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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 603c05a1639f Merge tag 'nfs-for-6.19-2' of git://git.linux..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16dbf052580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c160236e1ef1e401
dashboard link: https://syzkaller.appspot.com/bug?extid=975bf42808160192ed5e
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/04a4e867f4c0/disk-603c05a1.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/4ad8770d21bd/vmlinux-603c05a1.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6cd2ba5db9c7/bzImage-603c05a1.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+975bf4...@syzkaller.appspotmail.com

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

read to 0xffff888155bc8a48 of 1 bytes by task 13848 on cpu 0:
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+0x602/0xe70 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]
shmem_file_read_iter+0x2d6/0x540 mm/shmem.c:3438
copy_splice_read+0x442/0x660 fs/splice.c:363
do_splice_read fs/splice.c:981 [inline]
splice_direct_to_actor+0x290/0x680 fs/splice.c:1086
do_splice_direct_actor fs/splice.c:1204 [inline]
do_splice_direct+0xda/0x150 fs/splice.c:1230
do_sendfile+0x380/0x650 fs/read_write.c:1370
__do_sys_sendfile64 fs/read_write.c:1431 [inline]
__se_sys_sendfile64 fs/read_write.c:1417 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1417
x64_sys_call+0x2db1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

write to 0xffff888155bc8a08 of 66 bytes by task 13854 on cpu 1:
memcpy_from_iter lib/iov_iter.c:85 [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_from_iter lib/iov_iter.c:261 [inline]
copy_folio_from_iter_atomic+0x760/0x1150 lib/iov_iter.c:491
generic_perform_write+0x2c2/0x490 mm/filemap.c:4332
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3490
iter_file_splice_write+0x66b/0xa20 fs/splice.c:738
do_splice_from fs/splice.c:938 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1161
splice_direct_to_actor+0x312/0x680 fs/splice.c:1105
do_splice_direct_actor fs/splice.c:1204 [inline]
do_splice_direct+0xda/0x150 fs/splice.c:1230
do_sendfile+0x380/0x650 fs/read_write.c:1370
__do_sys_sendfile64 fs/read_write.c:1431 [inline]
__se_sys_sendfile64 fs/read_write.c:1417 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1417
x64_sys_call+0x2db1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 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: 13854 Comm: syz.1.2788 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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