general protection fault in put_fs_context

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b179f0826c6a Add linux-next specific files for 20181029
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1243d1c5400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6b2b22387bad67fc
dashboard link: https://syzkaller.appspot.com/bug?extid=cefb6ac96fc431886ec2
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14394183400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14624fc5400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+cefb6ac96fc431886ec2@xxxxxxxxxxxxxxxxxxxxxxxxx

RBP: 00000000006cb018 R08: 0000000000000000 R09: 0000000000000035
R10: 0000000000000020 R11: 0000000000000246 R12: 0000000000000003
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
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: 5687 Comm: syz-executor385 Not tainted 4.19.0-next-20181029+  
#100
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:put_fs_context+0x31/0x5a0 fs/fs_context.c:487
Code: 56 41 55 41 54 53 48 89 fb 4c 8d ab 98 00 00 00 48 83 ec 18 e8 10 25  
9f ff 4c 89 ea 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f  
85 03 05 00 00 4c 8b a3 98 00 00 00 4d 85 e4 74 66
RSP: 0018:ffff8801ba447bd8 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: fffffffffffffff4 RCX: ffffffff81d99d2e
RDX: 0000000000000011 RSI: ffffffff81e04360 RDI: fffffffffffffff4
RBP: ffff8801ba447c18 R08: ffff8801d59e0680 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801c938ac30
R13: 000000000000008c R14: ffff8801c92784c0 R15: 0000000000000000
FS:  0000000000a61880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000455210 CR3: 00000001d7169000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 do_remount fs/namespace.c:2428 [inline]
 do_mount+0xda4/0x1ff0 fs/namespace.c:3020
 ksys_mount+0x12d/0x140 fs/namespace.c:3245
 __do_sys_mount fs/namespace.c:3259 [inline]
 __se_sys_mount fs/namespace.c:3256 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3256
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440339
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 5b 14 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe2209f3b8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440339
RDX: 0000000020000180 RSI: 0000000020000080 RDI: 0000000020000040
RBP: 00000000006cb018 R08: 0000000000000000 R09: 0000000000000035
R10: 0000000000000020 R11: 0000000000000246 R12: 0000000000000003
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
---[ end trace 9653fb4ba53c3c1b ]---
RIP: 0010:put_fs_context+0x31/0x5a0 fs/fs_context.c:487
Code: 56 41 55 41 54 53 48 89 fb 4c 8d ab 98 00 00 00 48 83 ec 18 e8 10 25  
9f ff 4c 89 ea 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f  
85 03 05 00 00 4c 8b a3 98 00 00 00 4d 85 e4 74 66
RSP: 0018:ffff8801ba447bd8 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: fffffffffffffff4 RCX: ffffffff81d99d2e
RDX: 0000000000000011 RSI: ffffffff81e04360 RDI: fffffffffffffff4
RBP: ffff8801ba447c18 R08: ffff8801d59e0680 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801c938ac30
R13: 000000000000008c R14: ffff8801c92784c0 R15: 0000000000000000
FS:  0000000000a61880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000455210 CR3: 00000001d7169000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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