======================================
| [  174.152220] ------------[ cut here ]------------
| [  174.152494] refcount_t: underflow; use-after-free.
| [  174.152759] WARNING: CPU: 1 PID: 68 at lib/refcount.c:28 refcount_warn_saturate (lib/refcount.c:28 (discriminator 3)) 
| [  174.153194] Modules linked in: xfrm_user psample macsec fou ip6_gre ip_gre gre sch_htb vxlan ip6_udp_tunnel udp_tunnel nft_chain_nat xt_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_compat nf_tables libcrc32c act_mirred cls_u32 sch_ingress ifb poly1305_generic libpoly1305 chacha_generic libchacha chacha20poly1305 tls [last unloaded: netdevsim]
[  174.155156] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[  174.155732] Workqueue: netns cleanup_net
[  174.155946] RIP: 0010:refcount_warn_saturate (lib/refcount.c:28 (discriminator 3)) 
[ 174.156214] Code: 0b 90 90 c3 cc cc cc cc 80 3d 69 d4 67 01 00 0f 85 75 ff ff ff c6 05 5c d4 67 01 01 90 48 c7 c7 e8 37 33 93 e8 a1 26 b4 ff 90 <0f> 0b 90 90 c3 cc cc cc cc 80 3d 3c d4 67 01 00 0f 85 4a ff ff ff
All code
========
   0:	0b 90 90 c3 cc cc    	or     -0x33333c70(%rax),%edx
   6:	cc                   	int3
   7:	cc                   	int3
   8:	80 3d 69 d4 67 01 00 	cmpb   $0x0,0x167d469(%rip)        # 0x167d478
   f:	0f 85 75 ff ff ff    	jne    0xffffffffffffff8a
  15:	c6 05 5c d4 67 01 01 	movb   $0x1,0x167d45c(%rip)        # 0x167d478
  1c:	90                   	nop
  1d:	48 c7 c7 e8 37 33 93 	mov    $0xffffffff933337e8,%rdi
  24:	e8 a1 26 b4 ff       	call   0xffffffffffb426ca
  29:	90                   	nop
  2a:*	0f 0b                	ud2		<-- trapping instruction
  2c:	90                   	nop
  2d:	90                   	nop
  2e:	c3                   	ret
  2f:	cc                   	int3
  30:	cc                   	int3
  31:	cc                   	int3
  32:	cc                   	int3
  33:	80 3d 3c d4 67 01 00 	cmpb   $0x0,0x167d43c(%rip)        # 0x167d476
  3a:	0f 85 4a ff ff ff    	jne    0xffffffffffffff8a

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	90                   	nop
   3:	90                   	nop
   4:	c3                   	ret
   5:	cc                   	int3
   6:	cc                   	int3
   7:	cc                   	int3
   8:	cc                   	int3
   9:	80 3d 3c d4 67 01 00 	cmpb   $0x0,0x167d43c(%rip)        # 0x167d44c
  10:	0f 85 4a ff ff ff    	jne    0xffffffffffffff60
[  174.157163] RSP: 0018:ffffc114c0263d78 EFLAGS: 00010286
[  174.157433] RAX: 0000000000000000 RBX: ffff9e61c29ee000 RCX: 00000000ffffdfff
[  174.157817] RDX: 0000000000000000 RSI: 00000000ffffffea RDI: 0000000000000001
[  174.158178] RBP: ffff9e61c29ee610 R08: ffffffff93778508 R09: 00000000ffffdfff
[  174.158538] R10: ffffffff93698520 R11: ffffffff93748520 R12: ffff9e61c29ee000
[  174.158904] R13: ffffc114c0263e08 R14: ffff9e61c29eeb80 R15: ffffffff94c78f90
[  174.159266] FS:  0000000000000000(0000) GS:ffff9e61fec80000(0000) knlGS:0000000000000000
[  174.159674] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  174.159971] CR2: 000055b4c6308dec CR3: 00000000049b4003 CR4: 0000000000770ef0
[  174.160331] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  174.160692] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  174.161063] PKRU: 55555554
[  174.161207] Call Trace:
[  174.161342]  <TASK>
[  174.161458] ? __warn (kernel/panic.c:677) 
[  174.161633] ? refcount_warn_saturate (lib/refcount.c:28 (discriminator 3)) 
[  174.161880] ? report_bug (lib/bug.c:201 lib/bug.c:219) 
[  174.162075] ? handle_bug (arch/x86/kernel/traps.c:238) 
[  174.162262] ? exc_invalid_op (arch/x86/kernel/traps.c:259 (discriminator 1)) 
[  174.162461] ? asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:568) 
[  174.162685] ? refcount_warn_saturate (lib/refcount.c:28 (discriminator 3)) 
[  174.162930] ? refcount_warn_saturate (lib/refcount.c:28 (discriminator 3)) 
[  174.163169] br_sysfs_delbr (net/bridge/br_sysfs_br.c:1086) 
[  174.163363] br_dev_delete (net/bridge/br_if.c:396) 
[  174.163554] br_net_exit_batch_rtnl (net/bridge/br.c:369) 
[  174.163783] cleanup_net (net/core/net_namespace.c:630) 
[  174.163979] ? process_one_work (kernel/workqueue.c:2606) 
[  174.164199] process_one_work (kernel/workqueue.c:2633) 
[  174.164411] worker_thread (kernel/workqueue.c:2700 kernel/workqueue.c:2787) 
[  174.164608] ? __pfx_worker_thread (kernel/workqueue.c:2733) 
[  174.164840] kthread (kernel/kthread.c:388) 
[  174.165009] ? __pfx_kthread (kernel/kthread.c:341) 
[  174.165207] ret_from_fork (arch/x86/kernel/process.c:147) 
[  174.165398] ? __pfx_kthread (kernel/kthread.c:341) 
[  174.165594] ret_from_fork_asm (arch/x86/entry/entry_64.S:250) 
| [  174.168137] ------------[ cut here ]------------
| [  174.168380] sysfs group 'bridge' not found for kobject 'test-br0'
| [  174.168707] WARNING: CPU: 1 PID: 68 at fs/sysfs/group.c:282 sysfs_remove_group (fs/sysfs/group.c:282 (discriminator 1)) 
| [  174.169128] Modules linked in: xfrm_user psample macsec fou ip6_gre ip_gre gre sch_htb vxlan ip6_udp_tunnel udp_tunnel nft_chain_nat xt_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_compat nf_tables libcrc32c act_mirred cls_u32 sch_ingress ifb poly1305_generic libpoly1305 chacha_generic libchacha chacha20poly1305 tls [last unloaded: netdevsim]
[  174.171144] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[  174.171731] Workqueue: netns cleanup_net
[  174.171943] RIP: 0010:sysfs_remove_group (fs/sysfs/group.c:282 (discriminator 1)) 
[ 174.172192] Code: 48 89 ef 5d 41 5c e9 7a ad ff ff 48 89 ef e8 12 9b ff ff eb d1 90 49 8b 14 24 48 8b 33 48 c7 c7 d0 cd 32 93 e8 ac 67 c5 ff 90 <0f> 0b 90 90 5b 5d 41 5c c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00
All code
========
   0:	48 89 ef             	mov    %rbp,%rdi
   3:	5d                   	pop    %rbp
   4:	41 5c                	pop    %r12
   6:	e9 7a ad ff ff       	jmp    0xffffffffffffad85
   b:	48 89 ef             	mov    %rbp,%rdi
   e:	e8 12 9b ff ff       	call   0xffffffffffff9b25
  13:	eb d1                	jmp    0xffffffffffffffe6
  15:	90                   	nop
  16:	49 8b 14 24          	mov    (%r12),%rdx
  1a:	48 8b 33             	mov    (%rbx),%rsi
  1d:	48 c7 c7 d0 cd 32 93 	mov    $0xffffffff9332cdd0,%rdi
  24:	e8 ac 67 c5 ff       	call   0xffffffffffc567d5
  29:	90                   	nop
  2a:*	0f 0b                	ud2		<-- trapping instruction
  2c:	90                   	nop
  2d:	90                   	nop
  2e:	5b                   	pop    %rbx
  2f:	5d                   	pop    %rbp
  30:	41 5c                	pop    %r12
  32:	c3                   	ret
  33:	cc                   	int3
  34:	cc                   	int3
  35:	cc                   	int3
  36:	cc                   	int3
  37:	66                   	data16
  38:	66                   	data16
  39:	2e                   	cs
  3a:	0f                   	.byte 0xf
  3b:	1f                   	(bad)
  3c:	84 00                	test   %al,(%rax)
	...

