Category Archives: Commentary

A community member shares a viewpoint

Q&A: Xen Project Release Strengthens Security and Pushes New Use Cases

The following Q&A with Lars Kurth, the Xen Project chairperson, was first published on

Xen Project technology supports more than 10 million users and is a staple in some of the largest clouds in production today, including Amazon Web Service, Tencent, and Alibaba’s Aliyun. Recently, the project announced the arrival of Xen Project Hypervisor 4.7. This new release focuses on improving code quality, security hardening and features, and support for the latest hardware. It is also the first release of the project’s fixed-term June – December release cycles. The fixed-term release cycles provide more predictability making it easier for consumers of Xen to plan ahead.

We recently sat down with the Xen Project chairperson, Lars Kurth, to talk about some of the key features of the release and the future of Xen Project technology. Lars will be discussing this topic and more during Xen Project’s Developer Summit in Toronto, CA from August 25-26 — the conference is directly after LinuxCon North America.

Q: What was the focus on this release?

Lars Kurth: There were five areas that we focused on for this release (full details are in our blog). In summary, we focused on security features, migration support, performance and workloads, support for new hardware features, and drivers and devices (Linux, FreeBSD and other).

Security is consistently something that we focus on in all of our releases. There are a lot of people that rely on Xen Project technology and security is our top concern in any release as well as how we organize our process around security disclosures.

Q: What was the biggest feature coming out of this release?

Lars: The biggest feature for us is live patching, which is a technology that enables re-boot free deployment for security patches to minimize disruption and downtime during security upgrades for cloud admins. It essentially eliminates all cloud reboots, making cloud providers and their users much more safe. It also eliminates a lot of headaches for system and DevOps admins of the world.

Q: Xen is often associated with the cloud, but are there additional use cases that you see growing around this technology, if so why?

Lars: We are seeing a lot of growth in terms of contributions, as well as many different use cases emerging, including automotive, aviation, embedded scenarios, security, and also IoT. In addition, we continue to grow within the public cloud sector and traditional server virtualization.

On the security front, for example, a number of vendors such as A1Logic, Bitdefender, Star Lab and Zentific have released or are working on new Xen Project-based security solutions. In addition, the security focused and Xen-based OpenXT project has started to work more closely with the Xen Project community.

Long-time contributors to the Xen Project, such as DornerWorks – a premier provider of electronic engineering services for the aerospace, medical, automotive, and industrial markets – have expanded their scope and are now providing support for the Xen Xilinx Zynq Distribution targeting embedded use-cases. We have also seen an increasing number of POCs and demos of automotive solutions, which include Xen as a virtualization solution.

Growth in these sectors is largely due to the Xen Project’s flexibility, extensibility, customisability and a clear lead when it comes to security-related technologies. Over the last year, we have also seen contributions increase from developers with strong security and embedded backgrounds. In fact, this totaled nearly 17 percent of the overall contributions in this release cycle, up from 9 percent in the previous release.

Q: How did you address these uses cases in this latest release?

Lars: We introduced the ability to remove core Xen Project Hypervisor features at compile via KCONFIG. This creates a more lightweight hypervisor and eliminates extra attack surfaces that are beneficial in security-first environments and microservice architectures. Users will still be able to get the core hypervisor functions, but they won’t receive all the drivers, schedulers, components or features that might not fit their use case.

Essentially it gives people an “a la carte” feature set. They can decide what they need for compliance, safety or performance reasons.

Q: Were there any new contributors for this release that surprised you?

Lars: We had three new companies contributing to the project: Star Lab, Bosch and Netflix. I met engineers from Star Lab for the first time at the 2015 Developer Summit less than a year ago, and helped introduce them to the Project’s culture. In that short period of time, Doug Goldstein from Star Lab has moved into the top five contributors and top 10 code reviewers for the Project.

I was surprised about Netflix’s contributions; I didn’t even know the company used Xen. Netflix improved and secured the VPMU feature, which is incredibly useful for system tuning and performance monitoring. Bosch Car Multimedia GmbH added some new ARM functionality. In addition, we have seen quite a bit of Xen related development in upstream and downstream projects such as Linux, FreeBSD, NetBSD, OpenBSD, QEMU and Libvirt.

Q: What’s next for Xen Project? Where do you think the technology is heading in the future and why?

