kernel BUG at fs/super.c:LINE!

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    526674536360 Add linux-next specific files for 20180706
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1443b168400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c8d1cfc0cb798e48
dashboard link: https://syzkaller.appspot.com/bug?extid=0977fcb74b8a12a967b8
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=159481b2400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=142bca2c400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0977fcb74b8a12a967b8@xxxxxxxxxxxxxxxxxxxxxxxxx

RBP: 0000000000000004 R08: 0000000020000180 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
Filesystem sysfs get_tree() didn't set fc->root
------------[ cut here ]------------
kernel BUG at fs/super.c:1761!
invalid opcode: 0000 [#1] SMP KASAN
CPU: 1 PID: 4470 Comm: syz-executor042 Not tainted  
4.18.0-rc3-next-20180706+ #1
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:vfs_get_tree.cold.21+0x4f/0x60 fs/super.c:1756
Code: 37 00 48 c1 e0 2a 48 89 da 48 c1 ea 03 80 3c 02 00 74 08 48 89 df e8  
30 f0 f4 ff 48 8b 33 48 c7 c7 a0 5a f3 87 e8 05 36 9d ff <0f> 0b 4c 89 ef  
e8 17 f0 f4 ff eb c0 90 90 90 90 90 55 48 89 e5 53
RSP: 0018:ffff8801aac07be0 EFLAGS: 00010286
RAX: 000000000000002f RBX: ffffffff8906e5e0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff81634381 RDI: 0000000000000001
RBP: ffff8801aac07c10 R08: ffff8801a6cd6440 R09: ffffed003b5e4fc0
R10: ffffed003b5e4fc0 R11: ffff8801daf27e07 R12: 0000000000000000
R13: ffff8801bf72bdc8 R14: 0000000000000020 R15: ffff8801bf72bdd8
FS:  0000000001bf0880(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004b4f7c CR3: 00000001c4ae4000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 do_new_mount fs/namespace.c:2567 [inline]
 do_mount+0x6c1/0x1fb0 fs/namespace.c:2889
 ksys_mount+0x12d/0x140 fs/namespace.c:3105
 __do_sys_mount fs/namespace.c:3119 [inline]
 __se_sys_mount fs/namespace.c:3116 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3116
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440c89
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 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 bb 10 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe59b66978 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007ffe59b66990 RCX: 0000000000440c89
RDX: 0000000020000280 RSI: 00000000200000c0 RDI: 0000000020000000
RBP: 0000000000000004 R08: 0000000020000180 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
---[ end trace 07cdba975902c41a ]---
RIP: 0010:vfs_get_tree.cold.21+0x4f/0x60 fs/super.c:1756
Code: 37 00 48 c1 e0 2a 48 89 da 48 c1 ea 03 80 3c 02 00 74 08 48 89 df e8  
30 f0 f4 ff 48 8b 33 48 c7 c7 a0 5a f3 87 e8 05 36 9d ff <0f> 0b 4c 89 ef  
e8 17 f0 f4 ff eb c0 90 90 90 90 90 55 48 89 e5 53
RSP: 0018:ffff8801aac07be0 EFLAGS: 00010286
RAX: 000000000000002f RBX: ffffffff8906e5e0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff81634381 RDI: 0000000000000001
RBP: ffff8801aac07c10 R08: ffff8801a6cd6440 R09: ffffed003b5e4fc0
R10: ffffed003b5e4fc0 R11: ffff8801daf27e07 R12: 0000000000000000
R13: ffff8801bf72bdc8 R14: 0000000000000020 R15: ffff8801bf72bdd8
FS:  0000000001bf0880(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004b4f7c CR3: 00000001c4ae4000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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