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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d60ec36cab33 Merge tag 'mm-hotfixes-stable-2026-05-25-16-2..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=142ce056580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d830408e2d151f21
dashboard link: https://syzkaller.appspot.com/bug?extid=104d34bdb460529ac8f5
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/bc5554f13e6c/disk-d60ec36c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d0317a873b98/vmlinux-d60ec36c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6ceafe75f601/bzImage-d60ec36c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+104d34...@syzkaller.appspotmail.com

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

write to 0xffff888131894000 of 1024 bytes by task 49 on cpu 1:
memcpy_to_iter lib/iov_iter.c:77 [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+0x602/0xea0 lib/iov_iter.c:197
copy_page_to_iter+0x178/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
shmem_file_read_iter+0x29e/0x4b0 mm/shmem.c:3426
lo_rw_aio+0x67d/0x730 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1925 [inline]
loop_process_work+0x56c/0xac0 drivers/block/loop.c:1960
loop_workfn+0x31/0x40 drivers/block/loop.c:1984
process_one_work kernel/workqueue.c:3314 [inline]
process_scheduled_works+0x4f0/0x9c0 kernel/workqueue.c:3397
worker_thread+0x58a/0x780 kernel/workqueue.c:3478
kthread+0x22a/0x280 kernel/kthread.c:436
ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff888131894000 of 512 bytes by task 3290 on cpu 0:
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+0x178/0x210 lib/iov_iter.c:374
copy_folio_to_iter include/linux/uio.h:204 [inline]
filemap_read+0x407/0xa10 mm/filemap.c:2857
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/0x3b0 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: 3290 Comm: udevd Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================


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