Code starting with the faulting instruction
===========================================
   0:	0f 0b                	ud2
   2:	90                   	nop
   3:	90                   	nop
   4:	5b                   	pop    %rbx
   5:	5d                   	pop    %rbp
   6:	41 5c                	pop    %r12
   8:	c3                   	ret
   9:	cc                   	int3
   a:	cc                   	int3
   b:	cc                   	int3
   c:	cc                   	int3
   d:	66                   	data16
   e:	66                   	data16
   f:	2e                   	cs
  10:	0f                   	.byte 0xf
  11:	1f                   	(bad)
  12:	84 00                	test   %al,(%rax)
	...
[  174.173121] RSP: 0018:ffffc114c0263d70 EFLAGS: 00010286
[  174.173391] RAX: 0000000000000000 RBX: ffffffff92f472c0 RCX: 00000000ffffdfff
[  174.173750] RDX: 0000000000000000 RSI: 00000000ffffffea RDI: 0000000000000001
[  174.174116] RBP: 0000000000000000 R08: ffffffff93778508 R09: 00000000ffffdfff
[  174.174476] R10: ffffffff93698520 R11: ffffffff93748520 R12: ffff9e61c29ee610
[  174.174843] R13: ffffc114c0263e08 R14: ffff9e61c29eeb80 R15: ffffffff94c78f90
[  174.175204] FS:  0000000000000000(0000) GS:ffff9e61fec80000(0000) knlGS:0000000000000000
[  174.175610] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  174.175910] CR2: 000055b4c6308dec CR3: 00000000049b4003 CR4: 0000000000770ef0
[  174.176273] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  174.176637] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  174.177014] PKRU: 55555554
[  174.177159] Call Trace:
[  174.177292]  <TASK>
[  174.177409] ? __warn (kernel/panic.c:677) 
[  174.177582] ? sysfs_remove_group (fs/sysfs/group.c:282 (discriminator 1)) 
[  174.177807] ? report_bug (lib/bug.c:201 lib/bug.c:219) 
[  174.178002] ? handle_bug (arch/x86/kernel/traps.c:238) 
[  174.178188] ? exc_invalid_op (arch/x86/kernel/traps.c:259 (discriminator 1)) 
[  174.178388] ? asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:568) 
[  174.178612] ? sysfs_remove_group (fs/sysfs/group.c:282 (discriminator 1)) 
[  174.178835] br_dev_delete (net/bridge/br_if.c:396) 
[  174.179026] br_net_exit_batch_rtnl (net/bridge/br.c:369) 
[  174.179257] cleanup_net (net/core/net_namespace.c:630) 
[  174.179449] ? process_one_work (kernel/workqueue.c:2606) 
[  174.179665] process_one_work (kernel/workqueue.c:2633) 
[  174.179884] worker_thread (kernel/workqueue.c:2700 kernel/workqueue.c:2787) 
[  174.180083] ? __pfx_worker_thread (kernel/workqueue.c:2733) 
[  174.180307] kthread (kernel/kthread.c:388) 
[  174.180474] ? __pfx_kthread (kernel/kthread.c:341) 
[  174.180669] ret_from_fork (arch/x86/kernel/process.c:147) 
[  174.180866] ? __pfx_kthread (kernel/kthread.c:341) 


Finger prints:
refcount_warn_saturate:br_sysfs_delbr:br_dev_delete:br_net_exit_batch_rtnl
sysfs_remove_group:br_dev_delete:br_net_exit_batch_rtnl:cleanup_net