Gentoo Hardened on the move


Sven Vermeulen Thu 26 July 2012

Gentoo Hardened is thriving and going forward. For those that don't exactly know what Gentoo Hardened is - it is a Gentoo project dedicated to bring Gentoo in a shape ready for highly secure, high stability production server environments. This is what we live by, and why we do what we do. To accomplish this goal, we use a great community of developers & users that work on several subprojects: the implementation of kernel hardening features such as grSecurity, memory-based protection schemes such as PaX, toolchain updates to harden against buffer overflows and memory attacks, mandatory access control schemes such as SELinux and RSBAC.

In Gentoo Hardened we then integrate these technologies in Gentoo Linux so that it is usable by a larger community, well documented and supported. I'm myself heavily working on the SELinux integration & documentation aspects, and am hoping to contribute even further - but more about that in a minute.

Today, we had an online meeting where developers present their current "state of affairs" and the upcoming things they are going to work on. This is done about once every month in the IRC chat channel #gentoo-hardened on the freenode network. Of course, most of the developers are available on the chat channel on an (almost) daily basis.

Todays meeting gave us feedback on the following (and remind you, this is one month of volunteer-driven work)...


When we talk about the toolchain, we mean the set of tools and libraries needed to build a (hardened) system. We put most focus on the GCC compiler because it contains most of the changes we support (like stack smashing protection, position independent code/executable changes, etc.) but work on libraries like glibc and uclibc are on their way as well.

Zorry (yeah, I'm going to use nicknames here so you know who you're talking to on IRC ;-) is working on getting our patches upstream (meaning that the main GCC development can incorporate our patches). Sending and working together with the main projects is very important as it provides not only continuity on the patches (once they are upstream, more people are maintaining the code than just you/us), but also gives a multi-eye view on the code: is it of high quality? Does it comply with the proper security guidelines? What about impact of the code on things we don't or haven't considered yet?

On the library part, blueness (one of our Gentoo Hardened developers and - imho - an expert in many fields) has been working on Hardened support on ARM (armv7a) with uclibc. He has put up stage4 files for armv7a softfloat uclibc hardened and is working on those for hardfloat. This means that ARM with uclibc+hardened or ARM with glibc+hardened are working - he has even tested an xfce4 desktop on ARM with uclibc and hardened toolchain.

ARM support is becoming more and more important in the technology field. Other major processor players like SPARC, Itanium, PowerPC, ... are slowly seeing less and less market share, whereas ARM - albeit currently still a very small player - is rapidly gaining momentum. You all know ARM from the smartphones and other embedded-like platforms, but ARM on servers is coming faster than you expect. Being a simple platform with low energy consumption and good commercial backing (both on CPU level as well as platform support), we can see ARM becoming a major player on this - and Gentoo Hardened is actively working towards it.


Within Gentoo Hardened, we support the grSecurity and PaX kernel patches for a more hardened Linux kernel. But this additional hardening can also sometimes interfere with the normal functioning of systems. To help users in their configuration quest, grSecurity allows users to select a few "prebuilt configuration types" in the kernel build.

Previously, these types where one of the following label: "virtualization", "workstation" or "server". Based on these labels, the security settings that did not negatively effect the functioning of the system were selected. Recently, the labels have changed into a question-based configuration: is it a server or not? will you use it for virtualization and if so, on host or guest? Is performance for you more important than security? These questions are now also integrated in our hardened-sources.

While working and testing one of the kernel settings (KERNEXEC - kernel non-executable pages, to protect non-code containing memory pages from being used to run (potentially hostile/injected) code from) in a virtualized environment, prometheanfire (another Gentoo Hardened developer) noticed a possible regression on the performance of guests if the host had KERNEXEC set. A severe performance hit is to be expected if the host processor doesn't support hardware-assisted nested page tables (a method for supporting memory page virtualization), but this also seemed to occur on systems with nested page tables (/proc/cpuinfo flag ept for Intel, or rvi for AMD). So more testing (from others as well) is therefore needed to confirm and work on this.


