Final Points

Managing a large UML server requires attention to a number of areas that aren't of great concern with a smaller server. Security requires some care. In order to run a secure installation, I recommend the following guidelines.

• The host should be running a fairly recent kernel. This will give you the performance enhancements that are trickling into the mainline kernel. Also consider applying some of the other patches I have mentioned. In particular, the file-punching patch, which is currently MADV_TRUNCATE, creates a number of new possibilities for UML management and hosting policy.

• Configure the UML instances carefully. Loadable module support should definitely be disabled, as should tt mode support. If access to host filesystems is provided, those filesystems should be bind-mounted into the UML jail. They should also be read-only if possible.

• Jail the UML instances tightly. The jail should be as minimal as you can make it, consistent with your other goals. I expect that the jailing will never be exercised since I know of no way for anyone to break out of a properly configured UML instance. However, a good jail will provide another level of security in the event of a configuration error or an exploitable escape from UML.