BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff835a6e8c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD b7615067 PUD b9235067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 11915 Comm: syz-executor6 Not tainted 4.4.152-ge5c5f1f #89
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800a4208000 task.stack: ffff8800b3160000
RIP: 0010:[<ffffffff835a6e8c>]  [<ffffffff835a6e8c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8800b3167cd0  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801d8575900 RCX: 0000000000000000
RDX: 1ffff100169879c0 RSI: ffffffff835a6e61 RDI: ffff8800b4c3ce00
RBP: ffff8800b3167cf0 R08: ffff8800a4208900 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800b4c3cc80
R13: ffff8801d8575908 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000bd8940(0063) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000000b1add000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801bdd14c80 ffff8801d8575900 ffff8801bdd15050 ffff8801bdd14f58
 ffff8800b3167d18 ffffffff835b0907 ffff8801bdd14c80 ffffffff835b0830
 ffff8801bb27aee0 ffff8800b3167d40 ffffffff82f3839c ffff8801bdd14c80
Call Trace:
 [<ffffffff835b0907>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835b0907>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f3839c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f3885f>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f38a60>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835b16da>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835b16da>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f210c9>] __sock_release+0xd9/0x260 net/socket.c:592
 [<ffffffff82f21269>] sock_close+0x19/0x20 net/socket.c:1050
 [<ffffffff81528875>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81528db5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118ee4f>] task_work_run+0x10f/0x190 kernel/task_work.c:115
 [<ffffffff8100362d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100362d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:253
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:349
 [<ffffffff838cc6f5>] 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 a5 d6 da fd e8 a0 d6 da fd 4c 
RIP  [<ffffffff835a6e8c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8800b3167cd0>
CR2: 0000000000000080
---[ end trace cfb3d5b880a81f53 ]---