BUG: sleeping function called from invalid context at net/core/sock.c:2502
in_atomic(): 1, irqs_disabled(): 0, pid: 17, name: ksoftirqd/1
1 lock held by ksoftirqd/1/17:
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_reclaim kernel/rcu/rcu.h:108 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_process_callbacks+0x98e/0x12b0 kernel/rcu/tree.c:3037
Preemption disabled at:[ 1265.422277] [<ffffffff83a0848d>] __do_softirq+0xdd/0x940 kernel/softirq.c:265
CPU: 1 PID: 17 Comm: ksoftirqd/1 Not tainted 4.9.124-g09eb2ba #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d9abfa60 ffffffff81eb95e9 ffffffff83a0848d 0000000000000000
 0000000000000100 ffff8801d9aa3000 ffff8801d9aa3000 ffff8801d9abfa98
 ffffffff81426851 ffff8801d9aa3000 ffffffff840f88a0 00000000000009c6
Call Trace:
 [<ffffffff81eb95e9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb95e9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81426851>] ___might_sleep.cold.123+0x1bc/0x1f5 kernel/sched/core.c:7988
 [<ffffffff811bd9a5>] __might_sleep+0x95/0x1a0 kernel/sched/core.c:7945
 [<ffffffff8302b774>] lock_sock_nested+0x34/0x120 net/core/sock.c:2502
 [<ffffffff83428dd9>] lock_sock include/net/sock.h:1404 [inline]
 [<ffffffff83428dd9>] inet_shutdown+0x69/0x360 net/ipv4/af_inet.c:823
 [<ffffffff836cb740>] pppol2tp_session_close+0xa0/0xe0 net/l2tp/l2tp_ppp.c:441
 [<ffffffff836c4f71>] l2tp_tunnel_closeall+0x231/0x350 net/l2tp/l2tp_core.c:1368
 [<ffffffff836c57f2>] l2tp_tunnel_destruct+0x2f2/0x590 net/l2tp/l2tp_core.c:1324
 [<ffffffff830281f5>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff812899ee>] __rcu_reclaim kernel/rcu/rcu.h:118 [inline]
 [<ffffffff812899ee>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 [<ffffffff812899ee>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 [<ffffffff812899ee>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 [<ffffffff812899ee>] rcu_process_callbacks+0x8ae/0x12b0 kernel/rcu/tree.c:3037
 [<ffffffff83a085c0>] __do_softirq+0x210/0x940 kernel/softirq.c:288
 [<ffffffff8114d320>] run_ksoftirqd+0x30/0x60 kernel/softirq.c:680
 [<ffffffff811aa5a9>] smpboot_thread_fn+0x5c9/0x900 kernel/smpboot.c:163
 [<ffffffff8119f3cd>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff83a01b9c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373

=================================
[ INFO: inconsistent lock state ]
4.9.124-g09eb2ba #31 Tainted: G        W      
---------------------------------
inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
ksoftirqd/1/17 [HC0[0]:SC1[3]:HE1:SE0] takes:
 (sk_lock-AF_PPPOX){+.?.+.}, at: [<ffffffff83428dd9>] lock_sock include/net/sock.h:1404 [inline]
 (sk_lock-AF_PPPOX){+.?.+.}, at: [<ffffffff83428dd9>] inet_shutdown+0x69/0x360 net/ipv4/af_inet.c:823
  mark_held_locks+0xc7/0x130 kernel/locking/lockdep.c:2660
  __trace_hardirqs_on_caller kernel/locking/lockdep.c:2689 [inline]
  trace_hardirqs_on_caller+0x38b/0x590 kernel/locking/lockdep.c:2736
  trace_hardirqs_on+0xd/0x10 kernel/locking/lockdep.c:2743
  __local_bh_enable_ip+0x6a/0xd0 kernel/softirq.c:190
  local_bh_enable include/linux/bottom_half.h:31 [inline]
  lock_sock_nested+0xdc/0x120 net/core/sock.c:2512
  lock_sock include/net/sock.h:1404 [inline]
  pppol2tp_release+0x50/0x2e0 net/l2tp/l2tp_ppp.c:476
  __sock_release+0xd7/0x260 net/socket.c:605
  sock_close+0x19/0x20 net/socket.c:1059
  __fput+0x263/0x700 fs/file_table.c:208
  ____fput+0x15/0x20 fs/file_table.c:244
  task_work_run+0x10c/0x180 kernel/task_work.c:116
  tracehook_notify_resume include/linux/tracehook.h:191 [inline]
  exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
  prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
  syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
  do_syscall_64+0x364/0x490 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_swapgs+0x5d/0xdb
irq event stamp: 9887730
hardirqs last  enabled at (9887730): [<ffffffff83a02369>] restore_regs_and_iret+0x0/0x1d
hardirqs last disabled at (9887729): [<ffffffff83a022db>] common_interrupt+0x9b/0xa0 arch/x86/entry/entry_64.S:461
softirqs last  enabled at (9886908): [<ffffffff83a08825>] __do_softirq+0x475/0x940 kernel/softirq.c:314
softirqs last disabled at (9886913): [<ffffffff8114d320>] run_ksoftirqd+0x30/0x60 kernel/softirq.c:680

