=================================
[ INFO: inconsistent lock state ]
4.9.124-g09eb2ba #31 Tainted: G        W      
---------------------------------
inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
syz-executor5/19726 [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_connect+0xd9/0x18f0 net/l2tp/l2tp_ppp.c:590
  SYSC_connect+0x1b8/0x300 net/socket.c:1575
  SyS_connect+0x24/0x30 net/socket.c:1556
  do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
  entry_SYSCALL_64_after_swapgs+0x5d/0xdb
irq event stamp: 842
hardirqs last  enabled at (842): [<ffffffff83a02369>] restore_regs_and_iret+0x0/0x1d
hardirqs last disabled at (841): [<ffffffff83a0332b>] apic_timer_interrupt+0x9b/0xb0 arch/x86/entry/entry_64.S:648
softirqs last  enabled at (572): [<ffffffff8302bdeb>] spin_unlock_bh include/linux/spinlock.h:352 [inline]
softirqs last  enabled at (572): [<ffffffff8302bdeb>] __release_sock+0xcb/0x390 net/core/sock.c:2052
softirqs last disabled at (631): [<ffffffff8114d554>] invoke_softirq kernel/softirq.c:368 [inline]
softirqs last disabled at (631): [<ffffffff8114d554>] irq_exit+0x114/0x150 kernel/softirq.c:409

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 ***

3 locks held by syz-executor5/19726:
 #0:  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff8337342d>] lock_sock include/net/sock.h:1404 [inline]
 #0:  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff8337342d>] tcp_sendmsg+0xbd/0x3040 net/ipv4/tcp.c:1140
 #1:  (rcu_read_lock){......}, at: [<ffffffff8362746f>] inet6_csk_xmit+0xff/0x4d0 net/ipv6/inet6_connection_sock.c:165
 #2:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_reclaim kernel/rcu/rcu.h:108 [inline]
 #2:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_do_batch kernel/rcu/tree.c:2789 [inline]
 #2:  (rcu_callback){......}, at: [<ffffffff81289ace>] invoke_rcu_callbacks kernel/rcu/tree.c:3053 [inline]
 #2:  (rcu_callback){......}, at: [<ffffffff81289ace>] __rcu_process_callbacks kernel/rcu/tree.c:3020 [inline]
 #2:  (rcu_callback){......}, at: [<ffffffff81289ace>] rcu_process_callbacks+0x98e/0x12b0 kernel/rcu/tree.c:3037

