pktgen: kernel_thread() failed for cpu 0
pktgen: Cannot create thread for cpu 0 (-4)
pktgen: kernel_thread() failed for cpu 1
pktgen: Cannot create thread for cpu 1 (-4)
pktgen: Initialization failed for all threads
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
PGD 1c8038067 PUD 1d7e7f067 PMD 0 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 14860 Comm: syz-executor4 Not tainted 4.4.145-g2241aa9 #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8800bb0e1800 task.stack: ffff8801bb9a8000
RIP: 0010:[<ffffffff835a2dfc>]  [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
RSP: 0018:ffff8801bb9afc90  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8800ac8b6280 RCX: ffffc90003639000
RDX: 1ffff10015916dc0 RSI: ffffffff835a2dd1 RDI: ffff8800ac8b6e00
RBP: ffff8801bb9afcb0 R08: ffffffff85304470 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8800ac8b6c80
R13: ffff8800ac8b6288 R14: 0000000000000000 R15: ffffffff82f1eea0
FS:  0000000000000000(0000) GS:ffff8801db200000(0063) knlGS:00000000f5735b40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001ca43a000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801c92d7700 ffff8800ac8b6280 ffff8801c92d7ad0 ffff8801c92d79d8
 ffff8801bb9afcd8 ffffffff835ac877 ffff8801c92d7700 ffffffff835ac7a0
 ffff8800ab82e660 ffff8801bb9afd00 ffffffff82f33f1c ffff8801c92d7700
Call Trace:
 [<ffffffff835ac877>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:293 [inline]
 [<ffffffff835ac877>] pppol2tp_session_destruct+0xd7/0x110 net/l2tp/l2tp_ppp.c:477
 [<ffffffff82f33f1c>] sk_destruct+0x4c/0x4c0 net/core/sock.c:1447
 [<ffffffff82f343df>] __sk_free+0x4f/0x220 net/core/sock.c:1480
 [<ffffffff82f345e0>] sk_free+0x30/0x40 net/core/sock.c:1491
 [<ffffffff835ad64a>] sock_put include/net/sock.h:1639 [inline]
 [<ffffffff835ad64a>] pppol2tp_release+0x26a/0x310 net/l2tp/l2tp_ppp.c:518
 [<ffffffff82f1ed76>] sock_release+0x96/0x1c0 net/socket.c:586
 [<ffffffff82f1eeb6>] sock_close+0x16/0x20 net/socket.c:1037
 [<ffffffff81525155>] __fput+0x235/0x6f0 fs/file_table.c:208
 [<ffffffff81525695>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8118e00f>] 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
 [<ffffffff8100708e>] prepare_exit_to_usermode arch/x86/entry/common.c:284 [inline]
 [<ffffffff8100708e>] syscall_return_slowpath arch/x86/entry/common.c:349 [inline]
 [<ffffffff8100708e>] do_syscall_32_irqs_on arch/x86/entry/common.c:399 [inline]
 [<ffffffff8100708e>] do_fast_syscall_32+0x61e/0x8b0 arch/x86/entry/common.c:460
 [<ffffffff838c9e43>] sysenter_flags_fixed+0xd/0x1a
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 09 db fd e8 60 09 db fd 4c 
RIP  [<ffffffff835a2dfc>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1671
 RSP <ffff8801bb9afc90>
CR2: 0000000000000080
---[ end trace 8517a3f76fd14cc0 ]---