[moderation] [exfat?] KCSAN: data-race in copy_folio_from_iter_atomic / fat12_ent_put (5)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 665159e24674 Merge tag 'probes-fixes-v7.2-rc1' of git://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=103319de580000
kernel config: https://syzkaller.appspot.com/x/.config?x=84b3039e8461eef5
dashboard link: https://syzkaller.appspot.com/bug?extid=a1c4f205c9bb8442e497
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
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/31c1c90dee17/disk-665159e2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d073e04a63f4/vmlinux-665159e2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0421e67defd8/bzImage-665159e2.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a1c4f2...@syzkaller.appspotmail.com

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

write to 0xffff88811a96602b of 1 bytes by task 12275 on cpu 0:
fat12_ent_put+0x74/0x180 fs/fat/fatent.c:168
fat_alloc_clusters+0x55e/0xc40 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+0x252/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
block_write_begin fs/buffer.c:2234 [inline]
cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
cont_expand_zero fs/buffer.c:2524 [inline]
cont_write_begin+0x18d/0x920 fs/buffer.c:2586
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
fat_fallocate+0x177/0x1c0 fs/fat/file.c:305
vfs_fallocate+0x3ac/0x400 fs/open.c:338
ksys_fallocate fs/open.c:362 [inline]
__do_sys_fallocate fs/open.c:367 [inline]
__se_sys_fallocate fs/open.c:365 [inline]
__x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
x64_sys_call+0x298e/0x3020 arch/x86/include/generated/asm/syscalls_64.h:286
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811a966000 of 512 bytes by task 270 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+0x728/0x10a0 lib/iov_iter.c:491
generic_perform_write+0x2c4/0x490 mm/filemap.c:4376
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3414
lo_rw_aio+0x6a9/0x760 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1921 [inline]
loop_process_work+0x567/0xac0 drivers/block/loop.c:1956
loop_workfn+0x31/0x40 drivers/block/loop.c:1980
process_one_work kernel/workqueue.c:3322 [inline]
process_scheduled_works+0x4d4/0x9a0 kernel/workqueue.c:3405
worker_thread+0x569/0x750 kernel/workqueue.c:3486
kthread+0x221/0x270 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: 270 Comm: kworker/u8:5 Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: loop3 loop_workfn
==================================================================


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