======================================
| [    4.177745] ------------[ cut here ]------------
| [    4.177833] refcount_t: underflow; use-after-free.
| [    4.177912] WARNING: CPU: 3 PID: 322 at lib/refcount.c:28 refcount_warn_saturate (lib/refcount.c:28 (discriminator 3))
| [    4.178012] Modules linked in: xfrm_user l2tp_ip6 l2tp_eth l2tp_ip l2tp_netlink l2tp_core
[    4.178213] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[    4.178280] Workqueue: l2tp l2tp_session_del_work [l2tp_core]
[    4.178361] RIP: 0010:refcount_warn_saturate (lib/refcount.c:28 (discriminator 3))
[    4.178423] Code: 95 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 95 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:	95                   	xchg   %eax,%ebp
   1:	e8 e8 54 ba ff       	call   0xffffffffffba54ee
   6:	90                   	nop
   7:	0f 0b                	ud2
   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 95 	mov    $0xffffffff95f548f8,%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
[    4.178606] RSP: 0018:ffffbac9c078fda8 EFLAGS: 00010286
[    4.178659] RAX: 0000000000000000 RBX: ffffbac9c078fdb8 RCX: 00000000ffffdfff
[    4.178742] RDX: 0000000000000000 RSI: 00000000ffffffea RDI: 0000000000000001
[    4.178822] RBP: ffffbac9c078f8b0 R08: ffffffff9675ba28 R09: 00000000ffffdfff
[    4.178904] R10: ffffffff9667ba40 R11: ffffffff9672ba40 R12: dead000000000122
[    4.178986] R13: 00000000fffb7bee R14: 0000000000000001 R15: ffff983a0871e000
[    4.179073] FS:  0000000000000000(0000) GS:ffff983aa7f11000(0000) knlGS:0000000000000000
[    4.179162] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    4.179239] CR2: 00007fff52184938 CR3: 0000000008598001 CR4: 0000000000772ef0
[    4.179326] PKRU: 55555554
[    4.179352] Call Trace:
[    4.179384]  <TASK>
[    4.179420]  netdev_run_todo (./include/linux/list.h:381 net/core/dev.c:11670)
[    4.179475] l2tp_eth_delete (net/l2tp/l2tp_eth.c:158 net/l2tp/l2tp_eth.c:145) l2tp_eth
[    4.179541] l2tp_session_del_work (net/l2tp/l2tp_core.c:1748) l2tp_core
[    4.179606]  process_one_work (kernel/workqueue.c:3268)
[    4.179669]  worker_thread (kernel/workqueue.c:3340 kernel/workqueue.c:3427)
[    4.179725]  ? rescuer_thread (kernel/workqueue.c:3373)
[    4.179771]  kthread (kernel/kthread.c:463)
[    4.179823]  ? kthreads_online_cpu (kernel/kthread.c:412)
[    4.179882]  ? kthreads_online_cpu (kernel/kthread.c:412)
[    4.179934]  ret_from_fork (arch/x86/kernel/process.c:164)
[    4.179998]  ? 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