Lars: In the last three releases, we introduced several major new features such as PVH, COLO, new schedulers, VMI, Live Patching, Graphics Virtualization, etc. and significant re-work of existing features such as Migration and the Xen Security Modules (XSM). Looking at trends within the community, I expect that stepwise evolution of large new features to continue.

Some new capabilities, such as restartable Dom0’s, and additional techniques to provide more isolation and security, are also likely to appear. In addition, it looks likely that we will see some GPU virtualization capabilities for GPUs that target the ARM ecosystem, although it is not yet clear whether these will be available as open source. I also expect that both Intel and ARM hardware features will be closely tracked.

Some areas, such as new schedulers, XSM, PVH and Live Patching, will see significant efforts to harden and improve existing functionality. The goal is to ensure their swift adoption in commercial products and Linux and BSD distributions. Some features, which are not enabled by default are likely to become part of the Xen Project Hypervisor’s default configuration.

Will Docker Replace Virtual Machines?

Docker is certainly the most influential open source project of the moment. Why is Docker so successful? Is it going to replace Virtual Machines? Will there be a big switch? If so, when?

Let’s look at the past to understand the present and predict the future. Before virtual machines, system administrators used to provision physical boxes to their users. The process was cumbersome, not completely automated, and it took hours if not days. When something went wrong, they had to run to the server room to replace the physical box.

With the advent of virtual machines, DevOps could install any hypervisor on all their boxes, then they could simply provision new virtual machines upon request from their users. Provisioning a VM took minutes instead of hours and could be automated. The underlying hardware made less of a difference and was mostly commoditized. If one needed more resources, it would just create a new VM. If a physical machine broke, the admin just migrated or resumed her VMs onto a different host.

Finer-grained deployment models became viable and convenient. Users were not forced to run all their applications on the same box anymore, to exploit the underlying hardware capabilities to the fullest. One could run a VM with the database, another with middleware and a third with the webserver without worrying about hardware utilization. The people buying the hardware and the people architecting the software stack could work independently in the same company, without interference. The new interface between the two teams had become the virtual machine. Solution architects could cheaply deploy each application on a different VM, reducing their maintenance costs significantly. Software engineers loved it. This might have been the biggest innovation introduced by hypervisors.

A few years passed and everybody in the business got accustomed to working with virtual machines. Startups don’t even buy server hardware anymore, they just shop on Amazon AWS. One virtual machine per application is the standard way to deploy software stacks.

Application deployment hasn’t changed much since the ’90s though. Up until then, it still involved installing a Linux distro, mostly built for physical hardware, installing the required deb or rpm packages, and finally installing and configuring the application that one actually wanted to run.

In 2013 Docker came out with a simple, yet effective tool to create, distribute and deploy applications wrapped in a nice format to run in independent Linux containers. It comes with a registry that is like an app store for these applications, which I’ll call “cloud apps” for clarity. Deploying the Nginx webserver had just become one “docker pull nginx” away. This is much quicker and simpler than installing the latest Ubuntu LTS. Docker cloud apps come preconfigured and without any unnecessary packages that are unavoidably installed by Linux distros. In fact the Nginx Docker cloud app is produced and distributed by the Nginx community directly, rather than Canonical or Red Hat.

Docker’s outstanding innovations are the introduction of a standard format for cloud applications, including the registry. Instead of using VMs to run cloud apps, Linux containers are used instead. Containers had been available for years, but they weren’t quite popular outside Google and few other circles. Although they offer very good performance, they have fewer features and weaker isolation compared to virtual machines. As a rising star, Docker made Linux containers suddenly popular, but containers were not the reason behind Docker’s success. It was incidental.

What is the problem with containers? Their live-migration support is still very green and they cannot run non-native workloads (Windows on Linux or Linux on Windows). Furthermore, the primary challenge with containers is security: the surface of attack is far larger compared to virtual machines. In fact, multi-tenant container deployments are strongly discouraged by Docker, CoreOS, and anybody else in the industry. With virtual machines you don’t have to worry about who is going to use it or how it will be used. On the other hand, only containers that belong to the same user should be run on the same host. Amazon and Google offer container hosting, but they both run each container on top of a separate virtual machine for isolation and security. Maybe inefficient but certainly simple and effective.

People are starting to notice this. At the beginning of the year a few high profile projects launched to bring the benefits of virtual machines to Docker, in particular Clear Linux by Intel and Hyper. Both of them use conventional virtual machines to run Docker cloud applications directly (no Linux containers are involved). We did a few tests with Xen: tuning the hypervisor for this use case allowed us to reach the same startup times offered by Linux containers, retaining all the other features. A similar effort by Intel for Xen is being presented at the Xen Developer Summit and Hyper is also presenting their work.

