FreeBSD: VID-90071333-fbe5-11f0-a13f-bc241121aa0a (CVE-2025-15547): FreeBSD -- Jail escape by a privileged user via nullfs

Problem Description: By default, jailed processes cannot mount filesystems, including nullfs(4). However, the allow.mount.nullfs option enables mounting nullfs filesystems, subject to privilege checks. If a privileged user within a jail is able to nullfs-mount directories, a limitation of the kernel's path lookup logic allows that user to escape the jail's chroot, yielding access to the full filesystem of the host or parent jail. Impact: In a jail configured to allow nullfs(4) mounts from within the jail, the jailed root user can escape the jail's filesystem root.