[moderation] [exfat?] KCSAN: data-race in __fat_write_inode / copy_folio_from_iter_atomic (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 080ffb4bec4d Merge tag 'i3c/for-6.18' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13ceb858580000
kernel config: https://syzkaller.appspot.com/x/.config?x=30018395ac5e2350
dashboard link: https://syzkaller.appspot.com/bug?extid=46fcf86d675442061f44
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
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/a950c30cb9ec/disk-080ffb4b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/588803080702/vmlinux-080ffb4b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b616d95a98af/bzImage-080ffb4b.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+46fcf8...@syzkaller.appspotmail.com

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

write to 0xffff8881219a5c1c of 4 bytes by task 5461 on cpu 1:
__fat_write_inode+0x236/0x520 fs/fat/inode.c:-1
fat_sync_inode+0x1a/0x30 fs/fat/inode.c:931
fat_free fs/fat/file.c:332 [inline]
fat_truncate_blocks+0x1da/0x550 fs/fat/file.c:394
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
do_iter_readv_writev+0x49e/0x540 fs/read_write.c:-1
vfs_writev+0x2df/0x8b0 fs/read_write.c:1057
do_pwritev fs/read_write.c:1153 [inline]
__do_sys_pwritev2 fs/read_write.c:1211 [inline]
__se_sys_pwritev2+0xfc/0x1c0 fs/read_write.c:1202
__x64_sys_pwritev2+0x67/0x80 fs/read_write.c:1202
x64_sys_call+0x2c59/0x3000 arch/x86/include/generated/asm/syscalls_64.h:329
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 0xffff8881219a5c00 of 1024 bytes by task 37 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:306 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
__copy_from_iter lib/iov_iter.c:249 [inline]
copy_folio_from_iter_atomic+0x7fc/0x11b0 lib/iov_iter.c:479
generic_perform_write+0x2c2/0x490 mm/filemap.c:4193
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3528
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:1919 [inline]
loop_process_work+0x52d/0xa60 drivers/block/loop.c:1954
loop_workfn+0x31/0x40 drivers/block/loop.c:1978
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0x4cb/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x11f/0x1b0 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: 37 Comm: kworker/u8:2 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
Workqueue: loop0 loop_workfn
==================================================================


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