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BUG: KASAN: use-after-free in userfaultfd_release+0x5c1/0x6e0 fs/userfaultfd.c:824
Read of size 8 at addr ffff880051fe9f88 by task syz-executor7/9052

CPU: 1 PID: 9052 Comm: syz-executor7 Not tainted 4.13.0-rc7-next-20170831+ #12
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 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:246
 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
 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:158
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:266 [inline]
 do_syscall_64+0x65c/0x8c0 arch/x86/entry/common.c:292
 entry_SYSCALL64_slow_path+0x25/0x25
RIP: 0033:0x447299
RSP: 002b:00007f7788cd5c08 EFLAGS: 00000282 ORIG_RAX: 0000000000000038
RAX: fffffffffffffdff RBX: 0000000000000000 RCX: 0000000000447299
RDX: 0000000020f42000 RSI: 0000000020025000 RDI: 0000000000000000
RBP: 0000000000708210 R08: 0000000020ef4ffc R09: 0000000000000000
R10: 0000000020a6bffc R11: 0000000000000282 R12: 00000000ffffffff
R13: 00000000000003b0 R14: 00000000006e2470 R15: 0000000020025000

Allocated by task 9037:
 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:1171 [inline]
 copy_mm+0xa38/0x1310 kernel/fork.c:1225
 copy_process.part.35+0x1eae/0x4af0 kernel/fork.c:1727
 copy_process kernel/fork.c:1540 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2019
 SYSC_clone kernel/fork.c:2129 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2123
 do_syscall_64+0x26c/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 9037:
 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:1171 [inline]
 copy_mm+0xe9b/0x1310 kernel/fork.c:1225
 copy_process.part.35+0x1eae/0x4af0 kernel/fork.c:1727
 copy_process kernel/fork.c:1540 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2019
 SYSC_clone kernel/fork.c:2129 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2123
 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 ffff880051fe9e00
 which belongs to the cache userfaultfd_ctx_cache of size 400
The buggy address is located 392 bytes inside of
 400-byte region [ffff880051fe9e00, ffff880051fe9f90)
The buggy address belongs to the page:
page:ffffea000147fa40 count:1 mapcount:0 mapping:ffff880051fe9000 index:0xffff88006c9b0bc0
flags: 0x500000000000100(slab)
raw: 0500000000000100 ffff880051fe9000 ffff88006c9b0bc0 0000000100000008
raw: ffff88006b367950 ffffea0001a58ea0 ffff88006b368c80 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff880051fe9e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880051fe9f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff880051fe9f80: fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff880051fea000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880051fea080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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