About a year ago I started working on my masters project. The topic turned out to be … not interesting enough to hold my attention I guess. This ended up being for the best since shortly after losing interest in my first project I got picked up on a project at work that I managed to dual-purpose for both work and school. I’m at a point where all I need to do to graduate is write this work up.
This first post (with a new tag) is a brief introduction of what this work is and why you should care. Future posts on this topic will cover the technical background and an implementation, both a simulation and code targeted at a specific Xen-based platform.
SELinux and Virtualization
I’ve been playing with SELinux for a while now and more recently I’ve had the need to work with the Xen hypervisor. Specifically I’ve been tasked with getting an SELinux policy running in the management VM (a.k.a. dom0). Most people savvy on SELinux would simply use a Linux distro that supports SELinux out of the box as their dom0 like Debian, Fedora, CentOS / RedHat. This is a valid argument but a minimal Debian install has a root file system that’s about about a gigabyte in size and this was too big for our purposes.
The project was actually in full swing when I was tasked with SELinux integration and the team had already rolled their own dom0 using openembedded . Getting SELinux up and running on an OE system was a pain but it’s done and that work isn’t all that interesting. What was interesting was implementing some policy and machinery to separate the instances of QEMU  that provide emulation for the HVM Windows guests .
SELinux does a great job of formalizing the interactions between processes. Any interactions that aren’t specified are prevented. I won’t go into a detailed explanation of the SELinux policy language  or Domain and Type Enforcement . These are well documented elsewhere .
What’s interesting is that on an SELinux system each process runs with a label defined by the label of it’s parent (the process that caused the execution) and the label on the programs binary file on disk. This means that a process performing multiple fork/exec calls on the same binary will produce child processes all with the same SELinux label. Under nearly all circumstances this is the desired effect. On our specific platform there is a case where this isn’t what we want.
As our platform is Xen, xend is the daemon responsible for managing VMs. When xend starts an HVM client it executes QEMU. This causes all QEMU instances running to have the same SELinux label. In SELinux speak we would say these QEMU instances are all running “in the same domain” which is equivalent to them all having the same permissions, or more precisely, the same access to system resources.
At this point understanding the separation goals of Xen is important. Where an OS kernel aims to keep running processes separate, Xen aims to keep running VMs separate. Having instances of QEMU operating on behalf of what are effectively untrusted client VMs all with the same SELinux label undermines this goal of separation.
Separating QEMU Instances
The sVirt  project specifically addressed this problem with a prototype implementation some time back. Eventually this was integrated with libvirt so if you’re running libvirt with the SELinux security driver loaded then presumably you’ve got these protections in place. But again due to the embedded nature of our dom0 libvirt was way more software than we needed.
It became necessary to implement the basic sVirt design but integrated directly into our management stack. This isn’t a line-for-line re-implementation of the libvirt SELinux code though. I’ve made a number of changes that I will discuss in detail in the following posts though the design goals remain the same: keep instances of QEMU separate. The metric we’re using to judge the usefulness of this work is our answer to the question “what could a compromised QEMU instance access?”. If the answer to this is “all of the virtual hard disks on the system” then obviously we’ve failed. Instead we’re aiming to confine a QEMU instance to the resources it needs to function, like the virtual disks belonging to the VM it is providing services to.
Now that the basic problem is laid out the next step is to cover some background.
In my next post I’ll cover the background necessary to understand the specific pieces of the SELinux policy that this work uses to achieve these goals: the MCS policy.
1 the Open Embedded project: http://www.openembedded.org/index.php/Main_Page
2 the QEMU project: http://wiki.qemu.org/Main_Page
3 Xen-HVM: http://en.wikipedia.org/wiki/QEMU#Xen-HVM
4 SELinux policy language: http://selinuxproject.org/page/PolicyLanguage
5 Practical Domain and Type Enforcement for UNIX: http://portal.acm.org/citation.cfm?id=882491.884237
6 SELinux By Example: http://www.informit.com/store/product.aspx?isbn=0131963694
7 sVirt Requirements Document: http://selinuxproject.org/page/Svirt_requirements_v1.0
8 The libvirt Project: http://libvirt.org/
9 libvirt SELinux Driver: http://libvirt.org/drvqemu.html#securitysvirt
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