general protection fault in reconfigure_super

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    7c1b097f27bf Add linux-next specific files for 20180912
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=175570c6400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5980033172920ec0
dashboard link: https://syzkaller.appspot.com/bug?extid=87ccdc710b6af3fff875
compiler:       gcc (GCC) 8.0.1 20180413 (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+87ccdc710b6af3fff875@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: 19487 Comm: syz-executor4 Not tainted 4.19.0-rc3-next-20180912+  
#72
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:reconfigure_super+0x48e/0xbb0 fs/super.c:981
Code: 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 62 05 00 00 48 ba 00 00 00  
00 00 fc ff df 48 8b 03 48 8d 78 30 48 89 f9 48 c1 e9 03 <80> 3c 11 00 0f  
85 4d 05 00 00 48 8b 40 30 48 85 c0 48 89 85 20 ff
RSP: 0018:ffff8801c116f8d8 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff8801c116f9f8 RCX: 0000000000000006
RDX: dffffc0000000000 RSI: ffffffff81cd864b RDI: 0000000000000030
kobject: 'loop0' (00000000b8cccb9a): kobject_uevent_env
RBP: ffff8801c116f9c8 R08: ffff8801b75aa540 R09: fffffbfff12c24e5
R10: fffffbfff12c24e5 R11: ffffffff8961272b R12: ffff8801c87eae80
R13: 1ffff1003822df20 R14: 0000000000000000 R15: ffff8801c116fadc
FS:  00007fb276613700(0000) GS:ffff8801dac00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b32228000 CR3: 00000001c2282000 CR4: 00000000001406f0
kobject: 'loop0' (00000000b8cccb9a): fill_kobj_path: path  
= '/devices/virtual/block/loop0'
Call Trace:
 do_umount_root+0x2b1/0x320 fs/namespace.c:1435
 do_umount fs/namespace.c:1505 [inline]
 ksys_umount+0x8b6/0x1230 fs/namespace.c:1621
 __do_sys_umount fs/namespace.c:1632 [inline]
 __se_sys_umount fs/namespace.c:1630 [inline]
 __x64_sys_umount+0x54/0x80 fs/namespace.c:1630
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4572d9
Code: fd b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 cb b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fb276612c78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: ffffffffffffffda RBX: 00007fb2766136d4 RCX: 00000000004572d9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000140
RBP: 00000000009300a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004d7be0 R14: 00000000004caa6b R15: 0000000000000000
Modules linked in:
---[ end trace 95eb19dc6e15b099 ]---
RIP: 0010:reconfigure_super+0x48e/0xbb0 fs/super.c:981
kobject: 'loop2' (00000000c4272b7a): kobject_uevent_env
kobject: 'loop2' (00000000c4272b7a): fill_kobj_path: path  
= '/devices/virtual/block/loop2'
Code: 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 62 05 00 00 48 ba 00 00 00  
00 00 fc ff df 48 8b 03 48 8d 78 30 48 89 f9 48 c1 e9 03 <80> 3c 11 00 0f  
85 4d 05 00 00 48 8b 40 30 48 85 c0 48 89 85 20 ff
RSP: 0018:ffff8801c116f8d8 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff8801c116f9f8 RCX: 0000000000000006
RDX: dffffc0000000000 RSI: ffffffff81cd864b RDI: 0000000000000030
RBP: ffff8801c116f9c8 R08: ffff8801b75aa540 R09: fffffbfff12c24e5
R10: fffffbfff12c24e5 R11: ffffffff8961272b R12: ffff8801c87eae80
R13: 1ffff1003822df20 R14: 0000000000000000 R15: ffff8801c116fadc
FS:  00007fb276613700(0000) GS:ffff8801dad00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
sysfs: Unknown parameter './file0/bus'
CR2: 0000001b33822000 CR3: 00000001c2282000 CR4: 00000000001406e0


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