This week I have spent my time at LinuxCon Europe and KVM Forum 2012. I gave a talk titled “Building application sandboxes on top of KVM or LXC using libvirt”. For those who enquired afterwards, the slides are now available.
As mentioned previously, today I presented a talk at FOSDEM 2012, titled “Building application sandboxes on top of LXC and KVM with libvirt”. As promised I have now uploaded the PDF slides for public access. For further information about libvirt-sandbox, consult this previous blog post on the subject. Also keep an eye on this site for further blog posts in the future. Thanks to everyone who attended the talk. I look forward to returning again in a year’s time for another update.
As many readers are no doubt aware, the FOSDEM 2012 conference is taking place this weekend in Brussels. This year I was organized enough to submit a proposal for a talk and was very happy to be accepted. My talk is titled “Building app sandboxes on top of LXC and KVM with libvirt” and is part of the Virtualization & Cloud Dev Room. As you can guess from the title, I will be talking in some detail about the libvirt-sandbox project I recently announced. Richard Jones is also attending to provide a talk on libguestfs and how it is used in cloud projects like OpenStack. There will be three talks covering different aspects of the oVirt project, a general project overview, technical look at the management engine and a technical look at the node agent VDSM. Finally the GNOME Boxes project I mentioned a few weeks ago will also be represented in the CrossDesktop devroom.
Besides these virtualization related speakers, there are a great many other Red Hat people attending FOSDEM this year, so we put together a small flyer highlighting all their talks. In keeping with the spirit of FOSDEM, these talks will of course be community / technically focused, not corporate marketing ware :-) I look forward to meeting many people at FOSDEM this year, and if all goes well, make it a regular conference to attend.
I have mentioned in passing every now & then over the past few months, that I have been working on a tool for creating application sandboxes using libvirt, LXC and KVM. Last Thursday, I finally got around to creating a first public release of a package that is now called libvirt-sandbox. Before continuing it is probably worth defining what I consider the term “application sandbox” to mean. My working definition is that an “application sandbox” is simply a way to confine the execution environment of an application, limiting the access it has to OS resources. To me one notable point is that there is no need for a separate / special installation of the application to be confined. An application sandbox ought to be able to run any existing application installed in the OS.
For a few Fedora releases, users have had the SELinux sandbox command which will execute a command with a strictly confined SELinux context applied. It is also able to make limited use of the kernel filesystem namespace feature, to allow changes to the mount table inside the sandbox. For example, the common case is to put in place a different $HOME. The SELinux sandbox has been quite effective, but there is a limit to what can be done with SELinux policy alone, as evidenced by the need to create a setuid helper to enable use of the kernel namespace feature. Architecturally this gets even more problematic as new feature requests need to be dealt with.
As most readers are no doubt aware, libvirt provides a virtualization management API, with support for a wide variety of virtualization technologies. The KVM driver is easily the most advanced and actively developed driver for libvirt with a very wide array of features for machine based virtualization. In terms of container based virtualization, the LXC driver is the most advanced driver in libvirt, often getting new features “for free” since it shares alot of code with the KVM driver, in particular anything cgroup based. The LXC driver has always had the ability to pass arbitrary host filesystems through to the container, and the KVM driver gained similar capabilities last year with the inclusion of support for virtio 9p filesystems. One of the well known security features in libvirt is sVirt, which leverages MAC technology like SELinux to strictly confine the execution environment of QEMU. This has also now been adapted to work for the LXC driver.
Looking at the architecture of the SELinux sandbox command last year, it occurred to me that the core concepts mapped very well to the host filesystem passthrough & sVirt features in libvirt’s KVM & LXC drivers. In other words, it ought to be possible to create application sandboxes using the libvirt API and suitably advanced drivers like KVM or LXC. A few weeks hacking resulted in a proof of concept tool virt-sandbox which can run simple commands in sandboxes built on LXC or KVM.
A command line tool for running applications inside a sandbox is great, but even more useful would be an API for creating application sandboxes that programmers can use directly. While libvirt provides an API that is portable across different virtualization technologies, it cannot magically hide the differences in feature set or architecture between the technologies. Thus the decision was taken to create a new library called libvirt-sandbox that provides a higher level API for managing application sandboxes, built on top of libvirt. The virt-sandbox command from the proof of concept would then be re-implemented using this library API.
The libvirt-sandbox library is built using GObject to enable it to be accessible to any programming language via GObject Introspection. The basic idea is that programmer simply defines the desired characteristics of the sandbox, such as the command to be executed, any arguments, filesystems to be exposed from host, any bind mounts, private networking configuration, etc. From this configuration description, libvirt-sandbox will decide upon & construct a libvirt guest XML configuration that can actually provided the requested characteristics. In other words, the libvirt-sandbox API is providing a layer of policy avoid libvirt, to isolate the application developer from the implementation details of the underlying hypervisor.
