[Android 6.1] KASAN: use-after-free Read in f2fs_inode_synced

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 642656a36791 ANDROID: GKI: Honor add symbols to symbol list
git tree: android14-6.1
console+strace: https://syzkaller.appspot.com/x/log.txt?x=13654c60580000
kernel config: https://syzkaller.appspot.com/x/.config?x=63146b1c521b2f10
dashboard link: https://syzkaller.appspot.com/bug?extid=849174b2efaf0d8be6ba
compiler: Debian clang version 15.0.6, Debian LLD 15.0.6
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15139174580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=174dc568580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e51b7754573b/disk-642656a3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/007b00b65019/vmlinux-642656a3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fb492c4ce878/bzImage-642656a3.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/e9039915f837/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=13a992cc580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+849174...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in __list_del_entry_valid+0xa6/0x130 lib/list_debug.c:62
Read of size 8 at addr ffff8881005803b8 by task kworker/u4:0/8

CPU: 0 PID: 8 Comm: kworker/u4:0 Tainted: G W 6.1.129-syzkaller-00017-g642656a36791 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Workqueue: writeback wb_workfn (flush-7:0)
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x151/0x1b7 lib/dump_stack.c:106
print_address_description mm/kasan/report.c:316 [inline]
print_report+0x158/0x4e0 mm/kasan/report.c:427
kasan_report+0x13c/0x170 mm/kasan/report.c:531
__asan_report_load8_noabort+0x14/0x20 mm/kasan/report_generic.c:351
__list_del_entry_valid+0xa6/0x130 lib/list_debug.c:62
__list_del_entry include/linux/list.h:134 [inline]
list_del_init include/linux/list.h:206 [inline]
f2fs_inode_synced+0x100/0x2e0 fs/f2fs/super.c:1553
f2fs_update_inode+0x72/0x1c40 fs/f2fs/inode.c:588
f2fs_update_inode_page+0x135/0x170 fs/f2fs/inode.c:706
f2fs_write_inode+0x416/0x790 fs/f2fs/inode.c:734
write_inode fs/fs-writeback.c:1460 [inline]
__writeback_single_inode+0x4cf/0xb80 fs/fs-writeback.c:1677
writeback_sb_inodes+0xb32/0x1910 fs/fs-writeback.c:1903
__writeback_inodes_wb+0x118/0x3f0 fs/fs-writeback.c:1974
wb_writeback+0x3da/0xa00 fs/fs-writeback.c:2081
wb_check_background_flush fs/fs-writeback.c:2151 [inline]
wb_do_writeback fs/fs-writeback.c:2239 [inline]
wb_workfn+0xbba/0x1030 fs/fs-writeback.c:2266
process_one_work+0x73d/0xcb0 kernel/workqueue.c:2299
worker_thread+0xa60/0x1260 kernel/workqueue.c:2446
kthread+0x26d/0x300 kernel/kthread.c:386
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>

Allocated by task 298:
kasan_save_stack mm/kasan/common.c:45 [inline]
kasan_set_track+0x4b/0x70 mm/kasan/common.c:52
kasan_save_alloc_info+0x1f/0x30 mm/kasan/generic.c:505
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:333
kasan_slab_alloc include/linux/kasan.h:202 [inline]
slab_post_alloc_hook+0x53/0x2c0 mm/slab.h:768
slab_alloc_node mm/slub.c:3421 [inline]
slab_alloc mm/slub.c:3431 [inline]
__kmem_cache_alloc_lru mm/slub.c:3438 [inline]
kmem_cache_alloc_lru+0x102/0x270 mm/slub.c:3454
alloc_inode_sb include/linux/fs.h:3255 [inline]
f2fs_alloc_inode+0x2d/0x350 fs/f2fs/super.c:1437
alloc_inode fs/inode.c:261 [inline]
iget_locked+0x18c/0x7e0 fs/inode.c:1373
f2fs_iget+0x55/0x4ca0 fs/f2fs/inode.c:486
f2fs_lookup+0x3c1/0xb50 fs/f2fs/namei.c:484
__lookup_slow+0x2b9/0x3e0 fs/namei.c:1689
lookup_slow+0x5a/0x80 fs/namei.c:1706
walk_component+0x2e7/0x410 fs/namei.c:1997
lookup_last fs/namei.c:2454 [inline]
path_lookupat+0x16d/0x450 fs/namei.c:2478
filename_lookup+0x251/0x600 fs/namei.c:2507
vfs_statx+0x107/0x4b0 fs/stat.c:229
vfs_fstatat fs/stat.c:267 [inline]
vfs_lstat include/linux/fs.h:3434 [inline]
__do_sys_newlstat fs/stat.c:423 [inline]
__se_sys_newlstat+0xda/0x7c0 fs/stat.c:417
__x64_sys_newlstat+0x5b/0x70 fs/stat.c:417
x64_sys_call+0x52/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:7
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x3b/0x80 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

