[moderation] [exfat?] KCSAN: data-race in __fat_write_inode / _copy_to_iter (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8a30aeb0d1b4 Merge tag 'nfsd-7.0-2' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16f6c572580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=95435c683e0555b0567b
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/d210c1d5bea9/disk-8a30aeb0.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0f40c70c1c61/vmlinux-8a30aeb0.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a2ee2732326/bzImage-8a30aeb0.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+95435c...@syzkaller.appspotmail.com

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

write to 0xffff888109eadc1c of 4 bytes by task 19918 on cpu 0:
__fat_write_inode+0x236/0x530 fs/fat/inode.c:-1
fat_write_inode+0xbf/0xd0 fs/fat/inode.c:925
write_inode fs/fs-writeback.c:1582 [inline]
__writeback_single_inode+0x38e/0x860 fs/fs-writeback.c:1813
writeback_single_inode+0x16d/0x3f0 fs/fs-writeback.c:1869
sync_inode_metadata+0x6e/0xa0 fs/fs-writeback.c:2974
writeback_inode fs/fat/inode.c:1875 [inline]
fat_flush_inodes+0x77/0x150 fs/fat/inode.c:1895
fat_truncate_blocks+0x528/0x5a0 fs/fat/file.c:396
fat_write_failed fs/fat/inode.c:219 [inline]
fat_write_end+0xba/0x160 fs/fat/inode.c:247
generic_perform_write+0x311/0x490 mm/filemap.c:4340
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4436
generic_file_write_iter+0x8d/0x310 mm/filemap.c:4462
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/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888109eadc00 of 512 bytes by task 17614 on cpu 1:
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+0x18f/0x2d0 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x407/0xa10 mm/filemap.c:2856
blkdev_read_iter+0x22d/0x2e0 block/fops.c:855
new_sync_read fs/read_write.c:493 [inline]
vfs_read+0x6c0/0x7f0 fs/read_write.c:574
ksys_read+0xdc/0x1a0 fs/read_write.c:717
__do_sys_read fs/read_write.c:726 [inline]
__se_sys_read fs/read_write.c:724 [inline]
__x64_sys_read+0x40/0x50 fs/read_write.c:724
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+0x12c/0x370 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: 17614 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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