BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff8359d08c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD 1d9b25067 PUD 1da343067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 8272 Comm: syz-executor4 Not tainted 4.4.138-g226f96b #63
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800bb268000 task.stack: ffff8801b3468000
RIP: 0010:[<ffffffff8359d08c>]  [<ffffffff8359d08c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8801b346f8c8  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8800b5138780 RCX: 0000000000000000
RDX: 1ffff10015e33350 RSI: ffffffff8359d061 RDI: ffff8800af199a80
RBP: ffff8801b346f8e8 R08: ffffffff853012f0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800af199900
R13: ffff8800b5138788 R14: 0000000000000000 R15: ffffffff82f1a2b0
FS:  00007f864c82f700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001da219000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d0ca7700 ffff8800b5138780 ffff8801d0ca7ad0 ffff8801d0ca79d8
 ffff8801b346f910 ffffffff835a6b07 ffff8801d0ca7700 ffffffff835a6a30
 ffff8801ca7888a0 ffff8801b346f938 ffffffff82f2f31c ffff8801d0ca7700
Call Trace:
 [<ffffffff835a6b07>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835a6b07>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f2f31c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f2f7df>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f2f9e0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a78da>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a78da>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f1a186>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f1a2c6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522e05>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523345>] ____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
 [<ffffffff8100df8b>] do_signal+0x8b/0x1d30 arch/x86/kernel/signal.c:712
 [<ffffffff8100360a>] exit_to_usermode_loop+0x11a/0x160 arch/x86/entry/common.c:248
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:283 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:348
 [<ffffffff838c28b5>] int_ret_from_sys_call+0x25/0xa3
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 15 44 db fd e8 10 44 db fd 4c 
RIP  [<ffffffff8359d08c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8801b346f8c8>
CR2: 0000000000000080
---[ end trace 75f5be7319fae3c3 ]---