BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff8359f6ac>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD 1c88d6067 PUD 1d20e9067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 3272 Comm: syz-executor7 Not tainted 4.4.143-g7bbfac1 #13
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801c6ac3000 task.stack: ffff8801d8b08000
RIP: 0010:[<ffffffff8359f6ac>]  [<ffffffff8359f6ac>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8801d8b0fc90  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8800b1b14c80 RCX: ffffc90000c3c000
RDX: 1ffff10039e0ba60 RSI: ffffffff8359f681 RDI: ffff8801cf05d300
RBP: ffff8801d8b0fcb0 R08: ffffffff8531f6b0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801cf05d180
R13: ffff8800b1b14c88 R14: 0000000000000000 R15: ffffffff82f1c220
FS:  0000000000000000(0000) GS:ffff8801db200000(0063) knlGS:00000000f57bcb40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001c8ba5000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801d6430880 ffff8800b1b14c80 ffff8801d6430c50 ffff8801d6430b58
 ffff8801d8b0fcd8 ffffffff835a9127 ffff8801d6430880 ffffffff835a9050
 ffff8800b9f34020 ffff8801d8b0fd00 ffffffff82f3128c ffff8801d6430880
Call Trace:
 [<ffffffff835a9127>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835a9127>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f3128c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f3174f>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f31950>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835a9efa>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835a9efa>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f1c0f6>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f1c236>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81522ed5>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81523415>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118bdbf>] 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
 [<ffffffff81007090>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff81007090>] syscall_return_slowpath arch/x86/entry/common.c:349 [inline]
 [<ffffffff81007090>] do_syscall_32_irqs_on arch/x86/entry/common.c:399 [inline]
 [<ffffffff81007090>] do_fast_syscall_32+0x620/0x8b0 arch/x86/entry/common.c:460
 [<ffffffff838c66ea>] 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 65 1e db fd e8 60 1e db fd 4c 
RIP  [<ffffffff8359f6ac>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8801d8b0fc90>
CR2: 0000000000000080
---[ end trace 019136ed6dbff8a9 ]---