[syzbot] [mm?] KCSAN: data-race in _copy_to_iter / _copy_to_iter (3)

[syzbot <syzbot+68c0534b26fbda7ed84f@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    a804c5742bff Merge tag 'lsm-pr-20261016' of git://git.kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=170e93cd580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e0a09745349d136e
dashboard link: https://syzkaller.appspot.com/bug?extid=68c0534b26fbda7ed84f
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/171f09113e28/disk-a804c574.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0f8812b149fb/vmlinux-a804c574.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6349a85938ab/bzImage-a804c574.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+68c0534b26fbda7ed84f@syzkaller.appspotmail.com

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

write to 0xffff88810d40b000 of 1024 bytes by task 12 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+0x605/0xea0 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]
 shmem_file_read_iter+0x29c/0x4a0 mm/shmem.c:3391
 lo_rw_aio+0x6a6/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1949 [inline]
 loop_process_work+0x569/0xac0 drivers/block/loop.c:1984
 loop_workfn+0x31/0x40 drivers/block/loop.c:2008
 process_one_work kernel/workqueue.c:3396 [inline]
 process_scheduled_works+0x53f/0xa10 kernel/workqueue.c:3479
 worker_thread+0x569/0x750 kernel/workqueue.c:3560
 kthread+0x223/0x270 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 0xffff88810d40b000 of 512 bytes by task 13858 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+0x174/0x210 lib/iov_iter.c:374
 copy_folio_to_iter include/linux/uio.h:204 [inline]
 filemap_read+0x3f8/0xad0 mm/filemap.c:2875
 blkdev_read_iter+0x21e/0x2d0 block/fops.c:838
 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+0x2135/0x2550 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 13858 Comm: udevd Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
==================================================================


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