KASAN: use-after-free Read in __ext4_check_dir_entry

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
9dd2326890d89a5179967c947dab2bab34d7ddee (Fri Mar 30 17:29:47 2018 +0000)
Merge tag 'ceph-for-4.16-rc8' of git://github.com/ceph/ceph-client
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=1236ce66f79263e8a862

C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6171830884761600
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=5341633302233088
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6080954745487360
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-2760467897697295172
compiler: gcc (GCC) 7.1.1 20170620

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Reported-by: syzbot+1236ce66f79263e8a862@xxxxxxxxxxxxxxxxxxxxxxxxx
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==================================================================
BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x2de/0x320  
fs/ext4/dir.c:69
Read of size 2 at addr ffff8801b80bf000 by task syzkaller432828/4365

CPU: 1 PID: 4365 Comm: syzkaller432828 Not tainted 4.16.0-rc7+ #7
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+0x194/0x24d lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23c/0x360 mm/kasan/report.c:412
 __asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:431
 __ext4_check_dir_entry+0x2de/0x320 fs/ext4/dir.c:69
 ext4_readdir+0xd00/0x3600 fs/ext4/dir.c:237
 iterate_dir+0x1ca/0x530 fs/readdir.c:51
 SYSC_getdents64 fs/readdir.c:314 [inline]
 SyS_getdents64+0x221/0x420 fs/readdir.c:295
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x43fd69
RSP: 002b:00007ffc446ac118 EFLAGS: 00000203 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fd69
RDX: 00000000200015fc RSI: 0000000020001540 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000203 R12: 0000000000401690
R13: 0000000000401720 R14: 0000000000000000 R15: 0000000000000000

The buggy address belongs to the page:
page:ffffea0006e02fc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: ffffea0006c095a0 ffffea0006e03020 ffff8801be5f2a50 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801b80bef00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8801b80bef80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff8801b80bf000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff8801b80bf080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801b80bf100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


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