BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff8345ac1c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1667
PGD 80000001cbe40067 PUD 1cbfab067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 3957 Comm: syzkaller971971 Not tainted 4.4.115-gaa856bd #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801d8810000 task.stack: ffff8801d8f50000
RIP: 0010:[<ffffffff8345ac1c>]  [<ffffffff8345ac1c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1667
RSP: 0018:ffff8801d8f57cd8  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801cbe8c780 RCX: ffffffff8345abf1
RDX: 1ffff10039518750 RSI: 0000000000000001 RDI: ffff8801ca8c3a80
RBP: ffff8801d8f57cf8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 1ffff1003b1eaf5c R12: ffff8801ca8c3900
R13: ffff8801cbe8c788 R14: 0000000000000000 R15: ffffffff82de7b90
FS:  00007fb1bad61700(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001caa3e000 CR4: 0000000000160670
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d89e3b80 ffff8801cbe8c780 ffff8801d89e3f50 ffff8801d89e3e58
 ffff8801d8f57d20 ffffffff83464a13 ffff8801d89e3b80 ffffffff83464940
 ffff8800aba24ce0 ffff8801d8f57d48 ffffffff82dfca9a ffff8801d89e3b80
Call Trace:
 [<ffffffff83464a13>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff83464a13>] pppol2tp_session_destruct+0xd3/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82dfca9a>] sk_destruct+0x4a/0x4c0 net/core/sock.c:1447
 [<ffffffff82dfcf67>] __sk_free+0x57/0x230 net/core/sock.c:1480
 [<ffffffff82dfd170>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff834658ea>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff834658ea>] pppol2tp_release+0x27a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82de7a3d>] sock_release+0x8d/0x1e0 net/socket.c:586
 [<ffffffff82de7ba6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff815236a3>] __fput+0x233/0x6d0 fs/file_table.c:208
 [<ffffffff81523bc5>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118b9e4>] task_work_run+0x104/0x180 kernel/task_work.c:115
 [<ffffffff8100361d>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100361d>] exit_to_usermode_loop+0x13d/0x160 arch/x86/entry/common.c:251
 [<ffffffff81006535>] prepare_exit_to_usermode arch/x86/entry/common.c:282 [inline]
 [<ffffffff81006535>] syscall_return_slowpath+0x1b5/0x1f0 arch/x86/entry/common.c:347
 [<ffffffff837745a9>] 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 c3 00 00 00 4d 8b b4 24 80 01 00 00 <f0> 41 ff 8e 80 00 00 00 74 64 e8 55 61 f0 fd e8 50 61 f0 fd 4c 
RIP  [<ffffffff8345ac1c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1667
 RSP <ffff8801d8f57cd8>
CR2: 0000000000000080
---[ end trace 18adfd4322ec3827 ]---