[moderation] [exfat?] KCSAN: data-race in _copy_to_iter / fat16_ent_put (9)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 502d801f0ab0 Merge tag 'erofs-for-7.2-rc1' of git://git.ke..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12d5cfee580000
kernel config: https://syzkaller.appspot.com/x/.config?x=4d0f6c16a80c07db
dashboard link: https://syzkaller.appspot.com/bug?extid=5318655d41c7aedcaea8
compiler: Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
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/7a2ab34be1f7/disk-502d801f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/344e858ce175/vmlinux-502d801f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/269d734808e7/bzImage-502d801f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+531865...@syzkaller.appspotmail.com

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

write to 0xffff88812292528c of 2 bytes by task 10323 on cpu 0:
fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
fat_chain_add+0x174/0x4a0 fs/fat/misc.c:136
fat_add_cluster fs/fat/inode.c:113 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x468/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
block_write_begin fs/buffer.c:2234 [inline]
cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_perform_write+0x186/0x490 mm/filemap.c:4363
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4480
generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4506
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x57d/0x990 fs/read_write.c:687
ksys_write+0xdc/0x1a0 fs/read_write.c:739
__do_sys_write fs/read_write.c:750 [inline]
__se_sys_write fs/read_write.c:747 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:747
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+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888122925200 of 512 bytes by task 3288 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+0x174/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x3ed/0xa20 mm/filemap.c:2866
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: 1 UID: 0 PID: 3288 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
==================================================================


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