Building sandboxes using LXC is quite straightforward, since application confinement is a core competency of LXC. Thus I will move straight to the KVM implementation, which is where the real fun is. Booting up an entire virtual machine probably sounds like quite a slow process, but it really need not be particularly if you have a well constrained hardware definition which avoids any need for probing. People also generally assume that running a KVM guest, means having a guest operating system install. This is absolutely something that is not acceptable for application sandboxing, and indeed not actually necessary. In a nutshell, libvirt-sandbox creates a new initrd image containing a custom init binary. This init binary simply loads the virtio-9p kernel module and then mounts the host OS’ root filesystem as the guest’s root filesystem, readonly of course. It then hands off to a second boot strap process which runs the desired application binary and forwards I/O back to the host OS, until the sandboxed application exits. Finally the init process powers off the virtual machine. To get an idea of the overhead, the /bin/false binary can be executed inside a KVM sandbox with an overall execution time of 4 seconds. That is the total time for libvirt to start QEMU, QEMU to run its BIOS, the BIOS to load the kernel + initrd, the kenrel to boot up, /bin/false to run, and the kernel to shutdown & QEMU to exit. I think 3 seconds is pretty impressive todo all that. This is a constant overhead, so for a long running command like an MP3 encoder, it disappears into the background noise. With sufficient optimization, I’m fairly sure we could get the overhead down to approx 2 seconds.
The Fedora review of the libvirt-sandbox package was nice & straightforward, so the package is already available in rawhide for ready to test the VirtSandbox F17 feature. The virt-sandbox command is provided by the libvirt-sandbox RPM package
# yum install libvirt-sandbox
Assuming libvirt is already installed & able to run either LXC or KVM guests, everything is ready to use immediately.
A first example is to run the ‘/bin/date’ command inside a KVM sandbox:
$ virt-sandbox -c qemu:///session /bin/date Thu Jan 12 22:30:03 GMT 2012
You want proof that this really is running an entire KVM guest ? How about looking at the /proc/cpuinfo contents:
$ virt-sandbox -c qemu:///session /bin/cat /proc/cpuinfo processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 2 model name : QEMU Virtual CPU version 1.0 stepping : 3 cpu MHz : 2793.084 cache size : 4096 KB fpu : yes fpu_exception : yes cpuid level : 4 wp : yes flags : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm up rep_good nopl pni cx16 hypervisor lahf_lm bogomips : 5586.16 clflush size : 64 cache_alignment : 64 address sizes : 40 bits physical, 48 bits virtual power management:
How about using LXC instead of KVM, and providing an interactive console instead of just a one-shot command ? Yes, we can do that too:
$ virt-sandbox -c lxc:/// /bin/sh sh-4.2$ ps -axuwf USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.0 165436 3756 pts/0 Ss+ 22:31 0:00 libvirt-sandbox-init-lxc berrange 24 0.0 0.1 167680 4688 pts/0 S+ 22:31 0:00 libvirt-sandbox-init-common berrange 47 0.0 0.0 13852 1608 pts/1 Ss 22:31 0:00 \_ /bin/sh berrange 48 0.0 0.0 13124 996 pts/1 R+ 22:31 0:00 \_ ps -axuwf
Notice how we only see the processes from our sandbox, none from the host OS. There are many more examples I’d like to illustrate, but this post is already far too long.
This blog post might give the impression that every is complete & operational, but that is far from the truth. This is only the bare minimum functionality to enable some real world usage. Things that are yet to be dealt with include
For those attending FOSDEM this year, I will be giving a presentation about libvirt-sandbox in the virt/cloud track.
Oh and as well as the released tar.gz mentioned in the first paragraph, or the Fedora RPM, the code is all available in GIT
In the five years since the libvirt project started, alot has changed. The size of the libvirt API has increased dramatically; the number of languages you can access the API from has likewise grown to cover most important targets; libvirt has been translated to fit into several other object models; plugins have been developed to bind libvirt to other tools. At the same time many other libraries have grown up alongside libvirt, not least libguestfs, gtk-vnc and more recently spice-gtk. Together all these pieces provide a rich software development platform for people building virtualization management applications. A picture is worth 1000 words, so to keep this blog post short, here is the way I visualize the pieces in the virtualization tools platform, and a selection of the applications built on it (click to enlarge the image)
The language bindings for libvirt aim to be a 1-for-1 export of the libvirt C API into the corresponding language. They generally don’t attempt to change the way the libvirt API looks or is structured. There is generally completely interoperability between all language bindings, so you can trivially have part of your application written in Perl and another part written in Java and play nicely together.
The object mappings are distinct from language bindings, because they will often significantly change the structure of the libvirt API to fit in the requirement of the object system being targeted. Depending on the object systems involved, this translation might be lossless, thus an application generally has to pick one object system & stick with it. It is not a good idea to do a mixture of SNMP and QMF calls from the same application.
Many common infrastructure applications can be extended by adding plugins for new functionality. This particularly common with network monitoring or performance collection applications. libvirt can of course be used to create plugins for such applications
The development of a set of GObject based libraries came about after noticing that many users of the basic libvirt API were having to solve similar problems over & over. For example, every application wanted some programmatic way to extract info from XML documents. Many applications wanted libvirt translated into GObjects. Many applications needed a way to determine optimal hardware configuration for operating systems. The primary reasons for choosing to use GObject as the basis for these APIs was first to facilitate development of graphical desktop applications. With the advent of GObject Introspection, the even more compelling reason is that you get language bindings to all GObject libraries for free. Contrary to popular understanding, GObject is not solely for GTK based desktop applications. It is entirely independent of GTK and can be easily used from any conceivable application. If libvirt were to be started from scratch again today, it would probably go straight for GObject as the basis for the primary C library. It is that compelling.
There is undoubtably plenty of stuff I left out of this diagram & description. For example there are many other data center & cloud management projects that are based on libvirt, which I left out for clarity. There are plenty more libvirt plugins for other applications too, many I will never have heard about. No doubt our future plans will change too, as we adapt to new information. This should have given a good overview of how broad the open source virtualization tools software development ecosystem has become.