KASAN: use-after-free Read in locks_delete_block

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    442b8cea2477 Add linux-next specific files for 20181109
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=115dbad5400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2f72bdb11df9fbe8
dashboard link: https://syzkaller.appspot.com/bug?extid=a4a3d526b4157113ec6a
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+a4a3d526b4157113ec6a@xxxxxxxxxxxxxxxxxxxxxxxxx

device loop0 blocksize: 4096
__find_get_block_slow() failed. block=1, b_blocknr=8
b_state=0x00000029, b_size=512
device loop0 blocksize: 4096
==================================================================
BUG: KASAN: use-after-free in __list_del_entry_valid+0xf1/0x100  
lib/list_debug.c:51
Read of size 8 at addr ffff88017eb47b70 by task syz-executor3/13461

CPU: 0 PID: 13461 Comm: syz-executor3 Not tainted 4.20.0-rc1-next-20181109+  
#110
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+0x244/0x39d lib/dump_stack.c:113
 print_address_description.cold.7+0x9/0x1ff mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.8+0x242/0x309 mm/kasan/report.c:412
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
 __list_del_entry_valid+0xf1/0x100 lib/list_debug.c:51
 __list_del_entry include/linux/list.h:117 [inline]
 list_del_init include/linux/list.h:159 [inline]
 __locks_delete_block fs/locks.c:683 [inline]
 locks_delete_block+0xce/0x3d0 fs/locks.c:716
 locks_mandatory_area+0x48b/0x6a0 fs/locks.c:1398
 rw_verify_area+0x2f2/0x360 fs/read_write.c:386
 vfs_write+0x149/0x560 fs/read_write.c:544
 ksys_write+0x101/0x260 fs/read_write.c:598
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write fs/read_write.c:607 [inline]
 __x64_sys_write+0x73/0xb0 fs/read_write.c:607
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457569
Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ff2e8194c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000457569
RDX: 0000000000000010 RSI: 0000000020000180 RDI: 0000000000000006
RBP: 000000000072c0e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ff2e81956d4
R13: 00000000004c571f R14: 00000000004d9360 R15: 00000000ffffffff

The buggy address belongs to the page:
page:ffffea0005fad1c0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 ffffea0005fad1c8 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88017eb47a00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88017eb47a80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> ffff88017eb47b00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                                             ^
 ffff88017eb47b80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88017eb47c00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


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