[moderation] [exfat?] KCSAN: data-race in copy_folio_from_iter_atomic / fat12_ent_put (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2eba5e05d9bc Merge tag 'loongarch-fixes-6.18-2' of git://g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16ab18b4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=655255e3ef31c19b
dashboard link: https://syzkaller.appspot.com/bug?extid=27a590529b67234b2734
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/c8f29f4a69c2/disk-2eba5e05.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d84d88612df0/vmlinux-2eba5e05.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3c13e871004e/bzImage-2eba5e05.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+27a590...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in copy_folio_from_iter_atomic / fat12_ent_put

write to 0xffff888125de6403 of 1 bytes by task 6609 on cpu 1:
fat12_ent_put+0x74/0x170 fs/fat/fatent.c:168
fat_alloc_clusters+0x4d1/0xa80 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+0x400/0xf90 fs/buffer.c:2145
block_write_begin fs/buffer.c:2256 [inline]
cont_write_begin+0x5fc/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:229
generic_perform_write+0x184/0x490 mm/filemap.c:4254
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4371
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4397
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x52a/0x960 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+0x2802/0x3000 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 0xffff888125de6400 of 1024 bytes by task 3639 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:4272
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3466
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:1923 [inline]
loop_process_work+0x52d/0xa60 drivers/block/loop.c:1958
loop_workfn+0x31/0x40 drivers/block/loop.c:1982
process_one_work kernel/workqueue.c:3263 [inline]
process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3346
worker_thread+0x582/0x770 kernel/workqueue.c:3427
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x122/0x1b0 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 3639 Comm: kworker/u8:8 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Workqueue: loop1 loop_workfn
==================================================================


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