Freed by task 0:
kasan_save_stack mm/kasan/common.c:45 [inline]
kasan_set_track+0x4b/0x70 mm/kasan/common.c:52
kasan_save_free_info+0x2b/0x40 mm/kasan/generic.c:516
____kasan_slab_free+0x131/0x180 mm/kasan/common.c:241
__kasan_slab_free+0x11/0x20 mm/kasan/common.c:249
kasan_slab_free include/linux/kasan.h:178 [inline]
slab_free_hook mm/slub.c:1745 [inline]
slab_free_freelist_hook mm/slub.c:1771 [inline]
slab_free mm/slub.c:3686 [inline]
kmem_cache_free+0x291/0x560 mm/slub.c:3711
f2fs_free_inode+0x24/0x30 fs/f2fs/super.c:1584
i_callback+0x4b/0x70 fs/inode.c:250
rcu_do_batch+0x552/0xbe0 kernel/rcu/tree.c:2297
rcu_core+0x502/0xf40 kernel/rcu/tree.c:2557
rcu_core_si+0x9/0x10 kernel/rcu/tree.c:2574
handle_softirqs+0x1db/0x650 kernel/softirq.c:624
__do_softirq kernel/softirq.c:662 [inline]
invoke_softirq kernel/softirq.c:479 [inline]
__irq_exit_rcu+0x52/0xf0 kernel/softirq.c:711
irq_exit_rcu+0x9/0x10 kernel/softirq.c:723
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1118 [inline]
sysvec_apic_timer_interrupt+0xa9/0xc0 arch/x86/kernel/apic/apic.c:1118
asm_sysvec_apic_timer_interrupt+0x1b/0x20 arch/x86/include/asm/idtentry.h:691

Last potentially related work creation:
kasan_save_stack+0x3b/0x60 mm/kasan/common.c:45
__kasan_record_aux_stack+0xb4/0xc0 mm/kasan/generic.c:486
kasan_record_aux_stack_noalloc+0xb/0x10 mm/kasan/generic.c:496
__call_rcu_common kernel/rcu/tree.c:2807 [inline]
call_rcu+0xdc/0x10f0 kernel/rcu/tree.c:2926
destroy_inode fs/inode.c:316 [inline]
evict+0x87d/0x930 fs/inode.c:720
iput_final fs/inode.c:1834 [inline]
iput+0x616/0x690 fs/inode.c:1860
do_unlinkat+0x4e1/0x920 fs/namei.c:4396
__do_sys_unlink fs/namei.c:4437 [inline]
__se_sys_unlink fs/namei.c:4435 [inline]
__x64_sys_unlink+0x49/0x50 fs/namei.c:4435
x64_sys_call+0x289/0x9a0 arch/x86/include/generated/asm/syscalls_64.h:88
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x3b/0x80 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2

The buggy address belongs to the object at ffff888100580000
which belongs to the cache f2fs_inode_cache of size 1360
The buggy address is located 952 bytes inside of
1360-byte region [ffff888100580000, ffff888100580550)

