======================================
| [    3.533777] ------------[ cut here ]------------
| [    3.533848] refcount_t: underflow; use-after-free.
| [    3.533912] WARNING: CPU: 3 PID: 38 at lib/refcount.c:28 refcount_warn_saturate (lib/refcount.c:28 (discriminator 3))
| [    3.533988] Modules linked in: xfrm_user l2tp_ip6 l2tp_eth l2tp_ip l2tp_netlink l2tp_core
[    3.534128] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[    3.534181] Workqueue: l2tp l2tp_session_del_work [l2tp_core]
[    3.534239] RIP: 0010:refcount_warn_saturate (lib/refcount.c:28 (discriminator 3))
[    3.534282] Code: a2 e8 e8 54 ba ff 90 0f 0b 90 90 c3 80 3d 00 e6 2b 01 00 75 89 c6 05 f7 e5 2b 01 01 90 48 c7 c7 f8 48 f5 a2 e8 c5 54 ba ff 90 <0f> 0b 90 90 c3 80 3d db e5 2b 01 00 0f 85 62 ff ff ff c6 05 ce e5
All code
========
   0:	a2 e8 e8 54 ba ff 90 	movabs %al,0xb0f90ffba54e8e8
   7:	0f 0b 
   9:	90                   	nop
   a:	90                   	nop
   b:	c3                   	ret
   c:	80 3d 00 e6 2b 01 00 	cmpb   $0x0,0x12be600(%rip)        # 0x12be613
  13:	75 89                	jne    0xffffffffffffff9e
  15:	c6 05 f7 e5 2b 01 01 	movb   $0x1,0x12be5f7(%rip)        # 0x12be613
  1c:	90                   	nop
  1d:	48 c7 c7 f8 48 f5 a2 	mov    $0xffffffffa2f548f8,%rdi
  24:	e8 c5 54 ba ff       	call   0xffffffffffba54ee
  29:	90                   	nop
  2a:*	0f 0b                	ud2		<-- trapping instruction
  2c:	90                   	nop
  2d:	90                   	nop
  2e:	c3                   	ret
  2f:	80 3d db e5 2b 01 00 	cmpb   $0x0,0x12be5db(%rip)        # 0x12be611
  36:	0f 85 62 ff ff ff    	jne    0xffffffffffffff9e
  3c:	c6                   	.byte 0xc6
  3d:	05                   	.byte 0x5
  3e:	ce                   	(bad)
  3f:	e5                   	.byte 0xe5

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	90                   	nop
   3:	90                   	nop
   4:	c3                   	ret
   5:	80 3d db e5 2b 01 00 	cmpb   $0x0,0x12be5db(%rip)        # 0x12be5e7
   c:	0f 85 62 ff ff ff    	jne    0xffffffffffffff74
  12:	c6                   	.byte 0xc6
  13:	05                   	.byte 0x5
  14:	ce                   	(bad)
  15:	e5                   	.byte 0xe5
[    3.534414] RSP: 0018:ffffbc41c015bda8 EFLAGS: 00010286
[    3.534457] RAX: 0000000000000000 RBX: ffffbc41c015bdb8 RCX: 00000000ffffdfff
[    3.534516] RDX: 0000000000000000 RSI: 00000000ffffffea RDI: 0000000000000001
[    3.534570] RBP: ffff93098633e000 R08: ffffffffa375ba28 R09: 00000000ffffdfff
[    3.534630] R10: ffffffffa367ba40 R11: ffffffffa372ba40 R12: 0000000000000001
[    3.534698] R13: 00000000fffb7962 R14: 0000000000000001 R15: ffff93098633e000
[    3.534770] FS:  0000000000000000(0000) GS:ffff930a1af11000(0000) knlGS:0000000000000000
[    3.534837] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    3.534889] CR2: 00005586c22bfdec CR3: 00000000060b4004 CR4: 0000000000772ef0
[    3.534944] PKRU: 55555554
[    3.534967] Call Trace:
[    3.534987]  <TASK>
[    3.535011]  netdev_run_todo (./include/linux/list.h:381 net/core/dev.c:11670)
[    3.535052] l2tp_eth_delete (net/l2tp/l2tp_eth.c:158 net/l2tp/l2tp_eth.c:145) l2tp_eth
[    3.535093] l2tp_session_del_work (net/l2tp/l2tp_core.c:1748) l2tp_core
[    3.535135]  ? complete (./include/linux/list.h:226 ./include/linux/list.h:295 kernel/sched/swait.c:31 kernel/sched/swait.c:22 kernel/sched/completion.c:29 kernel/sched/completion.c:52)
[    3.535174]  process_one_work (kernel/workqueue.c:3268)
[    3.535214]  worker_thread (kernel/workqueue.c:3340 kernel/workqueue.c:3427)
[    3.535248]  ? rescuer_thread (kernel/workqueue.c:3373)
[    3.535281]  kthread (kernel/kthread.c:463)
[    3.535315]  ? kthreads_online_cpu (kernel/kthread.c:412)
[    3.535358]  ? kthreads_online_cpu (kernel/kthread.c:412)
[    3.535400]  ret_from_fork (arch/x86/kernel/process.c:164)
[    3.535439]  ? kthreads_online_cpu (kernel/kthread.c:412)


Finger prints:
refcount_warn_saturate:netdev_run_todo:l2tp_eth_delete:l2tp_session_del_work:process_one_work