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BUG: KASAN: use-after-free in handle_userfault+0x2076/0x23a0 fs/userfaultfd.c:371
Read of size 8 at addr ffff8801c668a588 by task syz-executor1/4475

CPU: 0 PID: 4475 Comm: syz-executor1 Not tainted 4.14.0-rc2-next-20170929+ #32
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
 handle_userfault+0x2076/0x23a0 fs/userfaultfd.c:371
 do_anonymous_page mm/memory.c:3115 [inline]
 handle_pte_fault mm/memory.c:3928 [inline]
 __handle_mm_fault+0x2d46/0x39c0 mm/memory.c:4054
 handle_mm_fault+0x334/0x8d0 mm/memory.c:4091
 __do_page_fault+0x5bd/0xd60 arch/x86/mm/fault.c:1444
 do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1520
 page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1066
RIP: 0033:0x4520b5
RSP: 002b:0000000020013000 EFLAGS: 00010217
RAX: 0000000000000000 RBX: 0000000000718000 RCX: 00000000004520a9
RDX: 0000000020059ffc RSI: 0000000020013000 RDI: 0000000000000400
RBP: 0000000000000450 R08: 0000000020058ffd R09: 0000000000000000
R10: 0000000020058ffc R11: 0000000000000216 R12: 00000000004b7454
R13: 00000000ffffffff R14: 0000000000000400 R15: 0000000020013000

Allocated by task 4460:
 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:3562
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:659
 dup_mmap kernel/fork.c:456 [inline]
 dup_mm kernel/fork.c:1244 [inline]
 copy_mm+0xa38/0x1310 kernel/fork.c:1298
 copy_process.part.36+0x1f79/0x4b90 kernel/fork.c:1802
 copy_process kernel/fork.c:1615 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2094
 SYSC_clone kernel/fork.c:2204 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2198
 do_syscall_64+0x26c/0x8d0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 4460:
 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:3504 [inline]
 kmem_cache_free+0x77/0x280 mm/slab.c:3764
 userfaultfd_ctx_put+0x501/0x720 fs/userfaultfd.c:165
 userfaultfd_event_wait_completion+0x788/0x9c0 fs/userfaultfd.c:605
 dup_fctx fs/userfaultfd.c:693 [inline]
 dup_userfaultfd_complete+0x2de/0x480 fs/userfaultfd.c:701
 dup_mmap kernel/fork.c:524 [inline]
 dup_mm kernel/fork.c:1244 [inline]
 copy_mm+0xe9b/0x1310 kernel/fork.c:1298
 copy_process.part.36+0x1f79/0x4b90 kernel/fork.c:1802
 copy_process kernel/fork.c:1615 [inline]
 _do_fork+0x1ef/0xfe0 kernel/fork.c:2094
 SYSC_clone kernel/fork.c:2204 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2198
 do_syscall_64+0x26c/0x8d0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

The buggy address belongs to the object at ffff8801c668a400
 which belongs to the cache userfaultfd_ctx_cache of size 400
The buggy address is located 392 bytes inside of
 400-byte region [ffff8801c668a400, ffff8801c668a590)
The buggy address belongs to the page:
page:ffffea000719a280 count:1 mapcount:0 mapping:ffff8801c668a000 index:0xffff8801c6b48600
flags: 0x200000000000100(slab)
raw: 0200000000000100 ffff8801c668a000 ffff8801c6b48600 0000000100000008
raw: ffff8801d56cc950 ffffea00072129a0 ffff8801d56cea80 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c668a480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c668a500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801c668a580: fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff8801c668a600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c668a680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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