High [CVE-2026-68448] 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 copy_file_range source with src mounter creds Commit 5dae222a5ff0c ("vfs: allow copy_file_range to copy across devices") allowed filesystems that implement the copy_file_range() f_op 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 ->copy_file_range() and do not support cross-sb copy at the time, namely, to ceph, fuse and nfs.

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

A flaw was found in the Linux kernel's overlayfs component. When copying files across different overlay filesystems, the system incorrectly checked read permissions using the destination filesystem's credentials instead of the source's.