stack backtrace:
CPU: 1 PID: 19726 Comm: syz-executor5 Tainted: G        W       4.9.124-g09eb2ba #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
 ffff8801db307a58 ffffffff81eb95e9 ffff8801c1f2e000 ffffffff855f7b90
 ffff8801c1f2e940 ffff8801c1f2e960 0000000000000000 ffff8801db307ac8
 ffffffff81429ccd 0000000000000003 0000000000000001 ffff880100000000
Call Trace:
 <IRQ> [  277.720784]  [<ffffffff81eb95e9>] __dump_stack lib/dump_stack.c:15 [inline]
 <IRQ> [  277.720784]  [<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
 [<ffffffff8114d554>] invoke_softirq kernel/softirq.c:368 [inline]
 [<ffffffff8114d554>] irq_exit+0x114/0x150 kernel/softirq.c:409
 [<ffffffff83a071a1>] exiting_irq arch/x86/include/asm/apic.h:669 [inline]
 [<ffffffff83a071a1>] smp_apic_timer_interrupt+0x81/0xa0 arch/x86/kernel/apic/apic.c:962
 [<ffffffff83a03330>] apic_timer_interrupt+0xa0/0xb0 arch/x86/entry/entry_64.S:648
 <EOI> [  277.922051]  [<ffffffff836835a6>] ? ip6_dst_hoplimit+0x86/0x370 net/ipv6/output_core.c:129
 [<ffffffff836835d0>] rcu_read_lock include/linux/rcupdate.h:873 [inline]
 [<ffffffff836835d0>] ip6_dst_hoplimit+0xb0/0x370 net/ipv6/output_core.c:132
 [<ffffffff83561889>] ip6_xmit+0xd69/0x1b80 net/ipv6/ip6_output.c:236
 [<ffffffff836275ec>] inet6_csk_xmit+0x27c/0x4d0 net/ipv6/inet6_connection_sock.c:178
 [<ffffffff833acd1f>] __tcp_transmit_skb+0x16af/0x2e00 net/ipv4/tcp_output.c:1041
 [<ffffffff833bc92f>] tcp_transmit_skb net/ipv4/tcp_output.c:1057 [inline]
 [<ffffffff833bc92f>] tcp_send_synack+0x76f/0xab0 net/ipv4/tcp_output.c:3069
 [<ffffffff833a0b4d>] tcp_rcv_synsent_state_process net/ipv4/tcp_input.c:5918 [inline]
 [<ffffffff833a0b4d>] tcp_rcv_state_process+0x1b0d/0x4160 net/ipv4/tcp_input.c:5997
 [<ffffffff83609ebd>] tcp_v6_do_rcv+0x79d/0x1340 net/ipv6/tcp_ipv6.c:1310
 [<ffffffff8302be5a>] sk_backlog_rcv include/net/sock.h:871 [inline]
 [<ffffffff8302be5a>] __release_sock+0x13a/0x390 net/core/sock.c:2059
 [<ffffffff8302c109>] release_sock+0x59/0x1c0 net/core/sock.c:2520
 [<ffffffff8342a341>] inet_wait_for_connect net/ipv4/af_inet.c:566 [inline]
 [<ffffffff8342a341>] __inet_stream_connect+0x471/0xbf0 net/ipv4/af_inet.c:652
 [<ffffffff833750a2>] tcp_sendmsg_fastopen net/ipv4/tcp.c:1116 [inline]
 [<ffffffff833750a2>] tcp_sendmsg+0x1d32/0x3040 net/ipv4/tcp.c:1145
 [<ffffffff83431af3>] inet_sendmsg+0x203/0x4d0 net/ipv4/af_inet.c:770
 [<ffffffff8301e0ac>] sock_sendmsg_nosec net/socket.c:648 [inline]
 [<ffffffff8301e0ac>] sock_sendmsg+0xcc/0x110 net/socket.c:658
 [<ffffffff8301ed9c>] SYSC_sendto+0x21c/0x370 net/socket.c:1683
 [<ffffffff830213c0>] SyS_sendto+0x40/0x50 net/socket.c:1651
 [<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
 [<ffffffff83a019d3>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff836c473c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1765
PGD 1d4e4f067 [  278.269977] PUD 1a2140067 
Oops: 0002 [#1] PREEMPT SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 19722 Comm: syz-executor3 Tainted: G        W       4.9.124-g09eb2ba #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801c4116000 task.stack: ffff8801d3cd8000
RIP: 0010:[<ffffffff836c473c>]  [<ffffffff836c473c>] l2tp_session_free+0x11c/0x200 net/l2tp/l2tp_core.c:1765
RSP: 0018:ffff8801d3cdfc98  EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801c1e35180 RCX: 0000000000000000
RDX: 1ffff10038c95080 RSI: ffffffff836c4711 RDI: ffff8801c64a8400
RBP: ffff8801d3cdfcb8 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801c64a8280
R13: ffff8801c1e35188 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000001ee2940(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 00000001bfaa4000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
 ffff8801cb219100 ffff8801c1e35180 ffff8801cb2194d0 ffff8801cb2193d8
 ffff8801d3cdfce0 ffffffff836cc022 ffff8801cb219540 ffff8801cb219100
 ffffffff836cbf50 ffff8801d3cdfd18 ffffffff830281f5 ffff8801cb219540
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 <ffff8801d3cdfc98>
CR2: 0000000000000080
---[ end trace 4a1f1adfa55dd093 ]---