[moderation] [exfat?] KCSAN: data-race in _copy_to_iter / fat16_ent_put (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a989fde763f4 Merge tag 'libnvdimm-fixes-7.0-rc5' of git://..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13e324da580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=ebe9b4642879ca388ba8
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/534ea8813066/disk-a989fde7.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f2863b8f3695/vmlinux-a989fde7.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9c8a91db3162/bzImage-a989fde7.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ebe9b4...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in _copy_to_iter / fat16_ent_put

write to 0xffff888135980568 of 2 bytes by task 5836 on cpu 0:
fat16_ent_put+0x28/0x60 fs/fat/fatent.c:183
fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:503
fat_add_cluster fs/fat/inode.c:108 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2142
block_write_begin fs/buffer.c:2253 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2591
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
generic_perform_write+0x183/0x490 mm/filemap.c:4319
__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 0xffff888135980400 of 1024 bytes by task 3304 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: 3304 Comm: udevd Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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