do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
 invalid_op+0x22/0x40 arch/x86/entry/entry_64.S:1085
RIP: 0010:refcount_inc+0x47/0x50 lib/refcount.c:153
RSP: 0018:ffff8801d9077788 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: ffff8801d94f48c0 RCX: ffffffff8159dade
RDX: 0000000000000000 RSI: 1ffff1003b20eeac RDI: ffff8801d9077490
RBP: ffff8801d9077790 R08: 1ffff1003b20ee6e R09: 0000000000000000
R10: ffff8801d9077658 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: ffff8801bc2540c0 R15: ffff8801d2cb6800
 sock_hold include/net/sock.h:620 [inline]
 l2tp_session_register+0x9cb/0xe20 net/l2tp/l2tp_core.c:348
 pppol2tp_connect+0x134b/0x1dd0 net/l2tp/l2tp_ppp.c:761
 SYSC_connect+0x213/0x4a0 net/socket.c:1621
 SyS_connect+0x24/0x30 net/socket.c:1602
 do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
 do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
 entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129
RIP: 0023:0xf7fbac79
RSP: 002b:00000000ff9c653c EFLAGS: 00000207 ORIG_RAX: 000000000000016a
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00000000205fafd2
RDX: 000000000000002e RSI: 0000000000000167 RDI: 0000000000000018
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..