The buggy address belongs to the physical page:
page:ffffea0004016000 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x100580
head:ffffea0004016000 order:3 compound_mapcount:0 compound_pincount:0
flags: 0x4000000000010200(slab|head|zone=1)
raw: 4000000000010200 0000000000000000 dead000000000122 ffff8881002c2f00
raw: 0000000000000000 0000000080160016 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Reclaimable, gfp_mask 0xd2050(__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_RECLAIMABLE), pid 298, tgid 298 (syz-executor399), ts 24637948684, free_ts 0
set_page_owner include/linux/page_owner.h:33 [inline]
post_alloc_hook+0x213/0x220 mm/page_alloc.c:2637
prep_new_page+0x1b/0x110 mm/page_alloc.c:2644
get_page_from_freelist+0x3a98/0x3b10 mm/page_alloc.c:4539
__alloc_pages+0x234/0x610 mm/page_alloc.c:5837
alloc_slab_page+0x6c/0xf0 include/linux/gfp.h:-1
allocate_slab mm/slub.c:1962 [inline]
new_slab+0x90/0x3e0 mm/slub.c:2015
___slab_alloc+0x6f9/0xb80 mm/slub.c:3203
__slab_alloc+0x5d/0xa0 mm/slub.c:3302
slab_alloc_node mm/slub.c:3387 [inline]
slab_alloc mm/slub.c:3431 [inline]
__kmem_cache_alloc_lru mm/slub.c:3438 [inline]
kmem_cache_alloc_lru+0x149/0x270 mm/slub.c:3454
alloc_inode_sb include/linux/fs.h:3255 [inline]
f2fs_alloc_inode+0x2d/0x350 fs/f2fs/super.c:1437
alloc_inode fs/inode.c:261 [inline]
iget_locked+0x18c/0x7e0 fs/inode.c:1373
f2fs_iget+0x55/0x4ca0 fs/f2fs/inode.c:486
f2fs_fill_super+0x5360/0x6dc0 fs/f2fs/super.c:4488
mount_bdev+0x282/0x3b0 fs/super.c:1445
f2fs_mount+0x34/0x40 fs/f2fs/super.c:4743
legacy_get_tree+0xf1/0x190 fs/fs_context.c:632
page_owner free stack trace missing

Memory state around the buggy address:
ffff888100580280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888100580300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888100580380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff888100580400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888100580480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cd8e74fa0fa3 Merge tag 'android12-5.4.292_r00' into androi..
git tree: android12-5.4
console+strace: https://syzkaller.appspot.com/x/log.txt?x=11dde368580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b0da08c76d10c2ad
dashboard link: https://syzkaller.appspot.com/bug?extid=ca3e11229c92c86da445
compiler: Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=152170d4580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15379374580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1e3bf13bb6c0/disk-cd8e74fa.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/aebd9df08c05/vmlinux-cd8e74fa.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6474ecafb172/bzImage-cd8e74fa.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/a1b5a3d2ce6e/mount_0.gz
fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=12ab6374580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:

Reported-by: syzbot+ca3e11...@syzkaller.appspotmail.com

RBP: 00007ffc58c2a2a0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000100 R11: 0000000000000206 R12: 00007ffc58c2b390
R13: 00005555847f6700 R14: 0000000000000001 R15: 431bde82d7b634db
---[ end trace 68e4ad7cb2ed9303 ]---
==================================================================
BUG: KASAN: use-after-free in __list_del_entry_valid+0xa6/0x120 lib/list_debug.c:62
Read of size 8 at addr ffff8881dcba5fd0 by task syz-executor362/358

CPU: 1 PID: 358 Comm: syz-executor362 Tainted: G W 5.4.292-syzkaller-00021-gcd8e74fa0fa3 #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025

