TCP: request_sock_TCPv6: Possible SYN flooding on port 20002. Sending cookies.  Check SNMP counters.
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff8359d19c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD 1c884e067 PUD 1c70a6067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 12305 Comm: syz-executor3 Not tainted 4.4.136-ge4798d7 #58
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d6cbb000 task.stack: ffff8801d9610000
RIP: 0010:[<ffffffff8359d19c>]  [<ffffffff8359d19c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8801d9617880  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8800b5f8ef00 RCX: 0000000000000000
RDX: 1ffff10038f09080 RSI: ffffffff8359d171 RDI: ffff8801c7848400
RBP: ffff8801d96178a0 R08: ffffffff853504f0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801c7848280
R13: ffff8800b5f8ef08 R14: 0000000000000000 R15: ffffffff82f1a280
FS:  0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001d7c09000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8800b461b300 ffff8800b5f8ef00 ffff8800b461b6d0 ffff8800b461b5d8
 ffff8801d96178c8 ffffffff835a6c17 ffff8800b461b300 ffffffff835a6b40
 ffff8800ab758ce0 ffff8801d96178f0 ffffffff82f2f2ec ffff8800b461b300
Call Trace:
 [<ffffffff835a6c17>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835a6c17>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f2f2ec>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2f7af>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f2f9b0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a79ea>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a79ea>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f1a156>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f1a296>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522da5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff815232e5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bd7f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff81135285>] exit_task_work include/linux/task_work.h:21 [inline]
 [<ffffffff81135285>] do_exit+0x9e5/0x26b0 kernel/exit.c:759
 [<ffffffff8113b1d1>] do_group_exit+0x111/0x330 kernel/exit.c:889
 [<ffffffff8115e5cc>] get_signal+0x4ec/0x14b0 kernel/signal.c:2321
 [<ffffffff8100e02b>] do_signal+0x8b/0x1d30 arch/x86/kernel/signal.c:712
 [<ffffffff8100360a>] exit_to_usermode_loop+0x11a/0x160 arch/x86/entry/common.c:248
 [<ffffffff81007090>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81007090>] syscall_return_slowpath arch/x86/entry/common.c:348 [inline]
 [<ffffffff81007090>] do_syscall_32_irqs_on arch/x86/entry/common.c:398 [inline]
 [<ffffffff81007090>] do_fast_syscall_32+0x620/0x8b0 arch/x86/entry/common.c:459
 [<ffffffff838c416a>] sysenter_flags_fixed+0xd/0x17
Code: 49 8d bc 24 80 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 d0 00 00 00 4d 8b b4 24 80 01 00 00 <f0> 41 ff 8e 80 00 00 00 74 64 e8 05 43 db fd e8 00 43 db fd 4c 
RIP  [<ffffffff8359d19c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8801d9617880>
CR2: 0000000000000080
---[ end trace 7f657cd5e15e42ef ]---