[moderation] [exfat?] KCSAN: data-race in _copy_to_iter / fat16_ent_put (6)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a6923c06a3b2 Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10451f82580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c5b1727c1bf978fe
dashboard link: https://syzkaller.appspot.com/bug?extid=d9c2626690ae6a0e0c48
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/80a9e117b116/disk-a6923c06.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d4fd8567b841/vmlinux-a6923c06.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fd488fa2e276/bzImage-a6923c06.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d9c262...@syzkaller.appspotmail.com

udevd[3288]: inotify_add_watch(7, /dev/loop2p4, 10) failed: No such file or directory
==================================================================
BUG: KCSAN: data-race in _copy_to_iter / fat16_ent_put

write to 0xffff88811e87131e of 2 bytes by task 6053 on cpu 0:
fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
fat_alloc_clusters+0x4ce/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+0x3fd/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+0x181/0x490 mm/filemap.c:4175
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4292
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4318
do_iter_readv_writev+0x421/0x4c0 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+0x2c55/0x2ff0 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 0xffff88811e871200 of 512 bytes by task 3288 on cpu 1:
instrument_copy_to_user include/linux/instrumented.h:113 [inline]
copy_to_user_iter lib/iov_iter.c:24 [inline]
iterate_ubuf include/linux/iov_iter.h:30 [inline]
iterate_and_advance2 include/linux/iov_iter.h:300 [inline]
iterate_and_advance include/linux/iov_iter.h:328 [inline]
_copy_to_iter+0x12d/0xe30 lib/iov_iter.c:185
copy_page_to_iter+0x18f/0x2d0 lib/iov_iter.c:362
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x407/0xa00 mm/filemap.c:2763
blkdev_read_iter+0x22d/0x2e0 block/fops.c:849
new_sync_read fs/read_write.c:491 [inline]
vfs_read+0x5ca/0x6f0 fs/read_write.c:572
ksys_read+0xda/0x1a0 fs/read_write.c:715
__do_sys_read fs/read_write.c:724 [inline]
__se_sys_read fs/read_write.c:722 [inline]
__x64_sys_read+0x40/0x50 fs/read_write.c:722
x64_sys_call+0x27bc/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:1
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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3288 Comm: udevd Tainted: G W 6.16.0-syzkaller-11105-ga6923c06a3b2 #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
==================================================================


---
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.