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BUG: KASAN: stack-out-of-bounds in __list_del_entry_valid+0x10b/0x150 lib/list_debug.c:51
Read of size 8 at addr ffff8801c375fb50 by task syz-executor0/13340

CPU: 1 PID: 13340 Comm: syz-executor0 Not tainted 4.14.0-rc2-next-20170928+ #31
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:52
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
 __list_del_entry_valid+0x10b/0x150 lib/list_debug.c:51
 __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+0x535/0x9c0 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:745 [inline]
 dup_mm kernel/fork.c:1198 [inline]
 copy_mm+0xe9b/0x1310 kernel/fork.c:1252
 copy_process.part.35+0x1f0c/0x4b50 kernel/fork.c:1756
 copy_process kernel/fork.c:1569 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2048
 SYSC_clone kernel/fork.c:2158 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2152
 do_syscall_64+0x26c/0x8d0 arch/x86/entry/common.c:287
 entry_SYSCALL64_slow_path+0x25/0x25
RIP: 0033:0x4520a9
RSP: 002b:00007ff8ccfc5c08 EFLAGS: 00000216 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 0000000000718000 RCX: 00000000004520a9
RDX: 0000000020001ffc RSI: 0000000020001f37 RDI: 0000000000000000
RBP: 0000000000000450 R08: 0000000020001000 R09: 0000000000000000
R10: 0000000020001000 R11: 0000000000000216 R12: 00000000004b7454
R13: 00000000ffffffff R14: 0000000000000000 R15: 0000000020001f37

The buggy address belongs to the page:
page:ffffea00070dd7c0 count:0 mapcount:0 mapping:          (null) index:0x0
flags: 0x200000000000000()
raw: 0200000000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: dead000000000100 dead000000000200 0000000000000000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c375fa00: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8801c375fa80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
>ffff8801c375fb00: f1 f1 04 f2 f2 f2 f2 f2 f2 f2 04 f2 f2 f2 f2 f2
                                                 ^
 ffff8801c375fb80: f2 f2 00 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2 f2 f2
 ffff8801c375fc00: f2 f2 00 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2 f2 f2
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