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(sk_lock-AF_PPPOX);
  <Interrupt>
    lock(sk_lock-AF_PPPOX);

 *** DEADLOCK ***

1 lock held by ksoftirqd/1/17:
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_reclaim kernel/rcu/rcu.h:108 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 #0:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_process_callbacks+0x98e/0x12b0 kernel/rcu/tree.c:3037

stack backtrace:
CPU: 1 PID: 17 Comm: ksoftirqd/1 Tainted: G        W       4.9.124-g09eb2ba #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801d9abf7d8 ffffffff81eb95e9 ffff8801d9aa3000 ffffffff85602960
 ffff8801d9aa38f0 ffff8801d9aa3910 0000000000000000 ffff8801d9abf848
 ffffffff81429ccd 0000000000000003 0000000000000001 ffff880100000000
Call Trace:
 [<ffffffff81eb95e9>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81eb95e9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff81429ccd>] print_usage_bug.cold.57+0x327/0x421 kernel/locking/lockdep.c:2387
 [<ffffffff81237726>] valid_state kernel/locking/lockdep.c:2400 [inline]
 [<ffffffff81237726>] mark_lock_irq kernel/locking/lockdep.c:2602 [inline]
 [<ffffffff81237726>] mark_lock+0xcc6/0x1280 kernel/locking/lockdep.c:3065
 [<ffffffff812390f0>] mark_irqflags kernel/locking/lockdep.c:2923 [inline]
 [<ffffffff812390f0>] __lock_acquire+0xd40/0x4070 kernel/locking/lockdep.c:3302
 [<ffffffff8123ce90>] lock_acquire+0x130/0x3e0 kernel/locking/lockdep.c:3756
 [<ffffffff8302b806>] lock_sock_nested+0xc6/0x120 net/core/sock.c:2511
 [<ffffffff83428dd9>] lock_sock include/net/sock.h:1404 [inline]
 [<ffffffff83428dd9>] inet_shutdown+0x69/0x360 net/ipv4/af_inet.c:823
 [<ffffffff836cb740>] pppol2tp_session_close+0xa0/0xe0 net/l2tp/l2tp_ppp.c:441
 [<ffffffff836c4f71>] l2tp_tunnel_closeall+0x231/0x350 net/l2tp/l2tp_core.c:1368
 [<ffffffff836c57f2>] l2tp_tunnel_destruct+0x2f2/0x590 net/l2tp/l2tp_core.c:1324
 [<ffffffff830281f5>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff812899ee>] __rcu_reclaim kernel/rcu/rcu.h:118 [inline]
 [<ffffffff812899ee>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 [<ffffffff812899ee>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 [<ffffffff812899ee>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 [<ffffffff812899ee>] rcu_process_callbacks+0x8ae/0x12b0 kernel/rcu/tree.c:3037
 [<ffffffff83a085c0>] __do_softirq+0x210/0x940 kernel/softirq.c:288
 [<ffffffff8114d320>] run_ksoftirqd+0x30/0x60 kernel/softirq.c:680
 [<ffffffff811aa5a9>] smpboot_thread_fn+0x5c9/0x900 kernel/smpboot.c:163
 [<ffffffff8119f3cd>] kthread+0x26d/0x300 kernel/kthread.c:211
 [<ffffffff83a01b9c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff836c473c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1765
PGD 1ab16c067 [ 1266.185885] PUD 19ba74067 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 7306 Comm: syz-executor1 Tainted: G        W       4.9.124-g09eb2ba #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cfd46000 task.stack: ffff8801ca050000
RIP: 0010:[<ffffffff836c473c>]  [<ffffffff836c473c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1765
RSP: 0018:ffff8801ca057c98  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801cd2bfb80 RCX: 0000000000000000
RDX: 1ffff1003b0253a0 RSI: ffffffff836c4711 RDI: ffff8801d8129d00
RBP: ffff8801ca057cb8 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801d8129b80
R13: ffff8801cd2bfb88 R14: 0000000000000000 R15: 0000000000000000
FS:  000000000201b940(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001cb4d0000 CR4: 00000000001606f0
DR0: 0000000020000000 DR1: 0000000020000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Stack:
 ffff8801a00c3300 ffff8801cd2bfb80 ffff8801a00c36d0 ffff8801a00c35d8
 ffff8801ca057ce0 ffffffff836cc022 ffff8801a00c3740 ffff8801a00c3300
 ffffffff836cbf50 ffff8801ca057d18 ffffffff830281f5 ffff8801a00c3740
Call Trace:
 [<ffffffff836cc022>] l2tp_session_dec_refcount_1 net/l2tp/l2tp_core.h:297 [inline]
 [<ffffffff836cc022>] pppol2tp_session_destruct+0xd2/0x110 net/l2tp/l2tp_ppp.c:460
 [<ffffffff830281f5>] __sk_destruct+0x55/0x590 net/core/sock.c:1428
 [<ffffffff8302fc83>] sk_destruct+0x63/0x80 net/core/sock.c:1463
 [<ffffffff8302fcef>] __sk_free+0x4f/0x220 net/core/sock.c:1471
 [<ffffffff8302feeb>] sk_free+0x2b/0x40 net/core/sock.c:1482
 [<ffffffff836cf329>] sock_put include/net/sock.h:1588 [inline]
 [<ffffffff836cf329>] pppol2tp_release+0x239/0x2e0 net/l2tp/l2tp_ppp.c:501
 [<ffffffff83018357>] __sock_release+0xd7/0x260 net/socket.c:605
 [<ffffffff830184f9>] sock_close+0x19/0x20 net/socket.c:1059
 [<ffffffff8157ca73>] __fput+0x263/0x700 fs/file_table.c:208
 [<ffffffff8157cf95>] ____fput+0x15/0x20 fs/file_table.c:244
 [<ffffffff8119a66c>] task_work_run+0x10c/0x180 kernel/task_work.c:116
 [<ffffffff8100559c>] tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 [<ffffffff8100559c>] exit_to_usermode_loop+0xfc/0x120 arch/x86/entry/common.c:161
 [<ffffffff810064d4>] prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 [<ffffffff810064d4>] syscall_return_slowpath arch/x86/entry/common.c:260 [inline]
 [<ffffffff810064d4>] do_syscall_64+0x364/0x490 arch/x86/entry/common.c:287
 [<ffffffff83a019d3>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
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 c6 00 00 00 4d 8b b4 24 80 01 00 00 <f0> 41 ff 8e 80 00 00 00 74 69 e8 a5 c5 c9 fd 4c 89 ea 48 b8 00 
RIP  [<ffffffff836c473c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1765
 RSP <ffff8801ca057c98>
CR2: 0000000000000080
---[ end trace 73c79503a7f34aa5 ]---
------------[ cut here ]------------
WARNING: CPU: 0 PID: 7317 at net/ipv4/af_inet.c:167 inet_sock_destruct+0x598/0x760 net/ipv4/af_inet.c:167