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BUG: KASAN: use-after-free in userfaultfd_release+0x57f/0x5f0 fs/userfaultfd.c:879
Read of size 8 at addr ffff88809d6f9f88 by task syz-executor.4/12469

CPU: 1 PID: 12469 Comm: syz-executor.4 Not tainted 5.8.0-rc4-next-20200708-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x18f/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 userfaultfd_release+0x57f/0x5f0 fs/userfaultfd.c:879
 __fput+0x33c/0x880 fs/file_table.c:281
 task_work_run+0xdd/0x190 kernel/task_work.c:135
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop arch/x86/entry/common.c:239 [inline]
 __prepare_exit_to_usermode+0x1e9/0x1f0 arch/x86/entry/common.c:269
 do_syscall_64+0x6c/0xe0 arch/x86/entry/common.c:393
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45cba9
Code: Bad RIP value.
RSP: 002b:00007f6fab908c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000143
RAX: ffffffffffffffe8 RBX: 000000000050c520 RCX: 000000000045cba9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 000000000078bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000cab R14: 00000000004cf0b7 R15: 00007f6fab9096d4

Allocated by task 12469:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:461
 slab_post_alloc_hook mm/slab.h:536 [inline]
 slab_alloc mm/slab.c:3312 [inline]
 kmem_cache_alloc+0x148/0x550 mm/slab.c:3482
 __do_sys_userfaultfd+0x96/0x4b0 fs/userfaultfd.c:2026
 do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 12469:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
 kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
 __kasan_slab_free+0xd8/0x120 mm/kasan/common.c:422
 __cache_free mm/slab.c:3418 [inline]
 kmem_cache_free.part.0+0x67/0x1f0 mm/slab.c:3693
 __do_sys_userfaultfd+0x3cf/0x4b0 fs/userfaultfd.c:2061
 do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The buggy address belongs to the object at ffff88809d6f9e00
 which belongs to the cache userfaultfd_ctx_cache of size 408
The buggy address is located 392 bytes inside of
 408-byte region [ffff88809d6f9e00, ffff88809d6f9f98)
The buggy address belongs to the page:
page:ffffea000275be40 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff88809e50b780
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffff8880a4799950 ffff8880a4799950 ffff888219737a00
raw: ffff88809e50b780 ffff88809d6f9000 0000000100000008 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88809d6f9e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88809d6f9f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88809d6f9f80: fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff88809d6fa000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88809d6fa080: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
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