BUG: KASAN: use-after-free in userfaultfd_release+0x5c1/0x6e0 fs/userfaultfd.c:824
Read of size 8 at addr ffff8801c90cb388 by task syz-executor0/4268

CPU: 0 PID: 4268 Comm: syz-executor0 Not tainted 4.13.0-mm1+ #7
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:824
 __fput+0x333/0x7f0 fs/file_table.c:210
 ____fput+0x15/0x20 fs/file_table.c:244
 task_work_run+0x199/0x270 kernel/task_work.c:112
 exit_task_work include/linux/task_work.h:21 [inline]
 do_exit+0xa52/0x1b40 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:968
 SYSC_exit_group kernel/exit.c:979 [inline]
 SyS_exit_group+0x1d/0x20 kernel/exit.c:977
 entry_SYSCALL_64_fastpath+0x1f/0xbe
RIP: 0033:0x4520a9
RSP: 002b:00007ffd274e6a38 EFLAGS: 00000202 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000001096914 RCX: 00000000004520a9
RDX: 00000000004520a9 RSI: 0000000000760618 RDI: 0000000000000000
RBP: 0000000000000082 R08: 00000000007180b0 R09: 00000000000010a7
R10: 0000000000760610 R11: 0000000000000202 R12: 0000000000000045
R13: 00000000007180b0 R14: 00000000007180dc R15: 0000000000000016

Allocated by task 4270:
 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+0x12e/0x760 mm/slab.c:3561
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:653
 dup_mmap kernel/fork.c:658 [inline]
 dup_mm kernel/fork.c:1179 [inline]
 copy_mm+0xa38/0x1310 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:2138 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2132
 do_syscall_64+0x26c/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 4270:
 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:165
 userfaultfd_event_wait_completion+0x754/0x910 fs/userfaultfd.c:599
 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:2138 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2132
 do_syscall_64+0x26c/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

The buggy address belongs to the object at ffff8801c90cb200
 which belongs to the cache userfaultfd_ctx_cache of size 400
The buggy address is located 392 bytes inside of
 400-byte region [ffff8801c90cb200, ffff8801c90cb390)
The buggy address belongs to the page:
page:ffffea00072432c0 count:1 mapcount:0 mapping:ffff8801c90cb000 index:0xffff8801c91d5880
flags: 0x200000000000100(slab)
raw: 0200000000000100 ffff8801c90cb000 ffff8801c91d5880 0000000100000008
raw: ffffea000725b7a0 ffff8801d5690250 ffff8801d56f8c00 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c90cb280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c90cb300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801c90cb380: fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff8801c90cb400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8801c90cb480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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