net test error: general protection fault in kernfs_find_ns

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    064a1899 Merge tag 'mlx5-fixes-2019-11-20' of git://git.ke..
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=124aee22e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1aab6d4187ddf667
dashboard link: https://syzkaller.appspot.com/bug?extid=36275730ddc1625afdb5
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+36275730ddc1625afdb5@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.4.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: netns cleanup_net
RIP: 0010:kernfs_find_ns+0x36/0x380 fs/kernfs/dir.c:836
Code: 55 41 54 49 89 fc 53 48 83 ec 10 48 89 75 d0 e8 d0 7d 94 ff 49 8d 7c  
24 68 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f  
85 24 03 00 00 49 8d bc 24 90 00 00 00 49 8b 5c 24
RSP: 0018:ffff8880a9897898 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffffffff8848f7a0 RCX: 0000000000000000
RDX: 000000000000000d RSI: ffffffff81deda20 RDI: 0000000000000068
RBP: ffff8880a98978d0 R08: 1ffffffff1214718 R09: fffffbfff1214719
R10: ffff8880a98978d0 R11: ffffffff890a38c7 R12: 0000000000000000
R13: ffffffff8848f760 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c420800000 CR3: 00000000a8dc9000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 kernfs_find_and_get_ns+0x34/0x70 fs/kernfs/dir.c:913
 kernfs_find_and_get include/linux/kernfs.h:522 [inline]
 sysfs_remove_group+0x76/0x1b0 fs/sysfs/group.c:276
 netdev_queue_update_kobjects+0x261/0x3e0 net/core/net-sysfs.c:1505
 remove_queue_kobjects net/core/net-sysfs.c:1560 [inline]
 netdev_unregister_kobject+0x15e/0x1f0 net/core/net-sysfs.c:1710
 rollback_registered_many+0x7c6/0xfc0 net/core/dev.c:8546
 unregister_netdevice_many.part.0+0x1b/0x1f0 net/core/dev.c:9675
 unregister_netdevice_many+0x3b/0x50 net/core/dev.c:9674
 ip6gre_exit_batch_net+0x541/0x760 net/ipv6/ip6_gre.c:1604
 ops_exit_list.isra.0+0x10c/0x160 net/core/net_namespace.c:175
 cleanup_net+0x538/0xaf0 net/core/net_namespace.c:597
 process_one_work+0x9af/0x1740 kernel/workqueue.c:2269
 worker_thread+0x98/0xe40 kernel/workqueue.c:2415
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace b94ff3c5954003d3 ]---
RIP: 0010:kernfs_find_ns+0x36/0x380 fs/kernfs/dir.c:836
Code: 55 41 54 49 89 fc 53 48 83 ec 10 48 89 75 d0 e8 d0 7d 94 ff 49 8d 7c  
24 68 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f  
85 24 03 00 00 49 8d bc 24 90 00 00 00 49 8b 5c 24
RSP: 0018:ffff8880a9897898 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffffffff8848f7a0 RCX: 0000000000000000
RDX: 000000000000000d RSI: ffffffff81deda20 RDI: 0000000000000068
RBP: ffff8880a98978d0 R08: 1ffffffff1214718 R09: fffffbfff1214719
R10: ffff8880a98978d0 R11: ffffffff890a38c7 R12: 0000000000000000
R13: ffffffff8848f760 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c420800000 CR3: 00000000a7903000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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