BUG: KASAN: use-after-free in userfaultfd_release+0x5c1/0x6e0 fs/userfaultfd.c:802
Read of size 8 at addr ffff8801cd747da0 by task syz-executor0/4102

CPU: 1 PID: 4102 Comm: syz-executor0 Not tainted 4.13.0-rc6+ #46
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+0x24e/0x340 mm/kasan/report.c:409
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
 userfaultfd_release+0x5c1/0x6e0 fs/userfaultfd.c:802
 __fput+0x327/0x7e0 fs/file_table.c:210
 ____fput+0x15/0x20 fs/file_table.c:246
 task_work_run+0x18a/0x260 kernel/task_work.c:116
 exit_task_work include/linux/task_work.h:21 [inline]
 do_exit+0xa3a/0x1b10 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:969
 get_signal+0x7e8/0x17e0 kernel/signal.c:2334
 do_signal+0x94/0x1ee0 arch/x86/kernel/signal.c:808
 exit_to_usermode_loop+0x224/0x300 arch/x86/entry/common.c:157
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath+0x3a7/0x450 arch/x86/entry/common.c:263
 entry_SYSCALL_64_fastpath+0xbc/0xbe
RIP: 0033:0x40b871
RSP: 002b:00007f60fc9f1750 EFLAGS: 00000293 ORIG_RAX: 0000000000000002
RAX: fffffffffffffff5 RBX: cccccccccccccccd RCX: 000000000040b871
RDX: 0000000000000000 RSI: 0000000000002040 RDI: 00007f60fc9f1800
RBP: 0000000000005c90 R08: 0000000000000000 R09: 0000000000000000
R10: 00000000000f4242 R11: 0000000000000293 R12: 00000000004bac85
R13: 00000000ffffffff R14: 0000000020052000 R15: 000000000000007f

Allocated by task 4100:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
 kmem_cache_alloc+0x127/0x750 mm/slab.c:3561
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:631
 dup_mmap kernel/fork.c:654 [inline]
 dup_mm kernel/fork.c:1175 [inline]
 copy_mm+0xa27/0x1247 kernel/fork.c:1229
 copy_process.part.34+0x1ec4/0x4bd0 kernel/fork.c:1735
 copy_process kernel/fork.c:1545 [inline]
 _do_fork+0x1ef/0xfb0 kernel/fork.c:2026
 SYSC_clone kernel/fork.c:2136 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2130
 do_syscall_64+0x26c/0x800 arch/x86/entry/common.c:284
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 4100:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3503 [inline]
 kmem_cache_free+0x77/0x280 mm/slab.c:3763
 userfaultfd_ctx_put+0x50c/0x740 fs/userfaultfd.c:168
 userfaultfd_event_wait_completion+0x754/0x910 fs/userfaultfd.c:577
 dup_fctx fs/userfaultfd.c:665 [inline]
 dup_userfaultfd_complete+0x2de/0x480 fs/userfaultfd.c:673
 dup_mmap kernel/fork.c:716 [inline]
 dup_mm kernel/fork.c:1175 [inline]
 copy_mm+0xde2/0x1247 kernel/fork.c:1229
 copy_process.part.34+0x1ec4/0x4bd0 kernel/fork.c:1735
 copy_process kernel/fork.c:1545 [inline]
 _do_fork+0x1ef/0xfb0 kernel/fork.c:2026
 SYSC_clone kernel/fork.c:2136 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2130
 do_syscall_64+0x26c/0x800 arch/x86/entry/common.c:284
 return_from_SYSCALL_64+0x0/0x7a

The buggy address belongs to the object at ffff8801cd747c40
 which belongs to the cache userfaultfd_ctx_cache of size 360
The buggy address is located 352 bytes inside of
 360-byte region [ffff8801cd747c40, ffff8801cd747da8)
The buggy address belongs to the page:
page:ffffea000735d1c0 count:1 mapcount:0 mapping:ffff8801cd747000 index:0xffff8801cd747ff7
flags: 0x200000000000100(slab)
raw: 0200000000000100 ffff8801cd747000 ffff8801cd747ff7 0000000100000009
raw: ffffea000735d1a0 ffff8801d65fe348 ffff8801d660bc00 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801cd747c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801cd747d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801cd747d80: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
                               ^
 ffff8801cd747e00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8801cd747e80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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