Cappsule was released a few weeks ago and we're happy of the positive attention received. However, relying on a custom hypervisor make its usage quite difficult across various distros. This blogpost explains how the same goals can be achieved on Linux with usual software. Impatient readers can directly checkout NoFear's GitHub.

Cappsule's Goals

One of the main original Cappsule's goal was to provide transparent isolation to Windows and Linux thanks to hardware virtualization. On the fly virtualization seemed appealing to us since we thought that this idea could be implemented equally on Windows and Linux. Unfortunately one can admit that it was quite naive.

The development on Linux is almost straightforward and we quickly got a working PoC within a few weeks. Kernel source code and available literature were a great help to understand and circumvent the difficulties we encountered. Almost anything is possible with a few syscalls on Linux. For instance, the swap can easily be disabled with the swapoff command, and putting a CPU offline is just as simple as:

# echo 0 > /sys/devices/system/cpu/cpu6/online

The situation is completely different for Windows and we still don't have a rock solid solution to answer these two technical challenges, even after long weeks of reverse engineering.

As a reminder, the goals of Cappsule are:

  • instant launch of any software (cli or gui) in a hardware VM

  • transparently

  • with zero configuration

  • with sound and mic support

The rest of this blogpost is focused on Linux only and describes an alternative to Cappsule relying exclusively on free software components. The question on how to get a similar solution on Windows remains unanswered yet...

NoFear Internals

Although one doesn't have the possibility to tweak the Windows kernel binaries, the situation highly differs on Linux. For instance, the Intel® Clear Containers project shows that a Linux kernel can be booted in less than 45 milliseconds! A great LWN article [0] gives some technical details on how it's possible. If we only consider Linux operating system, on the fly virtualization isn't mandatory and one can use on-the-shelf software. One can take advantage of this to boot a minimal Linux kernel and run one single application inside it. Isolation made easy.

The NoFear project relies on 3 open source components: KVM, kvmtool and Xpra. All the hard work is done by each of these projects, and NoFear is only the glue (around 500 lines of code) that joins them together.

Hardware virtualization


KVM is included in mainline Linux kernel as of 2.6.20 and is available by default on major if not all distributions. For instance, the kernel modules are automatically loaded on Ubuntu 16.04 LTS if the CPU has hardware virtualization extensions:

$ lsmod | grep kvm
kvm_intel             172032  0
kvm                   540672  1 kvm_intel
irqbypass              16384  1 kvm

It works great and is known to be the hypervisor behind Google Compute Engine, with a solid security reputation [1].

Hypervisor interface

QEMU is the default interface for KVM but it doesn't fulfil exactly our requirements. It provides far more features that needed and isn't easy to modify. A few people wrote another software entitled kvmtool which seems more convenient to us. This LWN article explains the origin of the native KVM tool. It's worth noting that kvmtool is rkt's hypervisor [2]. Interestingly, the --sandbox option allows the execution of any command line in a new VM almost transparently.

We think that kvmtool doesn't get all the attention it deserves, but it might be related to the following observations. The first results for kvmtool on a search engine redirect to the Clear Containers' GitHub, while the official git repository is on Telling which kvmtool repository is the official one is indeed difficult, and a website would be great. Incidentally, there's no bugtracker and the mailing-list is KVM's, which make contributions difficult for non-kernel developers in my opinion.

Finally, the C programming language isn't memory safe and can easily lead to bugs and vulnerabilities; but we're not aware of other alternatives. novm is developed in Go but doesn't seem to be maintained anymore.


A custom Linux kernel is provided to ensure that required configuration options are enabled, but stock kernels of main distros should also work. Since no superfluous configuration (eg: driver support) option is enabled, the bzImage is built in less than 3 minutes on a recent computer. It doesn't matter for end users since the kernel image is provided by NoFear, but it's appreciated by its developers :)


Zero configuration

Cappsule makes use of detailed policies to describe which parts of the filesystem are shared between the host and the VMs. Unfortunately, these rules are pretty hard to write and are often broken after software updates. It's thus tempting for users to use the weakest policy, «unrestricted». Actually, these policies add a lot of complexity and may not be worth it. That's why we made the decision to completely get rid of any rules and policy files for NoFear.

During the creation of a new profile, /etc and /home are initialized with default configuration files (eg: /etc/passwd, /home/user/.bashrc). Once a VM is running, the following folders are shared with the host in read-only mode: /bin, /lib, /lib64, /opt, /sbin and /usr. It allows software installed in the host to be directly available to the VMs, but sensitive folders (/etc and /home) are not available not to leak any information to the guests.

