KASAN: stack-out-of-bounds Read in locks_delete

Title	Repro	Cause bisect	Fix bisect	Count	Last	Reported
KASAN: use-after-free Read in locks_delete_block fs	syz			4	2208d	2231d
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457569
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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:00007f48c594fc78 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
BUG: KASAN: stack-out-of-bounds in __list_del_entry_valid+0xf1/0x100 lib/list_debug.c:51
RAX: ffffffffffffffda RBX: 00007f48c594fc90 RCX: 0000000000457569
RDX: 0000000000000001 RSI: 00000000200000c0 RDI: 0000000000000004
Read of size 8 at addr ffff88018258fc70 by task syz-executor2/16421
RBP: 000000000072bf00 R08: 0000000020000240 R09: 000000000000001c

R10: 0000000000040844 R11: 0000000000000246 R12: 00007f48c59506d4
R13: 00000000004c3c1d R14: 00000000004d5e98 R15: 0000000000000006
CPU: 0 PID: 16421 Comm: syz-executor2 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
 locks_verify_truncate include/linux/fs.h:2361 [inline]
 do_sys_ftruncate+0x4b2/0x550 fs/open.c:190
 __do_sys_ftruncate fs/open.c:204 [inline]
 __se_sys_ftruncate fs/open.c:202 [inline]
 __x64_sys_ftruncate+0x59/0x80 fs/open.c:202
 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:00007f2bfb4c6c78 EFLAGS: 00000246 ORIG_RAX: 000000000000004d
RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 0000000000457569
RDX: 0000000000000000 RSI: 0000000000000039 RDI: 0000000000000004
RBP: 000000000072c040 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2bfb4c76d4
R13: 00000000004bde51 R14: 00000000004cd048 R15: 00000000ffffffff

The buggy address belongs to the page:
page:ffffea00060963c0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 ffffffff06090101 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88018258fb00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88018258fb80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff88018258fc00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                                             ^
 ffff88018258fc80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff88018258fd00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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