One of Gentoo Hardened's subprojects (and one I'm most actively working on) is its support for SELinux or Security Enhanced Linux. It offers a Mandatory Access Control implementation for Linux, ensuring that users cannot change the security settings that an administrator has set (which is Discretionary Access Control if they can), but also enforce that services/processes can not be forced to do things they are not meant to do. This provides reasonable protection against things like remote code execution exploits, or just limit what an administrator wants particular processes to do. With SELinux, you can even define roles to properly identify and segregate tasks, providing a method for "segregation of duties" on OS level.

Anyway, as I said, Gentoo Hardened is actively working on SELinux integration. First of all is stages support (providing a small, deployable system unit that users can use to install a SELinux-enabled system) as currently, users are forced to switch to SELinux after having installed Gentoo, which is a multi-step approach. By offering stages, we can simplify the deployment of Gentoo Hardened SELinux. Currently, building stages works but requires some manual steps (labeling mostly) which need to be removed before we can automatically build stages. The next steps here are to see if we can build SELinux stages on non-SELinux systems (as all we need is to link the proper files with the SELinux-supporting libraries, which should work regardless of SELinux being enabled or not). The fact that users need to relabel their system during deployment is just a minor inconvenience (and a one-command fix, so easy to document too).

Another item of progression made is a SELinux-enabled (well, Gentoo Hardened grSecurity with PaX and SELinux enforcing enabled) virtual image called "selinuxnode". This Qemu/KVM image is a simple Gentoo base installation but with those security features already enabled, allowing users to take a first look at SELinux before trying it out on their own system. But this image has the potential (and now roadmap ;-) to become much more:

  • Provide a play-ground for users to test things in. Try out hammering the SELinux policy, or reproducing potential issues before reporting them (to make sure they are easily reproduceable).
  • Become a Proof-of-Concept location for new enhancements: not only updates on SELinux, but also on other hardening measures and technologies that Gentoo Hardened can support. Implementing the technologies in the VM allow other developers to test and work on it without needing to sacrifice one of their own systems.
  • Become the main system for educational (course-like) documentation. If we develop HOWTO documents, using this VM as a base allows users to follow the instructions to the letter and try things out while keeping the documentation consistent. The documentation can, in the future, also contain instructions that users need to follow as a sort-of test. At the end of the test, a simple script can easily verify on the VM if the test was finished succesfully or not.

Even further down the road, it might evolve into a system for building appliance-like, hardened services based on Gentoo Hardened. But that's a milestone too far for now. But you can always dream ;-)

On the SELinux policy development side, I'm recently focusing on two aspects: the change towards /run (which already required a few "urgent" updates and will probably need a lot more) as well as confining popular attack surfaces like browsers. Not many SELinux users run their browser in a confined space, but I personally don't run anything in unconfined domains and feel that browsers are too popular in the security area to not put attention to. So I'm struggling to have the browsers (first focus is Chromium as that one has an open bug for it, and Firefox because that is my main browser platform) fully confined yet still flexible enough (using SELinux booleans) to support users that have other wishes on their browsers.

Speaking of policy development, in the meeting it was also brought forward to support a change of stabilization of SELinux policies from the standard 30-days towards a 14-day stabilization period. In most cases, this doesn't harm users as policies are usually enhanced (allow something that was denied before) and less about reducing privileges (as it is quite hard to find out why a rule was enabled in the first place, hence our reluctant approach to "quickly" update policies). For such updates, We're suggesting a 14-day stabilization period, while retaining 30 days for larger updates such as domain policy rewrites (which are sometimes needed if an application changes too much - think init and systemd - or when its segregated into multiple parts that each need (or can have) their own SELinux domain.

Finally, we gave a quick update on our status for upstream support (as I mentioned before, having patches supported and accepted upstream is very important for us): we have 116 changesets to the policy in comparison with the 20120215 refpolicy release (which is our "upstream"). Of those changesets, 45 have been accepted and implemented upstream, 12 are pending. 55 have not been sent yet (because they still need work or more documentation before they can be accepted) and 4 will not be sent (mostly because they are gentoo-only or deviate from upstream's acceptance guidelines but fit in Gentoo's approach).

grSecurity's PaX

Blueness worked on the xattr pax support implementation (using extended attributes to store and manage the PaX flags, rather than using the ELF header changes used in the past) within Gentoo Hardened. This is now production-ready, so the proper tools will be made generally available shortly whereas the older method (mainly chpax) will be decommissioned in the very near future.

