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BUG: KASAN: use-after-free in handle_userfault+0x21c1/0x24c0 fs/userfaultfd.c:371
Read of size 8 at addr ffff8801d0845a20 by task syz-executor2/6478

CPU: 0 PID: 6478 Comm: syz-executor2 Not tainted 4.15.0-rc5+ #237
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: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+0x21c1/0x24c0 fs/userfaultfd.c:371
 shmem_getpage_gfp+0x2992/0x3730 mm/shmem.c:1729
 shmem_fault+0x2b9/0x960 mm/shmem.c:1997
 __do_fault+0xeb/0x30f mm/memory.c:3196
 do_read_fault mm/memory.c:3606 [inline]
 do_fault mm/memory.c:3706 [inline]
 handle_pte_fault mm/memory.c:3937 [inline]
 __handle_mm_fault+0x1d8f/0x3ce0 mm/memory.c:4061
 handle_mm_fault+0x334/0x8d0 mm/memory.c:4098
 __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:1225
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: 00007f20798df6d4 R15: 0000000000000000

Allocated by task 6465:
 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:3544
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:659
 dup_mmap kernel/fork.c:662 [inline]
 dup_mm kernel/fork.c:1196 [inline]
 copy_mm+0xa38/0x131b kernel/fork.c:1250
 copy_process.part.38+0x1ee9/0x4b20 kernel/fork.c:1752
 copy_process kernel/fork.c:1565 [inline]
 _do_fork+0x1f7/0xfe0 kernel/fork.c:2044
 SYSC_clone kernel/fork.c:2154 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2148
 do_syscall_64+0x26c/0x920 arch/x86/entry/common.c:285
 return_from_SYSCALL_64+0x0/0x75

Freed by task 6465:
 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:3488 [inline]
 kmem_cache_free+0x83/0x2a0 mm/slab.c:3746
 userfaultfd_ctx_put+0x50c/0x740 fs/userfaultfd.c:165
 userfaultfd_event_wait_completion+0x86d/0xae0 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:729 [inline]
 dup_mm kernel/fork.c:1196 [inline]
 copy_mm+0xea6/0x131b kernel/fork.c:1250
 copy_process.part.38+0x1ee9/0x4b20 kernel/fork.c:1752
 copy_process kernel/fork.c:1565 [inline]
 _do_fork+0x1f7/0xfe0 kernel/fork.c:2044
 SYSC_clone kernel/fork.c:2154 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2148
 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 ffff8801d08458c0
 which belongs to the cache userfaultfd_ctx_cache of size 360
The buggy address is located 352 bytes inside of
 360-byte region [ffff8801d08458c0, ffff8801d0845a28)
The buggy address belongs to the page:
page:000000008e0c6f43 count:1 mapcount:0 mapping:0000000043f62174 index:0xffff8801d0845ff7
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d0845000 ffff8801d0845ff7 0000000100000009
raw: ffffea0006f75c60 ffffea000725d7e0 ffff8801d6a72300 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d0845900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d0845980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801d0845a00: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
                               ^
 ffff8801d0845a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d0845b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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