[moderation] [exfat?] KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 352af6a011d5 Merge tag 'rust-6.17' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17248042580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c5b1727c1bf978fe
dashboard link: https://syzkaller.appspot.com/bug?extid=b832b27219be386af310
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
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/3d0fcdd8b1bd/disk-352af6a0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e96d296c71ff/vmlinux-352af6a0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a3cb459fddb2/bzImage-352af6a0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b832b2...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic

write to 0xffff88813583a05c of 4 bytes by task 9010 on cpu 1:
__fat_write_inode+0x236/0x520 fs/fat/inode.c:-1
fat_write_inode+0xbf/0xd0 fs/fat/inode.c:924
write_inode fs/fs-writeback.c:1525 [inline]
__writeback_single_inode+0x319/0x7c0 fs/fs-writeback.c:1745
writeback_single_inode+0x167/0x3e0 fs/fs-writeback.c:1801
sync_inode_metadata+0x5b/0x90 fs/fs-writeback.c:2883
writeback_inode fs/fat/inode.c:1870 [inline]
fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1890
fat_truncate_blocks+0x4d8/0x550 fs/fat/file.c:395
fat_write_failed fs/fat/inode.c:218 [inline]
fat_write_end+0xba/0x160 fs/fat/inode.c:246
generic_perform_write+0x30f/0x490 mm/filemap.c:4196
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4292
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4318
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x4a0/0x8e0 fs/read_write.c:686
ksys_write+0xda/0x1a0 fs/read_write.c:738
__do_sys_write fs/read_write.c:749 [inline]
__se_sys_write fs/read_write.c:746 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:746
x64_sys_call+0x27fe/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88813583a000 of 2048 bytes by task 3657 on cpu 0:
memcpy_from_iter lib/iov_iter.c:73 [inline]
iterate_bvec include/linux/iov_iter.h:123 [inline]
iterate_and_advance2 include/linux/iov_iter.h:304 [inline]
iterate_and_advance include/linux/iov_iter.h:328 [inline]
__copy_from_iter lib/iov_iter.c:249 [inline]
copy_folio_from_iter_atomic+0x7fc/0x1170 lib/iov_iter.c:479
generic_perform_write+0x2c2/0x490 mm/filemap.c:4193
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3499
lo_rw_aio+0x6a0/0x760 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1910 [inline]
loop_process_work+0x52d/0xa60 drivers/block/loop.c:1945
loop_workfn+0x31/0x40 drivers/block/loop.c:1969
process_one_work kernel/workqueue.c:3236 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3319
worker_thread+0x582/0x770 kernel/workqueue.c:3400
kthread+0x489/0x510 kernel/kthread.c:464
ret_from_fork+0xdd/0x150 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3657 Comm: kworker/u8:21 Not tainted 6.16.0-syzkaller-11322-g352af6a011d5 #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Workqueue: loop7 loop_workfn
==================================================================


---
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

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.