Call Trace:
__dump_stack+0x1e/0x20 lib/dump_stack.c:77
dump_stack+0x15b/0x1b8 lib/dump_stack.c:118
print_address_description+0x8d/0x4c0 mm/kasan/report.c:384
__kasan_report+0xef/0x120 mm/kasan/report.c:516
kasan_report+0x30/0x60 mm/kasan/common.c:653
__asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
__list_del_entry_valid+0xa6/0x120 lib/list_debug.c:62
__list_del_entry include/linux/list.h:131 [inline]
list_del_init include/linux/list.h:190 [inline]
f2fs_inode_synced+0xf4/0x2d0 fs/f2fs/super.c:1363
f2fs_update_inode+0x77/0x1aa0 fs/f2fs/inode.c:589
f2fs_update_inode_page fs/f2fs/inode.c:703 [inline]
f2fs_write_inode+0x5a1/0x8c0 fs/f2fs/inode.c:731
write_inode fs/fs-writeback.c:1326 [inline]
__writeback_single_inode+0x4e2/0xc30 fs/fs-writeback.c:1524
writeback_single_inode+0x216/0x630 fs/fs-writeback.c:1578
sync_inode fs/fs-writeback.c:2591 [inline]
sync_inode_metadata+0xa8/0xf0 fs/fs-writeback.c:2611
f2fs_sync_inode_meta+0x167/0x280 fs/f2fs/checkpoint.c:1141
block_operations fs/f2fs/checkpoint.c:1246 [inline]
f2fs_write_checkpoint+0x9f0/0x1c50 fs/f2fs/checkpoint.c:1630
kill_f2fs_super+0x231/0x390 fs/f2fs/super.c:4342
deactivate_locked_super+0xa0/0x100 fs/super.c:335
deactivate_super+0x14f/0x1d0 fs/super.c:366
cleanup_mnt+0x446/0x500 fs/namespace.c:1102
__cleanup_mnt+0x19/0x20 fs/namespace.c:1109
task_work_run+0x146/0x170 kernel/task_work.c:113
exit_task_work include/linux/task_work.h:22 [inline]
do_exit+0xa43/0x2660 kernel/exit.c:861
do_group_exit+0x13e/0x300 kernel/exit.c:984
__do_sys_exit_group kernel/exit.c:995 [inline]
__se_sys_exit_group kernel/exit.c:993 [inline]
__x64_sys_exit_group+0x3f/0x40 kernel/exit.c:993
do_syscall_64+0xcf/0x170 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x5c/0xc1
RIP: 0033:0x7fa76f153e49
Code: Bad RIP value.
RSP: 002b:00007ffc58c29098 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fa76f153e49
RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000001
RBP: 00007fa76f1e3370 R08: ffffffffffffffb8 R09: 0000000000000000
R10: 0000000000000100 R11: 0000000000000246 R12: 00007fa76f1e3370
R13: 0000000000000000 R14: 00007fa76f1e5140 R15: 00007fa76f121c40

Allocated by task 360:
save_stack mm/kasan/common.c:70 [inline]
set_track mm/kasan/common.c:78 [inline]
__kasan_kmalloc+0x162/0x200 mm/kasan/common.c:529
kasan_slab_alloc+0x12/0x20 mm/kasan/common.c:537
slab_post_alloc_hook mm/slab.h:584 [inline]
slab_alloc_node mm/slub.c:2829 [inline]
slab_alloc mm/slub.c:2837 [inline]
kmem_cache_alloc+0xe2/0x270 mm/slub.c:2842
f2fs_alloc_inode+0x26/0x3b0 fs/f2fs/super.c:1241
alloc_inode fs/inode.c:231 [inline]
iget_locked+0x144/0x810 fs/inode.c:1273
f2fs_iget+0x55/0x4ac0 fs/f2fs/inode.c:499
f2fs_lookup+0x3ce/0xe00 fs/f2fs/namei.c:529
lookup_open fs/namei.c:3358 [inline]
do_last fs/namei.c:3471 [inline]
path_openat+0x133f/0x35c0 fs/namei.c:3683
do_filp_open+0x1ae/0x3f0 fs/namei.c:3713
do_sys_open+0x2bb/0x5d0 fs/open.c:1123
__do_sys_openat fs/open.c:1150 [inline]
__se_sys_openat fs/open.c:1144 [inline]
__x64_sys_openat+0xa2/0xb0 fs/open.c:1144
do_syscall_64+0xcf/0x170 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x5c/0xc1