This new direction has the potential to deliver the best of both worlds to our users: the convenience of Docker with the security of virtual machines. Soon Docker might not be fighting virtual machines at all, Docker could be the one deploying them.

A Chinese translation of the article is available here:

The Bare-Metal Hypervisor as a Platform for Innovation

In this industry, everyone seems to talk about innovation, but very few platforms exist which foster innovation.  More times than not, “innovation” is simply a buzzword used by some marketing campaign to hawk something about as novel as twenty-year-old accounting software.

Innovation does occur, of course.  But often real innovation leverages what already exists to create something which doesn’t yet exist.  It may borrow from the known, but it produces something previously unknown.  For example, the industry has been going wild over cloud computing in the past few years, but many of the core cloud computing concept are actually old mainframe concepts reimagined in the world of commodity servers.

Making a Place for Innovation to Thrive

A bare-metal hypervisor — like the one produced by the Xen Project — can be an excellent platform for innovation.  We think of hypervisors as old technology, plumbing for newer technologies like cloud — and, indeed, they are.  But the nature of the bare-metal hypervisor makes it an excellent platform for innovation to take place.

Continue reading

Hardening Hypervisors Against VENOM-Style Attacks

This is a guest blog post by Tamas K. Lengyel, a long-time open source enthusiast and Xen contributor. Tamas works as a Senior Security Researcher at Novetta, while finishing his PhD on the topic of malware analysis and virtualization security at the University of Connecticut.

The recent disclosure of the VENOM bug affecting major open-source hypervisors, such as KVM and Xen, has many reevaluating their risks when using cloud infrastructures. That is a very good thing indeed. Complex software systems are prone to bugs and errors.  Virtualization and the cloud have been erroneously considered by many to be a silver bullet against intrusions and malware. The fact is the cloud is anything but a safe place. There are inherent risks in the cloud and it’s very important to put those risks in their proper context.


VENOM is one of many vulnerabilities involving hypervisors (e.g. [Qemu], [ESX], [KVM], [Xen]). And, while Venom is indeed a serious bug and can result in a VM escape, which in turn can compromise all VMs on the host, it doesn’t have to be. In fact, we’ve known about VENOM-style attacks for a long time. Yet, there are easy-to-deploy counter-measures to mitigate the risk of such exploits natively available in both Xen and KVM (see RedHat’s blog post on the same topic).

What is the root cause of VENOM? Emulation

While modern systems come with a plethora of virtualization extensions, many components are still being emulated, usually devices such as your network card, graphics card and your hard drive. While Linux comes with paravirtual (virtualization-aware) drivers to create such devices, emulation is often the only solution to run operating systems that do not have kernel drivers. This has been traditionally the case with Windows (note that Citrix recently open-sourced its Windows PV drivers so this is no longer the case). This emulation layer has been implemented in QEMU, which caused VENOM and a handful of other VM-escape bugs in recent years. However, QEMU is not the only place where emulation is used within Xen. For a variety of interrupts and timers, such as RTC, PIT, HPET, PMTimer, PIC, IOAPIC and LAPIC, there is a baked-in emulator in Xen itself for performance and scalability reasons. As with QEMU, this emulator has been just as exploitable. This is simply because programming emulators properly is really complex and error-prone.

Available Mitigations

We’ve known how complicated emulation is for some time. In 2011, this Black Hat presentation on Virtunoid demonstrated a VM escape attack against KVM through QEMU. The author of Virtunoid even cautioned there would be many bugs of that nature found by researchers. Fast-forward four years and we now have VENOM.

The good news is it’s easy to mitigate all present and future QEMU bugs, which the recent Xen Security Advisory emphasized as well. Stubdomains can nip the whole class of vulnerabilities exposed by QEMU in the bud by moving QEMU into a de-privileged domain of its own. Instead of having QEMU run as root in dom0, a stubdomain has access only to the VM it is providing emulation for. Thus, an escape through QEMU will only land an attacker in a stubdomain, without access to critical resources. Furthermore, QEMU in a stubdomain runs on MiniOS, so an attacker would only have a very limited environment to run code in (as in return-to-libc/ROP-style), having exactly the same level of privilege as in the domain where the attack started. Nothing is to be gained for a lot of work, effectively making the system as secure as it would be if only PV drivers were used.

