[moderation] [mm?] KCSAN: data-race in _copy_to_iter / _copy_to_iter

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 3b08f56fbbb9 Merge tag 'x86-urgent-2025-09-20' of git://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=170ba712580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6e0c213d0735f5dd
dashboard link: https://syzkaller.appspot.com/bug?extid=c42bdcc6f021f53ce265
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [ak...@linux-foundation.org baoli...@linux.alibaba.com hu...@google.com linux-...@vger.kernel.org linu...@kvack.org]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/3fb2627999ce/disk-3b08f56f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/afcb046ecb69/vmlinux-3b08f56f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/52b4070d355f/bzImage-3b08f56f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c42bdc...@syzkaller.appspotmail.com

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

write to 0xffff88811d09e000 of 1024 bytes by task 51 on cpu 1:
memcpy_to_iter lib/iov_iter.c:65 [inline]
iterate_bvec include/linux/iov_iter.h:123 [inline]
iterate_and_advance2 include/linux/iov_iter.h:306 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
_copy_to_iter+0x602/0xe70 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]
shmem_file_read_iter+0x2d6/0x540 mm/shmem.c:3476
lo_rw_aio+0x69d/0x760 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1919 [inline]
loop_process_work+0x52d/0xa60 drivers/block/loop.c:1954
loop_workfn+0x31/0x40 drivers/block/loop.c:1978
process_one_work kernel/workqueue.c:3236 [inline]
process_scheduled_works+0x4cb/0x9d0 kernel/workqueue.c:3319
worker_thread+0x582/0x770 kernel/workqueue.c:3400
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x11f/0x1b0 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff88811d09e000 of 512 bytes by task 3534 on cpu 0:
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:302 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
_copy_to_iter+0x130/0xe70 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:852
new_sync_read fs/read_write.c:491 [inline]
vfs_read+0x64c/0x770 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: 0 UID: 0 PID: 3534 Comm: udevd Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
==================================================================


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