Freed by task 0:
save_stack mm/kasan/common.c:70 [inline]
set_track mm/kasan/common.c:78 [inline]
kasan_set_free_info mm/kasan/common.c:345 [inline]
__kasan_slab_free+0x1c3/0x280 mm/kasan/common.c:487
kasan_slab_free+0xe/0x10 mm/kasan/common.c:496
slab_free_hook mm/slub.c:1455 [inline]
slab_free_freelist_hook+0xb7/0x180 mm/slub.c:1494
slab_free mm/slub.c:3080 [inline]
kmem_cache_free+0x10c/0x2c0 mm/slub.c:3096
f2fs_free_inode+0x24/0x30 fs/f2fs/super.c:1397
i_callback+0x4c/0x70 fs/inode.c:220
__rcu_reclaim kernel/rcu/rcu.h:222 [inline]
rcu_do_batch+0x446/0x980 kernel/rcu/tree.c:2167
rcu_core+0x4bd/0xbd0 kernel/rcu/tree.c:2387
rcu_core_si+0x9/0x10 kernel/rcu/tree.c:2396
__do_softirq+0x236/0x660 kernel/softirq.c:292

The buggy address belongs to the object at ffff8881dcba5c70
which belongs to the cache f2fs_inode_cache of size 1264
The buggy address is located 864 bytes inside of
1264-byte region [ffff8881dcba5c70, ffff8881dcba6160)
The buggy address belongs to the page:
page:ffffea000772e800 refcount:1 mapcount:0 mapping:ffff8881f084ac80 index:0x0 compound_mapcount: 0
flags: 0x8000000000010200(slab|head)
raw: 8000000000010200 dead000000000100 dead000000000122 ffff8881f084ac80
raw: 0000000000000000 0000000080170017 00000001ffffffff 0000000000000000

page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Reclaimable, gfp_mask 0xd2050(__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_RECLAIMABLE)

set_page_owner include/linux/page_owner.h:31 [inline]
post_alloc_hook mm/page_alloc.c:2165 [inline]
prep_new_page+0x35e/0x370 mm/page_alloc.c:2171
get_page_from_freelist+0x1296/0x1310 mm/page_alloc.c:3794
__alloc_pages_nodemask+0x202/0x4b0 mm/page_alloc.c:4894
alloc_slab_page+0x3c/0x3b0 mm/slub.c:343
allocate_slab mm/slub.c:1683 [inline]
new_slab+0x93/0x420 mm/slub.c:1749
new_slab_objects mm/slub.c:2505 [inline]
___slab_alloc+0x29e/0x420 mm/slub.c:2667
__slab_alloc+0x63/0xa0 mm/slub.c:2707
slab_alloc_node mm/slub.c:2792 [inline]
slab_alloc mm/slub.c:2837 [inline]
kmem_cache_alloc+0x12c/0x270 mm/slub.c:2842
f2fs_alloc_inode+0x26/0x3b0 fs/f2fs/super.c:1241
alloc_inode fs/inode.c:231 [inline]
iget_locked+0x144/0x810 fs/inode.c:1273
f2fs_iget+0x55/0x4ac0 fs/f2fs/inode.c:499
f2fs_fill_super+0x44fc/0x7760 fs/f2fs/super.c:3963
mount_bdev+0x279/0x390 fs/super.c:1428
f2fs_mount+0x34/0x40 fs/f2fs/super.c:4316
legacy_get_tree+0xea/0x190 fs/fs_context.c:648
vfs_get_tree+0x89/0x260 fs/super.c:1558

page_owner free stack trace missing

Memory state around the buggy address:

ffff8881dcba5e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8881dcba5f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8881dcba5f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8881dcba6000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8881dcba6080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

[syzbot]

syzbot suspects this issue was fixed by commit:

commit 917ae5e280bc263f56c83fba0d0f0be2c4828083
Author: Chao Yu <ch...@kernel.org>
Date: Tue Jul 8 09:53:39 2025 +0000

f2fs: fix to avoid UAF in f2fs_sync_inode_meta()

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17ed3142580000
start commit: 642656a36791 ANDROID: GKI: Honor add symbols to symbol list
git tree: android14-6.1

kernel config: https://syzkaller.appspot.com/x/.config?x=63146b1c521b2f10
dashboard link: https://syzkaller.appspot.com/bug?extid=849174b2efaf0d8be6ba

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13177204580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17495ccc580000

If the result looks correct, please mark the issue as fixed by replying with:

#syz fix: f2fs: fix to avoid UAF in f2fs_sync_inode_meta()

For information about bisection process see: https://goo.gl/tpsmEJ#bisection