PaX markings allow the Linux kernel to toggle specific PaX settings on or off for processes so that the general state of the system can use the PaX protections while a very few set of programs that cannot work with these settings (often binary software or third party software, but some self-built software can have difficulties with PaX as well) can run without them (or with a lower set). This is much more flexible than an all-or-nothing approach. By using extended attributes, managing these markings can be done without modifying the binaries themselves. In Gentoo, proper support is also given through the paxctl-ng.eclass so developers can automatically set markings at deploy-time when needed.


In Gentoo, users select "profiles" as a way to define the defaults for their system. Profiles define stuff like the default kernel, C library, specific USE flags, toolchain, etc. For instance, users that want to use a Gentoo Hardened system with SELinux on an x86_64 system with no-multilib (all 64-bit only) select the hardened/linux/amd64/no-multilib/selinux profile.

In the last few weeks, blueness has been working on the uclibc-related profiles (hardened/linux/uclibc/\${ARCH}) using a clean slate. Gentoo supports profile inheritance, so you can "stack" one profile on top of the other. This is great for manageability, but when the profile is to support systems that are quite different from what Gentoo developers are used to, it makes sense to use a clean setup and start from there. And this is the case for hardened uclibc systems.

System integrity subproject

On this meeting the initial kick-off (after approval) was given of a new hardened subproject called system integrity. This project will focus on the implementation and support of integrity-related technologies such as (well, mainly) Linux IMA/EVM and its supporting userspace utilities and documentation. Integrity validation & enforcement is an important aspect of system security and, since I already work with SELinux, feel this is a natural improvement (since you need a MAC to enforce runtime security and use integrity to enforce detection and prevention of offline tampering).

We have great plans with IMA/EVM here, and can hopefully introduce the first few steps towards it in the selinuxnode virtual image soon ;-)


Of course, technologies are great, but documentation is always needed (even if nobody reads it (sic)). I have been documenting hardening of some settings/services using the XCCDF/OVAL languages (part of the SCAP set of standards) since not only do they provide the means to generate guides (we can generate guides in every language, XCCDF is probably the least flexible of them all) but they also support the validation of the settings in an automated manner.

By using XCCDF/OVAL-supporting software such as Open-SCAP (app-forensics/openscap in Gentoo) you can interpret these guides in an unattended manner, generating reports on the state of your services compared to these guides, and even have specific profiles (one system uses a different set of hardening guidelines than another). Since Gentoo Hardened is about supporting secure & stable production environments, it is logical that we can offer best practices on how to handle Gentoo-provided/supported services. And by using these within the SCAP standard, the guides might even be leveraged further than a regular online HOWTO could.

And all that from one project?

Not really. Gentoo Hardened here plays several roles: integrator for technologies that are managed in other (free software) projects, and development for technologies or settings that are either specific to Gentoo or not available to the public to the extend Gentoo Hardened believes is needed. You must understand this is possible thanks to the tremendous effort that all these projects perform. Gentoo Hardened here plays the role that every Linux distribution has: making all these technologies and advancements fit in a way that the users can easily work with it - integrated and well supported.

Thanks to the free software nature though, Gentoo Hardened does more than what "commercial integrators" do when they deal with closed, propriatary software: it updates the code, improves it and brings it back for broader re-use. As such, it also acts a bit as development within those projects to assist them in their quest. And in my book, users are more likely to believe in an integrator that can react code-wise rather than using workarounds or "helping create a service request".

The full excerpts of this meeting (the meeting minutes - well, actually an IRC chat log excerpt) will be sent out soon by the Gentoo Hardened project lead, Zorry. Big thanks to him (and the rest of the crew) to make all this happen! I love to be part of it, and hope I can remain so for a long, long time.

Edit: RSBAC, not grSecurity's RBAC.