Once again, we rely on kvmtool with a small patch to restrict which host folders can be shared. We found a few security issues in kvmtool's implementation of virto 9p [3] but they should be fixed soon, as we sent a patch series upstream a few days ago.


Changes to the filesystem are kept from one execution to another. Profiles are stored in ~/.lkvm/[profile-name]/ and contain every change made to the filesystem. If no -p/--profile argument is given on the command line, the default profile is used. A profile can be deleted with -d/--delete argument.

Technically, a sparse file is created during the creation of a new profile thanks to the dd's seek option:

# dd status=none if=/dev/zero of=/virt/target.disk bs=1 count=0 seek=1G

This file, /virt/target.disk, is shared between the host and the guest. Once created, an ext4 filesystem is built, and an overlay filesystem ensures that each modification is kept:

# mkfs.ext4 -q /virt/target.disk
# mount -o loop /virt/target.disk /virt/target
# mount -n -t overlay overlay \
      /virt/target/overlay \
      -o lowerdir=/host,\

All these operations are done inside the VM.


Xpra is a persistent remote display server and client for forwarding applications and desktop screens which works like a X11 proxy. It's under active development and sound features (microphone and speakers) are supported. Available on most distros and working out of the box, it's no coincidence that Firejail and Subgraph OS rely on Xpra for their GUI.

However, it didn't receive as much attention as Qubes OS GUI from a security point of view. Some tickets (eg: #11155 and #1217) give the impression that some parts of the project need a more thorough review. Hopefully, memory corruption bugs should be almost inexistent since Xpra is mostly developed in Python.

About performances, they're acceptable at the time and should even improve when virtio-sock will be available in distros' kernels.


Network works out of-the-box with kvmtool which supports several network modes. The user mode is used by default but it seems quite buggy and connection sometimes hangs for no reason. Thanks to virto-net, performances of tap mode are much better and it's also way more reliable.

In tap mode, root privileges are required during the install to:

  • set the cap_net_admin capability to the kvmtool binary to allow unprivileged user to launch VMs with network support

  • enable IP forwarding with sysctl -w net.ipv4.ip_forward=1

  • enable NAT with iptables -t nat -A POSTROUTING -j MASQUERADE

Distro Dependencies Issues

While the concepts behind NoFear are straightforward, slight differences across distros make it difficult to create a generic package. Below are the differences that we encountered during the beta-test.

The filesystem needs to be customized since usual packages are installed differently across Linux distros. For instance, on Debian-like systems a lot of usual files are actually symlinks in /etc/alternatives/ (man update-alternatives):

$ ls -l /etc/alternatives
lrwxrwxrwx   1 root root    29 Jul 19 09:27 google-chrome -> /usr/bin/google-chrome-stable
lrwxrwxrwx   1 root root    15 Jul 18 11:18 nc -> /bin/nc.openbsd
lrwxrwxrwx   1 root root    18 Jul 18 11:21 vim -> /usr/bin/vim.basic

If these symlinks aren't present in the VM filesystem, those commands are obviously not found. In a similar way, font files are located in different folders across distros. On Gentoo, mount fails to run (as root!) in the VM with a permission denied error, and we still need to investigate this issue.

Concerning Xpra, it works out of the box on most distros but the packaged version is often out-of-date. Xpra configuration varies widly between versions. For instance, --daemon option doesn't exist on Ubuntu 15.10 (Xpra v0.14.25) while it's supported on Ubuntu 16.04 (xpra v0.15.8). On Debian, the default configuration file seems to be broken...

What's Next?

These subtle distro specifics make it difficult to provide a NoFear version which runs anywhere. Anyway, NoFear is still a proof-of-concept but we hope that this blogpost will give it some attention, and that further testing will make it mature enough for distro packages. Checkout its GitHub repository, and report any issue you might encounter! We also would like to think that it can spread the idea that hardware virtualization isolation (while not being new nor a silver bullet) can be made way more accessible.

On a side note, a colleague of mine told me that NoFear works inside Docker (but I'm still skeptical about the concept, I don't get the point :) Regarding other architectures, KVM supports ARM and we would be curious to know if NoFear can run on Raspberry Pi (even if it seems to require a custom kernel).

Finally, thanks to each of my colleagues who reviewed this blog post and beta-tested NoFear (hi @doegox!)


If you would like to learn more about our security audits and explore how we can help you, get in touch with us!