While it might sound complex, it’s actually quite simple to take advantage of this protection. Simply add the following line to your Xen domain configuration, and you gain immediate protection against VENOM and the whole class of vulnerabilities brought to you by QEMU:

device_model_stubdomain_override = 1

However, as with most security systems, it comes at a cost. Running stubdomains requires a bit of extra memory as compared to the plain QEMU process in dom0. This in turn can limit the number of VMs a cloud provider may be able to sell, so they have little incentive to enable this feature by default on their end. However, this protection works best if all VMs on the server run with stubdomains. Otherwise you may end up protecting your neighbors against an escape attack from your domain, while not being protected from an escape attack from theirs. It’s no surprise that some users would not want to pay for this extra protection, even if it was available. But for private clouds and exclusive servers there is no reason why it shouldn’t be your default setting.

7 Unikernel Projects to Take On Docker in 2015

This is a reprint of a 3-part unikernel series published on In part one, Xen Project Advisory Board Chairman Lars Kurth takes a closer look at the rise of unikernels and several up-and-coming projects to keep close tabs on in the coming months.

Docker and Linux container technologies dominate headlines today as a powerful, easy way to package applications, especially as cloud computing becomes more mainstream. While still a work-in-progress, they offer a simple, clean and lean way to distribute application workloads.

With enthusiasm continuing to grow for container innovations, a related technology called unikernels is also beginning to attract attention. Known also for their ability to cleanly separate functionality at the component level, unikernels are developing a variety of new approaches to deploy cloud services.

Traditional operating systems run multiple applications on a single machine, managing resources and isolating applications from one another.  A unikernel runs a single application on a single virtual machine, relying instead on the hypervisor to isolate those virtual machines. Unikernels are constructed by using “library operating systems,” from which the developer selects only the minimal set of services required for an application to run. These sealed, fixed-purpose images run directly on a hypervisor without an intervening guest OS such as Linux.

unikernel illustration
Image credit: Xen Project.


As well as improving upon container technologies, unikernels are also able to deliver impressive flexibility, speed and versatility for cross-platform environments, big data analytics and scale-out cloud computing. Like container-based solutions, this technology fulfills the promise of easy deployment, but unikernels also offer an extremely tiny, specialized runtime footprint that is much less vulnerable to attack.

There are several up-and-coming open source projects to watch this year, including ClickOSClive,HaLVMLINGMirageOSRump Kernels and OSv among others, with each of them placing emphasis on a different aspect of the unikernel approach.  For example, MirageOS and HaLVM take a clean-slate approach and focus on safety and security, ClickOS emphasizes speed, while OSv and Rump kernels aim for compatibility with legacy software. Such flexible approaches are not possible with existing monolithic operating systems, which have decades of assumptions and trade-offs baked into them.

How are unikernels able to deliver better security? How do the various unikernel implementations differ in their approach? Who is using the technology today? What are the key benefits to cloud and data center operators? Will unikernels on hypervisors replace containers, or will enterprises use a mix of all three? If so, how and why?  Answers to these questions and insights from the key developers behind these exciting new projects will be covered in parts two and three of this series.

For those interested in learning more about unikernels, the entire series is available as a white paper titled “The Next Generation Cloud: The Rise of the Unikernel.”

XSA-108: Not the vulnerability you’re looking for

There has an unusual amount of media attention to XSA-108 during the embargo period (which ended Wednesday) — far more than any of the previous security issues the Xen Project has reported. It began when a blogger complained that Amazon was telling customers it would be rebooting VMs in certain regions before a specific date. Other media outlets picked it up and noticed that the date happened to coincide with the release of XSA-108, and conjectured that the reboots had something to do with that. Soon others were conjecturing that, because of the major impact to customers of rebooting, that it must be something very big and important, similar to the recent Heartbleed and Shell Shock vulnerabilities. Amazon confirmed that the reboots had to do with XSA-108, but could say nothing else because of the security embargo.

Unfortunately, because of the nature of embargoes, nobody with any actual knowledge of the vulnerability was allowed to say anything about it, and so the media was entirely free to speculate without any additional information to ground the discussion in reality.

Now that the embargo has lifted, we can talk in detail about the vulnerability; and I’m afraid that people looking for another Shell Shock or Heartbleed are going to be disappointed. No catchy name for this one.

Continue reading