WARNING in userfaultfd_ioctl

[syzbot <syzbot+06c7092e7d71218a2c16@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    d8f190ee836a Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13d5f293400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4602730af4f872ef
dashboard link: https://syzkaller.appspot.com/bug?extid=06c7092e7d71218a2c16
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=112711a3400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=169eb87d400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+06c7092e7d71218a2c16@syzkaller.appspotmail.com

audit: type=1800 audit(1543692860.969:30): pid=5864 uid=0 auid=4294967295  
ses=4294967295 subj==unconfined op=collect_data cause=failed(directio)  
comm="startpar" name="rmnologin" dev="sda1" ino=2423 res=0
IPVS: ftp: loaded support on port[0] = 21
WARNING: CPU: 1 PID: 6019 at fs/userfaultfd.c:1569 userfaultfd_unregister  
fs/userfaultfd.c:1569 [inline]
WARNING: CPU: 1 PID: 6019 at fs/userfaultfd.c:1569  
userfaultfd_ioctl+0x3d30/0x5610 fs/userfaultfd.c:1845
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 6019 Comm: syz-executor263 Not tainted 4.20.0-rc4+ #358
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x244/0x39d lib/dump_stack.c:113
  panic+0x2ad/0x55c kernel/panic.c:188
  __warn.cold.8+0x20/0x45 kernel/panic.c:540
  report_bug+0x254/0x2d0 lib/bug.c:186
  fixup_bug arch/x86/kernel/traps.c:178 [inline]
  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
  do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:290
  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:969
RIP: 0010:userfaultfd_unregister fs/userfaultfd.c:1569 [inline]
RIP: 0010:userfaultfd_ioctl+0x3d30/0x5610 fs/userfaultfd.c:1845
Code: 85 c0 f6 ff ff 48 c1 e8 03 42 80 3c 30 00 0f 84 a3 fa ff ff 48 8b bd  
c0 f6 ff ff e8 4a 60 db ff e9 92 fa ff ff e8 20 07 98 ff <0f> 0b e9 cd f7  
ff ff e8 14 07 98 ff 48 8b 95 f0 f6 ff ff b9 01 00
RSP: 0018:ffff8881c1417270 EFLAGS: 00010293
RAX: ffff8881c1e32300 RBX: 00000000080000d0 RCX: ffffffff81e77b7b
RDX: 0000000000000000 RSI: ffffffff81e783b0 RDI: 0000000000000007
RBP: ffff8881c1417c00 R08: ffff8881c1e32300 R09: 0000000000000008
R10: 0000000000003cb4 R11: ffff8881c1e32300 R12: 0000000020011000
R13: 0000000000000000 R14: dffffc0000000000 R15: ffff8881c2b6e460
  vfs_ioctl fs/ioctl.c:46 [inline]
  file_ioctl fs/ioctl.c:509 [inline]
  do_vfs_ioctl+0x1de/0x1790 fs/ioctl.c:696
  ksys_ioctl+0xa9/0xd0 fs/ioctl.c:713
  __do_sys_ioctl fs/ioctl.c:720 [inline]
  __se_sys_ioctl fs/ioctl.c:718 [inline]
  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440ad9
Code: 23 02 00 85 c0 b8 00 00 00 00 48 0f 44 c3 5b c3 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 0b 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00000000007dff68 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffc8f046ee0 RCX: 0000000000440ad9
RDX: 0000000020d62fe0 RSI: 000000008010aa01 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000004 R11: 0000000000000217 R12: 0000000000402270
R13: 0000000000402300 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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