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.