general protection fault in ovl_free_fs

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b36fdc6853a3 Merge tag 'gpio-v4.19-2' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13579cd1400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6c9564cd177daf0c
dashboard link: https://syzkaller.appspot.com/bug?extid=c75f181dc8429d2eb887
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+c75f181dc8429d2eb887@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
CPU: 0 PID: 12806 Comm: syz-executor3 Not tainted 4.19.0-rc2+ #224
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:ovl_free_fs+0x4d9/0x650 fs/overlayfs/super.c:226
Code: 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 67 01 00 00 48  
b8 00 00 00 00 00 fc ff df 4c 8b 23 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f  
85 56 01 00 00 49 8b 3c 24 e8 d4 0a 01 00 e9 37 fc
RSP: 0018:ffff8800aec7f7b8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff880111d7ef00 RCX: ffffc90003487000
RDX: 0000000000000000 RSI: ffffffff827c844a RDI: 0000000000000001
RBP: ffff8800aec7f810 R08: ffff8801b9cae080 R09: ffffed003b6046de
R10: 0000000000000003 R11: 0000000000000001 R12: 0000000000000000
R13: ffff880111d7ef20 R14: fffffffffffffff4 R15: ffff880111d7ef00
FS:  00007f715a1ab700(0000) GS:ffff8801db000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002242e80 CR3: 000000018698d000 CR4: 00000000001426f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
TCP: request_sock_TCPv6: Possible SYN flooding on port 20002. Sending  
cookies.  Check SNMP counters.
Call Trace:
 ovl_fill_super+0x4f1/0x3ffc fs/overlayfs/super.c:1508
 mount_nodev+0x6b/0x110 fs/super.c:1204
 ovl_mount+0x2c/0x40 fs/overlayfs/super.c:1516
 mount_fs+0xae/0x328 fs/super.c:1261
 vfs_kern_mount.part.35+0xdc/0x4f0 fs/namespace.c:961
 vfs_kern_mount fs/namespace.c:951 [inline]
 do_new_mount fs/namespace.c:2457 [inline]
 do_mount+0x581/0x30e0 fs/namespace.c:2787
 ksys_mount+0x12d/0x140 fs/namespace.c:3003
 __do_sys_mount fs/namespace.c:3017 [inline]
 __se_sys_mount fs/namespace.c:3014 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3014
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457099
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:00007f715a1aac78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f715a1ab6d4 RCX: 0000000000457099
RDX: 00000000200000c0 RSI: 0000000020000000 RDI: 0000000000000000
RBP: 00000000009300a0 R08: 0000000020000100 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 00000000004d3300 R14: 00000000004c8241 R15: 000000000000002a
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
---[ end trace a35d50b1706abe7c ]---
RIP: 0010:ovl_free_fs+0x4d9/0x650 fs/overlayfs/super.c:226
Code: 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 0f 85 67 01 00 00 48  
b8 00 00 00 00 00 fc ff df 4c 8b 23 4c 89 e2 48 c1 ea 03 <80> 3c 02 00 0f  
85 56 01 00 00 49 8b 3c 24 e8 d4 0a 01 00 e9 37 fc
RSP: 0018:ffff8800aec7f7b8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff880111d7ef00 RCX: ffffc90003487000
RDX: 0000000000000000 RSI: ffffffff827c844a RDI: 0000000000000001
RBP: ffff8800aec7f810 R08: ffff8801b9cae080 R09: ffffed003b6046de
R10: 0000000000000003 R11: 0000000000000001 R12: 0000000000000000
R13: ffff880111d7ef20 R14: fffffffffffffff4 R15: ffff880111d7ef00
FS:  00007f715a1ab700(0000) GS:ffff8801db000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000002242e80 CR3: 000000018698d000 CR4: 00000000001426f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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