BUG: KASAN: use-after-free in userfaultfd_release+0x5c1/0x6e0 fs/userfaultfd.c:806
Read of size 8 at addr ffff88006c477320 by task syz-executor0/3395

CPU: 3 PID: 3395 Comm: syz-executor0 Not tainted 4.13.0-rc5-next-20170817+ #5
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:806
 __fput+0x327/0x7e0 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+0xa52/0x1b30 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:968
 get_signal+0x7e8/0x17e0 kernel/signal.c:2330
 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+0x42f/0x500 arch/x86/entry/common.c:266
 entry_SYSCALL_64_fastpath+0xbc/0xbe
RIP: 0033:0x446739
RSP: 002b:00007fd44cc0cc08 EFLAGS: 00000292 ORIG_RAX: 0000000000000013
RAX: 0000000000000020 RBX: 0000000000000007 RCX: 0000000000446739
RDX: 0000000000000007 RSI: 0000000020053f90 RDI: 0000000000000007
RBP: 00000000007082a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000292 R12: 00000000ffffffff
R13: 0000000000003f10 R14: 00000000006e5fd0 R15: 0000000020053f90

Allocated by task 3392:
 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:635
 dup_mmap kernel/fork.c:657 [inline]
 dup_mm kernel/fork.c:1178 [inline]
 copy_mm+0xa27/0x1247 kernel/fork.c:1232
 copy_process.part.36+0x1ea3/0x4af0 kernel/fork.c:1734
 copy_process kernel/fork.c:1547 [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/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 3392:
 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:581
 dup_fctx fs/userfaultfd.c:669 [inline]
 dup_userfaultfd_complete+0x2de/0x480 fs/userfaultfd.c:677
 dup_mmap kernel/fork.c:719 [inline]
 dup_mm kernel/fork.c:1178 [inline]
 copy_mm+0xde2/0x1247 kernel/fork.c:1232
 copy_process.part.36+0x1ea3/0x4af0 kernel/fork.c:1734
 copy_process kernel/fork.c:1547 [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/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

The buggy address belongs to the object at ffff88006c4771c0
 which belongs to the cache userfaultfd_ctx_cache of size 360
The buggy address is located 352 bytes inside of
 360-byte region [ffff88006c4771c0, ffff88006c477328)
The buggy address belongs to the page:
page:ffffea0001b11dc0 count:1 mapcount:0 mapping:ffff88006c477000 index:0xffff88006c477ff7
flags: 0x500000000000100(slab)
raw: 0500000000000100 ffff88006c477000 ffff88006c477ff7 0000000100000009
raw: ffffea0001ac3aa0 ffffea0001a3bee0 ffff88006bfdd500 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88006c477200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88006c477280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88006c477300: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
                               ^
 ffff88006c477380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88006c477400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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