CVE-2026-68448: ovl: check access to copy_file_range source with src mounter creds

In the Linux kernel, the following vulnerability has been resolved:

ovl: check access to copyfilerange source with src mounter creds

Commit 5dae222a5ff0c ("vfs: allow copyfilerange to copy across devices") allowed filesystems that implement the copyfilerange() fop to decide if they want to access cross-sb copy from/to the same fs type.

The same commit added checks to verify same sb copy for filesystems that implement ->copyfilerange() and do not support cross-sb copy at the time, namely, to ceph, fuse and nfs.

The two remaining fs which implement ->copyfilerange(), cifs and overlayfs started to support cross-sb copy from this time.

While overlayfs does support cross-sb copy when the two underlying files are on the same base fs, the copy operation on the two real files from two different overalyfs filesystems is performed with the mounter creds of the destination overlayfs and the read permission access hook for the source file was called with the wrong creds.

This could cause either deny of access to copy which would otherwise be allowed (e.g. with splice) or allow read access to file which would otherwise be denied.

Fix the latter case by explicitly verifying read access to source file with the source overlayfs mounter creds.

The former case remains a quirk of cross-sb overlayfs copy, but userspace could fall back to regular copy so no harm done.