OSSN/OSSN-0028

Summary
When Nova is using the libvirt virtualization driver, the SMBIOS serial number supplied by libvirt is provided to the guest instances that are running on a compute node. This serial number may expose sensitive information about the underlying compute node hardware.

Affected Services / Software
Nova, Icehouse, Havana

Discussion
The 'serial' field in guest SMBIOS tables gets populated based on the libvirt reported UUID of the host hardware. The rationale is to allow correlation of guests running on the same host.

Unfortunately some hardware vendors use a subset of the host UUID as a key for retrieving hardware support contract information without requiring any authentication. In these cases, exposing the host UUID to the guest is an information leak for those vendors.

The exposed host UUID could theoretically be leveraged by a cloud user to get an approximate count of the number of unique hosts available to them in the cloud by launching many short lived VMs.

Recommended Actions
It is possible to override the use of the compute node's SMBIOS data by libvirt in /etc/libvirt/libvirtd.conf by setting the 'host_uuid' parameter. This allows setting an arbitrary UUID for identification purposes that doesn't leak any information about the real underlying hardware. It is advised to make use of this override ability to prevent potential exposure of information about the underlying compute node hardware.

In the Juno release of OpenStack, Nova's libvirt driver allows the source of the host UUID to be controlled via a new 'sysinfo_serial' config parameter. This new parameter allows the following values:


 * 'auto' - try /etc/machine-id, fallback to libvirt reported host UUID (new default)
 * 'hardware' - always use libvirt host UUID (old default)
 * 'os' - always use /etc/machine-id, error if missing
 * 'none' - do not report any value to the guest

In general, it is preferrable to use the /etc/machine-id UUID instead of the host hardware UUID. The former is a recent standard for Linux distros introduced by systemd to provide a UUID that is unique per operating system install. This means that even containers will see a separate /etc/machine-id value. This /etc/machine-id can be expected to be widely available in current and future distros. If this file is missing, it is still possible to fallback to the libvirt reported host UUID.

Administrators concerned about exposing the ability to identity an underlying compute node by it's serial number may wish to disable reporting of any sysinfo serial field at all by using the 'none' value.

Contacts / References

 * Author: Nathan Kinder, Red Hat
 * This OSSN : https://wiki.openstack.org/wiki/OSSN/OSSN-0028
 * Original LaunchPad Bug : https://bugs.launchpad.net/nova/+bug/1337349
 * OpenStack Security ML : openstack-security@lists.openstack.org
 * OpenStack Security Group : https://launchpad.net/~openstack-ossg