list_del corruption. prev->next should be ffff8801cf1b7080, but was           (null)
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:53!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 29658 Comm: syz-executor3 Not tainted 4.14.0-rc1+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801c3672400 task.stack: ffff8801cf1b0000
RIP: 0010:__list_del_entry_valid+0xef/0x150 lib/list_debug.c:51
RSP: 0018:ffff8801cf1b6e40 EFLAGS: 00010282
RAX: 0000000000000054 RBX: ffff8801c9e6f8f0 RCX: 0000000000000000
RDX: 0000000000000054 RSI: ffffffff815899fe RDI: ffffed0039e36dbc
RBP: ffff8801cf1b6e58 R08: ffff8801cf1b65b0 R09: 0000000000000000
R10: ffff8801cf1b6728 R11: 0000000000000000 R12: ffff8801c9e6f8f0
R13: dffffc0000000000 R14: ffff8801cf763000 R15: ffff8801cf1b7088
FS:  0000000000000000(0000) GS:ffff8801db300000(0063) knlGS:00000000f774db40
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00007f8f2bd92bc0 CR3: 00000001d86cb000 CR4: 00000000001426e0
Call Trace:
 __list_del_entry include/linux/list.h:116 [inline]
 list_del include/linux/list.h:124 [inline]
 __remove_wait_queue include/linux/wait.h:183 [inline]
 userfaultfd_event_wait_completion+0x519/0x910 fs/userfaultfd.c:591
 dup_fctx fs/userfaultfd.c:687 [inline]
 dup_userfaultfd_complete+0x2de/0x480 fs/userfaultfd.c:695
 dup_mmap kernel/fork.c:726 [inline]
 dup_mm kernel/fork.c:1179 [inline]
 copy_mm+0xe9b/0x1310 kernel/fork.c:1233
 copy_process.part.36+0x1eae/0x4af0 kernel/fork.c:1735
 copy_process kernel/fork.c:1548 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2027
 SYSC_clone kernel/fork.c:2137 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2131
 do_syscall_32_irqs_on arch/x86/entry/common.c:329 [inline]
 do_fast_syscall_32+0x3f2/0xeed arch/x86/entry/common.c:391
 entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:124
RIP: 0023:0xf7f51c79
RSP: 002b:00000000f774d05c EFLAGS: 00000296 ORIG_RAX: 0000000000000078
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000020179000
RDX: 0000000020f42000 RSI: 0000000020a6bffc RDI: 0000000020ef4ffc
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Code: 4c 89 e2 48 c7 c7 c0 d5 fd 84 e8 22 0d 1a ff 0f 0b 48 c7 c7 20 d6 fd 84 e8 14 0d 1a ff 0f 0b 48 c7 c7 80 d6 fd 84 e8 06 0d 1a ff <0f> 0b 48 c7 c7 e0 d6 fd 84 e8 f8 0c 1a ff 0f 0b 48 89 df 48 89 
RIP: __list_del_entry_valid+0xef/0x150 lib/list_debug.c:51 RSP: ffff8801cf1b6e40
---[ end trace 850facb5470d2b52 ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled