One of the cool new features included in Red Hat Enterprise Linux 5 was VFS polyinstantiation. This work was in support of the Multi Level Security configuration. It allows files to exist in a directory at different security classifications. The subset of files visible to the user depends on the user’s clearance. There is an excellent description of the functionality in both section 4.1.2 of Extending Linux for Multi-Level Security by Klaus Weidner, George Wilson and Loula Salem, as well as Russell Coker’s article Polyinstantiation of directories in an SELinux system.
Now there is an excellent new article on developerWorks by Robb Romans Improving Security with polyinstantiation which describes in simple and detailed terms how administrators can polyinstantiate /tmp (and other world writable directories) to help prevent attacks through /tmp. This technique usable whether or not SELinux is enabled. This article helps answer calls for the complete elimination of world writable directories so as to defeat resource exhaustion attacks (quotas were described as “non-optimal”). One can instead use the method described in this paper to polyinstantiate world writable directories to completely different devices to effectively eliminate the attack. (Yes, they grok TMPDIR. And, yes, unfortunately there are customers who won’t use SELinux.)
So if you were wondering how you can get your feet wet with polyinstantiation, give the steps described in Robb’s article a try.