[moderation] [exfat?] KCSAN: data-race in _copy_to_iter / fat_mirror_bhs (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 6bcdbd62bd56 Merge tag 'for-linus' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=170876a2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=691932535b45c95e
dashboard link: https://syzkaller.appspot.com/bug?extid=956e805354edd7fa192a
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/416b3ca8b200/disk-6bcdbd62.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/665cc80cecf2/vmlinux-6bcdbd62.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1c1d848fea08/bzImage-6bcdbd62.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+956e80...@syzkaller.appspotmail.com

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

write to 0xffff88810399c800 of 2048 bytes by task 4818 on cpu 0:
fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
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
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x49d/0x8e0 fs/read_write.c:686
ksys_write+0xda/0x1a0 fs/read_write.c:738
__do_sys_write fs/read_write.c:749 [inline]
__se_sys_write fs/read_write.c:746 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:746
x64_sys_call+0x27fe/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:2
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 0xffff88810399ce00 of 512 bytes by task 3497 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: 3497 Comm: udevd Not tainted 6.16.0-syzkaller-11743-g6bcdbd62bd56 #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.