[moderation] [exfat?] KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 02ddfb981de8 Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16120270580000
kernel config: https://syzkaller.appspot.com/x/.config?x=881e3ae9e3cc6f4e
dashboard link: https://syzkaller.appspot.com/bug?extid=018e6f695ad9bccb182a
compiler: Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
CC: [hiro...@mail.parknet.co.jp linki...@kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org sj155...@samsung.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/2bee38e5cde1/disk-02ddfb98.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/4f48b9d54eac/vmlinux-02ddfb98.xz
kernel image: https://storage.googleapis.com/syzbot-assets/195c60d459be/bzImage-02ddfb98.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+018e6f...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in fat16_ent_put / fat_mirror_bhs

write to 0xffff8881379354bc of 2 bytes by task 7549 on cpu 0:
fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
fat_alloc_clusters+0x4ce/0xa80 fs/fat/fatent.c:503
fat_add_cluster fs/fat/inode.c:107 [inline]
__fat_get_block fs/fat/inode.c:154 [inline]
fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
__block_write_begin_int+0x419/0xf70 fs/buffer.c:2116
block_write_begin fs/buffer.c:2227 [inline]
cont_write_begin+0x5a3/0x8e0 fs/buffer.c:2566
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
generic_perform_write+0x181/0x490 mm/filemap.c:4103
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4220
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4246
new_sync_write fs/read_write.c:591 [inline]
vfs_write+0x49d/0x8d0 fs/read_write.c:684
ksys_write+0xda/0x1a0 fs/read_write.c:736
__do_sys_write fs/read_write.c:747 [inline]
__se_sys_write fs/read_write.c:744 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:744
x64_sys_call+0x2cdd/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888137935400 of 512 bytes by task 7554 on cpu 1:
fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
fat_add_cluster fs/fat/inode.c:107 [inline]
__fat_get_block fs/fat/inode.c:154 [inline]
fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
__block_write_begin_int+0x419/0xf70 fs/buffer.c:2116
block_write_begin fs/buffer.c:2227 [inline]
cont_write_begin+0x5a3/0x8e0 fs/buffer.c:2566
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
generic_perform_write+0x181/0x490 mm/filemap.c:4103
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4220
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4246
aio_write+0x2e2/0x410 fs/aio.c:1633
__io_submit_one fs/aio.c:-1 [inline]
io_submit_one+0xac8/0x11b0 fs/aio.c:2052
__do_sys_io_submit fs/aio.c:2111 [inline]
__se_sys_io_submit+0xfb/0x280 fs/aio.c:2081
__x64_sys_io_submit+0x43/0x50 fs/aio.c:2081
x64_sys_call+0xfc4/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:210
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 7554 Comm: syz.3.872 Not tainted 6.15.0-rc3-syzkaller-00094-g02ddfb981de8 #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup