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BUG: KASAN: use-after-free in handle_userfault+0x22c4/0x2500 fs/userfaultfd.c:374
Read of size 8 at addr ffff8801cd5bc160 by task syz-executor2/1016

CPU: 1 PID: 1016 Comm: syz-executor2 Not tainted 4.15.0-rc4-mm1+ #49
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:355 [inline]
 kasan_report+0x23b/0x360 mm/kasan/report.c:413
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:434
 handle_userfault+0x22c4/0x2500 fs/userfaultfd.c:374
 do_huge_pmd_anonymous_page+0xe29/0x1b00 mm/huge_memory.c:707
 create_huge_pmd mm/memory.c:3838 [inline]
 __handle_mm_fault+0x1a0c/0x3ce0 mm/memory.c:4042
 handle_mm_fault+0x38f/0x930 mm/memory.c:4108
 __do_page_fault+0x5c9/0xc90 arch/x86/mm/fault.c:1429
 do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1504
 page_fault+0x4c/0x60 arch/x86/entry/entry_64.S:1243
RIP: 0033:0x452ad5
RSP: 002b:0000000020687000 EFLAGS: 00010217
RAX: 0000000000000000 RBX: 000000000071bea0 RCX: 0000000000452ac9
RDX: 0000000020b4c000 RSI: 0000000020687000 RDI: 0000000000000600
RBP: 0000000000000047 R08: 00000000207a4f71 R09: 0000000000000000
R10: 0000000020552ffc R11: 0000000000000212 R12: 00000000006ed748
R13: 00000000ffffffff R14: 00007f2cef61c6d4 R15: 0000000000000000

Allocated by task 1006:
 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:3541
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:662
 dup_mmap kernel/fork.c:460 [inline]
 dup_mm kernel/fork.c:1244 [inline]
 copy_mm+0xa38/0x131f kernel/fork.c:1298
 copy_process.part.37+0x1f56/0x4b60 kernel/fork.c:1806
 copy_process kernel/fork.c:1619 [inline]
 _do_fork+0x1f7/0xf70 kernel/fork.c:2100
 SYSC_clone kernel/fork.c:2208 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2202
 do_syscall_64+0x26c/0x920 arch/x86/entry/common.c:285
 return_from_SYSCALL_64+0x0/0x75

Freed by task 1006:
 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:3485 [inline]
 kmem_cache_free+0x83/0x2a0 mm/slab.c:3743
 userfaultfd_ctx_put+0x501/0x720 fs/userfaultfd.c:165
 userfaultfd_event_wait_completion+0x86d/0xae0 fs/userfaultfd.c:608
 dup_fctx fs/userfaultfd.c:696 [inline]
 dup_userfaultfd_complete+0x2de/0x480 fs/userfaultfd.c:704
 dup_mmap kernel/fork.c:528 [inline]
 dup_mm kernel/fork.c:1244 [inline]
 copy_mm+0xeaa/0x131f kernel/fork.c:1298
 copy_process.part.37+0x1f56/0x4b60 kernel/fork.c:1806
 copy_process kernel/fork.c:1619 [inline]
 _do_fork+0x1f7/0xf70 kernel/fork.c:2100
 SYSC_clone kernel/fork.c:2208 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2202
 do_syscall_64+0x26c/0x920 arch/x86/entry/common.c:285
 return_from_SYSCALL_64+0x0/0x75

The buggy address belongs to the object at ffff8801cd5bc000
 which belongs to the cache userfaultfd_ctx_cache of size 360
The buggy address is located 352 bytes inside of
 360-byte region [ffff8801cd5bc000, ffff8801cd5bc168)
The buggy address belongs to the page:
page:ffffea0007356f00 count:1 mapcount:0 mapping:ffff8801cd5bc000 index:0xffff8801cd5bcff7
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801cd5bc000 ffff8801cd5bcff7 0000000100000009
raw: ffffea000705b120 ffffea00070eb220 ffff8801d674a300 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801cd5bc000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801cd5bc080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801cd5bc100: fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc
                                                       ^
 ffff8801cd5bc180: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8801cd5bc200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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