Just update the operating system in order to fix the problem
On January 29, Canonical published in a security notice details about an OpenStack Nova vulnerability for its Ubuntu 12.10, Ubuntu 12.04 LTS, and Ubuntu 11.10 operating systems.According to Canonical, Nova volume could be made to expose volumes from other users.
It was discovered that nova-volume did not validate access to volumes. An authenticated attacker could exploit this to bypass intended access controls and boot from arbitrary volumes.
The security flaws can be fixed if you upgrade your system(s) to the latest nova-volume and python-nova packages, specific to each distribution. To apply the update, run the Update Manager application.
In general, a standard system update will make all the necessary changes. A system restart won't be necessary to implement the changes, but users have to restart the virtual machine.
To update your system, please follow these instructions: https://wiki.ubuntu.com/Security/Upgrades.