[moderation] [exfat?] KCSAN: data-race in __fat_write_inode / _copy_to_iter (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 5a66900afbd6 Merge tag 'drm-fixes-2026-06-27' of https://g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1269b07f980000
kernel config: https://syzkaller.appspot.com/x/.config?x=84b3039e8461eef5
dashboard link: https://syzkaller.appspot.com/bug?extid=db233d5750d0f9d0675f
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/9f572a1f8056/disk-5a66900a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5d71b2a419f6/vmlinux-5a66900a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/895502184655/bzImage-5a66900a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+db233d...@syzkaller.appspotmail.com

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

write to 0xffff888129696c1c of 4 bytes by task 15442 on cpu 1:
__fat_write_inode+0x235/0x540 fs/fat/inode.c:-1
fat_write_inode+0xbb/0xd0 fs/fat/inode.c:928
write_inode fs/fs-writeback.c:1607 [inline]
__writeback_single_inode+0x327/0x660 fs/fs-writeback.c:1850
writeback_single_inode+0x16d/0x400 fs/fs-writeback.c:1906
sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:3080
writeback_inode fs/fat/inode.c:1886 [inline]
fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1906
fat_truncate_blocks+0x536/0x5b0 fs/fat/file.c:399
fat_write_failed fs/fat/inode.c:219 [inline]
fat_write_begin+0xc0/0xe0 fs/fat/inode.c:234
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_setattr+0x2a4/0x850 fs/fat/file.c:551
notify_change+0x842/0x8c0 fs/attr.c:556
do_truncate fs/open.c:68 [inline]
do_ftruncate+0x344/0x410 fs/open.c:194
ksys_ftruncate fs/open.c:206 [inline]
__do_sys_ftruncate fs/open.c:211 [inline]
__se_sys_ftruncate fs/open.c:209 [inline]
__x64_sys_ftruncate+0x65/0xc0 fs/open.c:209
x64_sys_call+0x2beb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:78
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 0xffff888129696c00 of 512 bytes by task 14693 on cpu 0:
instrument_copy_to_user include/linux/instrumented.h:130 [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:302 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
_copy_to_iter+0x130/0xea0 lib/iov_iter.c:197
copy_page_to_iter+0x174/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x3ed/0xa20 mm/filemap.c:2865
blkdev_read_iter+0x21e/0x2d0 block/fops.c:830
new_sync_read fs/read_write.c:493 [inline]
vfs_read+0x699/0x7c0 fs/read_write.c:574
ksys_read+0xdc/0x1a0 fs/read_write.c:716
__do_sys_read fs/read_write.c:725 [inline]
__se_sys_read fs/read_write.c:723 [inline]
__x64_sys_read+0x40/0x50 fs/read_write.c:723
x64_sys_call+0x2886/0x3020 arch/x86/include/generated/asm/syscalls_64.h:1
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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 14693 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================


---
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