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BUG: KASAN: use-after-free in handle_userfault+0x21c1/0x24c0 fs/userfaultfd.c:371
Read of size 8 at addr ffff8801d9281788 by task syz-executor2/15466

CPU: 0 PID: 15466 Comm: syz-executor2 Not tainted 4.15.0-rc1+ #197
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
 do_anonymous_page mm/memory.c:3121 [inline]
 handle_pte_fault mm/memory.c:3934 [inline]
 __handle_mm_fault+0x2e8c/0x3ad0 mm/memory.c:4060
 handle_mm_fault+0x334/0x8d0 mm/memory.c:4097
 __do_page_fault+0x5c9/0xc90 arch/x86/mm/fault.c:1429
 do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1504
 page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1094
RIP: 0033:0x4529e5
RSP: 002b:000000002001efff EFLAGS: 00010217
RAX: 0000000000000000 RBX: 0000000000758020 RCX: 00000000004529d9
RDX: 0000000020059ffc RSI: 000000002001efff RDI: 0000000000000400
RBP: 0000000000000047 R08: 0000000020058ffd R09: 0000000000000000
R10: 000000002005d000 R11: 0000000000000212 R12: 00000000006ed748
R13: 00000000ffffffff R14: 00007fa96b1fc6d4 R15: 0000000000000000

Allocated by task 15451:
 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:3547
 dup_userfaultfd+0x21c/0x890 fs/userfaultfd.c:659
 dup_mmap kernel/fork.c:662 [inline]
 dup_mm kernel/fork.c:1197 [inline]
 copy_mm+0xa38/0x1310 kernel/fork.c:1251
 copy_process.part.36+0x1ec4/0x4ae0 kernel/fork.c:1753
 copy_process kernel/fork.c:1566 [inline]
 _do_fork+0x1ef/0xff0 kernel/fork.c:2045
 SYSC_clone kernel/fork.c:2155 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2149
 do_syscall_64+0x26c/0x920 arch/x86/entry/common.c:285
 return_from_SYSCALL_64+0x0/0x75

Freed by task 15451:
 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:3491 [inline]
 kmem_cache_free+0x77/0x280 mm/slab.c:3749
 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:730 [inline]
 dup_mm kernel/fork.c:1197 [inline]
 copy_mm+0xe9b/0x1310 kernel/fork.c:1251
 copy_process.part.36+0x1ec4/0x4ae0 kernel/fork.c:1753
 copy_process kernel/fork.c:1566 [inline]
 _do_fork+0x1ef/0xff0 kernel/fork.c:2045
 SYSC_clone kernel/fork.c:2155 [inline]
 SyS_clone+0x37/0x50 kernel/fork.c:2149
 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 ffff8801d9281600
 which belongs to the cache userfaultfd_ctx_cache of size 400
The buggy address is located 392 bytes inside of
 400-byte region [ffff8801d9281600, ffff8801d9281790)
The buggy address belongs to the page:
page:ffffea000764a040 count:1 mapcount:0 mapping:ffff8801d9281000 index:0xffff8801d926db40
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d9281000 ffff8801d926db40 0000000100000004
raw: ffff8801d5b6a940 ffffea0007530820 ffff8801d5b6bc40 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d9281680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d9281700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801d9281780: fb fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                      ^
 ffff8801d9281800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801d9281880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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