KASAN: use-after-scope in ext4_group_desc_csum

[Jan Glauber]

Hi,

I'm getting below warning when I enable CONFIG_KASAN_EXTRA=y on a arm64 ThunderX2 system.
As far as I can tell this is present since KASAN_EXTRA was introduced (4.16).

[   64.547333] ==================================================================
[   64.561933] BUG: KASAN: use-after-scope in ext4_es_lookup_extent+0x130/0x980
[   64.576105] Write of size 4 at addr ffff80222d81f0ec by task exe/4075

[   64.592044] CPU: 102 PID: 4075 Comm: exe Not tainted 4.19.0-rc6-jang+ #29
[   64.605690] Hardware name: To be filled by O.E.M. Saber/To be filled by O.E.M., BIOS 0ACKL018 03/30/2018
[   64.624750] Call trace:
[   64.629666]  dump_backtrace+0x0/0x360
[   64.637024]  show_stack+0x24/0x30
[   64.643687]  dump_stack+0x12c/0x1b4
[   64.650699]  print_address_description+0x68/0x2c8
[   64.660152]  kasan_report+0x130/0x300
[   64.667509]  __asan_store4+0x84/0xa8
[   64.674693]  ext4_es_lookup_extent+0x130/0x980
[   64.683623]  ext4_map_blocks+0xe0/0x990
[   64.691330]  _ext4_get_block+0x130/0x2b8
[   64.699211]  ext4_get_block+0x40/0x50
[   64.706571]  generic_block_bmap+0x104/0x178
[   64.714977]  ext4_bmap+0xc4/0x198
[   64.721636]  bmap+0x54/0x70
[   64.727250]  jbd2_journal_init_inode+0x2c/0x208
[   64.736355]  ext4_fill_super+0x5080/0x5c90
[   64.744587]  mount_bdev+0x1e0/0x228
[   64.751597]  ext4_mount+0x44/0x58
[   64.758255]  mount_fs+0x58/0x1b8
[   64.764740]  vfs_kern_mount.part.2+0xc0/0x2a8
[   64.773495]  do_mount+0x7a8/0x13e8
[   64.780327]  ksys_mount+0x9c/0x110
[   64.787160]  __arm64_sys_mount+0x70/0x88
[   64.795043]  el0_svc_handler+0xac/0x150
[   64.802749]  el0_svc+0x8/0xc

[   64.811521] The buggy address belongs to the page:
[   64.821149] page:ffff7e0088b607c0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
[   64.837249] flags: 0x1ffff00000000000()
[   64.844959] raw: 1ffff00000000000 ffff7e0088b607c8 ffff7e0088b607c8 0000000000000000
[   64.860527] raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
[   64.876093] page dumped because: kasan: bad access detected

[   64.890278] Memory state around the buggy address:
[   64.899907]  ffff80222d81ef80: f2 f2 f2 f2 00 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2
[   64.914426]  ffff80222d81f000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[   64.928945] >ffff80222d81f080: f8 f8 f8 f8 f8 f8 f1 f1 f1 f1 f8 f8 f8 f8 00 f2
[   64.943463]                                                           ^
[   64.956759]  ffff80222d81f100: f2 f2 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
[   64.971278]  ffff80222d81f180: f8 f8 f8 f8 f1 f1 f1 f1 00 00 00 f2 f8 f8 f8 f8
[   64.985795] ==================================================================
[   65.000312] Disabling lock debugging due to kernel taint
[   65.037509] EXT4-fs (sda2): mounted filesystem with ordered data mode. Opts: (null)

I'm not seeing any issues like filesystem corruption or misbehaviour that could be related
the warning.

Is this a false positive? Any thoughts?

--Jan