==================================================================
BUG: KASAN: use-after-free in handle_userfault+0x206f/0x2390 fs/userfaultfd.c:371
Read of size 8 at addr ffff88004e4c6188 by task syz-executor2/4638

CPU: 1 PID: 4638 Comm: syz-executor2 Not tainted 4.13.0-next-20170914+ #4
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
 handle_userfault+0x206f/0x2390 fs/userfaultfd.c:371
 do_anonymous_page mm/memory.c:3135 [inline]
 handle_pte_fault mm/memory.c:3908 [inline]
 __handle_mm_fault+0x3823/0x39c0 mm/memory.c:4034
 handle_mm_fault+0x334/0x8d0 mm/memory.c:4071
 __do_page_fault+0x4f6/0xb60 arch/x86/mm/fault.c:1445
 do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1520
 do_async_page_fault+0x72/0xc0 arch/x86/kernel/kvm.c:266
 async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069
RIP: 0010:copy_fxregs_to_user arch/x86/include/asm/fpu/internal.h:148 [inline]
RIP: 0010:copy_fpregs_to_sigframe arch/x86/kernel/fpu/signal.c:127 [inline]
RIP: 0010:copy_fpstate_to_sigframe+0x2f1/0x470 arch/x86/kernel/fpu/signal.c:175
RSP: 0000:ffff88004e887ba8 EFLAGS: 00010246
RAX: ffff88004ed7c040 RBX: 0000000020038d80 RCX: ffff88004e887c70
RDX: 0000000000000000 RSI: 0000000020038d80 RDI: ffff88004ed7d585
RBP: ffff88004e887bd0 R08: ffff88004e887f58 R09: 0000000000000000
R10: ffff88004ed7c758 R11: ffff88004e887cb0 R12: 0000000020038d80
R13: ffff88004ed7c040 R14: 0000000000000000 R15: 0000000020038f80
 get_sigframe arch/x86/kernel/signal.c:283 [inline]
 __setup_rt_frame arch/x86/kernel/signal.c:464 [inline]
 setup_rt_frame arch/x86/kernel/signal.c:698 [inline]
 handle_signal arch/x86/kernel/signal.c:742 [inline]
 do_signal+0x9f6/0x1ee0 arch/x86/kernel/signal.c:810
 exit_to_usermode_loop+0x224/0x300 arch/x86/entry/common.c:158
 prepare_exit_to_usermode+0x267/0x2c0 arch/x86/entry/common.c:197
 retint_user+0x8/0x23
RIP: 0033:          (null)
RSP: 002b:0000000020039008 EFLAGS: 00010217
RAX: 0000000000000000 RBX: 0000000000000400 RCX: 0000000000447299
RDX: 0000000020059ffc RSI: 0000000020039000 RDI: 0000000000000400
RBP: 0000000000708000 R08: 0000000020058ffd R09: 0000000000000000
R10: 0000000020058ffc R11: 0000000000000282 R12: 00000000ffffffff
R13: 0000000000000450 R14: 00000000006e3510 R15: 0000000020039000

Allocated by task 4624:
 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:2137 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2131
 do_syscall_64+0x26c/0x8c0 arch/x86/entry/common.c:287
 return_from_SYSCALL_64+0x0/0x7a

Freed by task 4624:
 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:2137 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2131
 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 ffff88004e4c6000
 which belongs to the cache userfaultfd_ctx_cache of size 400
The buggy address is located 392 bytes inside of
 400-byte region [ffff88004e4c6000, ffff88004e4c6190)
The buggy address belongs to the page:
page:ffffea0001393180 count:1 mapcount:0 mapping:ffff88004e4c6000 index:0xffff88006c59bac0
flags: 0x500000000000100(slab)
raw: 0500000000000100 ffff88004e4c6000 ffff88006c59bac0 0000000100000006
raw: ffff88006bac2140 ffff88006bac2140 ffff88006b945b00 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88004e4c6080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88004e4c6100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88004e4c6180: fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff88004e4c6200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88004e4c6280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================