[moderation] [exfat?] KCSAN: data-race in copy_folio_from_iter_atomic / fat12_ent_put (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 1f5ffc672165 Fix mismerge of the arm64 / timer-core interr..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=130e0702580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2d6dadc7c868d4dd
dashboard link: https://syzkaller.appspot.com/bug?extid=e083b60ad89b4401348b
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/d2e3333a19a1/disk-1f5ffc67.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/06193573dc60/vmlinux-1f5ffc67.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0852668436d9/bzImage-1f5ffc67.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e083b6...@syzkaller.appspotmail.com

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

write to 0xffff88815a352418 of 1 bytes by task 24388 on cpu 0:
fat12_ent_put+0x74/0x180 fs/fat/fatent.c:168
fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:508
fat_add_cluster fs/fat/inode.c:108 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x25e/0x5f0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2035
block_write_begin fs/buffer.c:2146 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2484
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_perform_write+0x183/0x490 mm/filemap.c:4324
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4441
generic_file_write_iter+0x8d/0x310 mm/filemap.c:4467
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x5a6/0x9f0 fs/read_write.c:688
ksys_write+0xdc/0x1a0 fs/read_write.c:740
__do_sys_write fs/read_write.c:751 [inline]
__se_sys_write fs/read_write.c:748 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:748
x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88815a352400 of 1024 bytes by task 68 on cpu 1:
memcpy_from_iter lib/iov_iter.c:85 [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:261 [inline]
copy_folio_from_iter_atomic+0x75f/0x1170 lib/iov_iter.c:491
generic_perform_write+0x2c1/0x490 mm/filemap.c:4342
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3504
lo_rw_aio+0x67d/0x730 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1925 [inline]
loop_process_work+0x56c/0xac0 drivers/block/loop.c:1960
loop_workfn+0x31/0x40 drivers/block/loop.c:1984
process_one_work kernel/workqueue.c:3288 [inline]
process_scheduled_works+0x4f0/0x9c0 kernel/workqueue.c:3371
worker_thread+0x58a/0x780 kernel/workqueue.c:3452
kthread+0x22a/0x280 kernel/kthread.c:436
ret_from_fork+0x146/0x330 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: 1 UID: 0 PID: 68 Comm: kworker/u8:4 Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Workqueue: loop5 loop_workfn
==================================================================


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