What's New in Qubes 4
In my recent article "The Refactor Factor", I talked about the new incarnation of Linux Journal in the context of a big software project doing a refactor:
Anyone who's been involved in the Linux community is familiar with a refactor. There's a long history of open-source project refactoring that usually happens around a major release. GNOME and KDE in particular both use .0 releases to rethink those desktop environments completely. Although that refactoring can cause complaints in the community, anyone who has worked on a large software project will tell you that sometimes you have to go in, keep what works, remove the dead code, make it more maintainable and rethink how your users use the software now and how they will use it in the future.
I've been using Qubes as my primary desktop for more than two years, and I've written about it previously in my Linux Journal column, so I was pretty excited to hear that Qubes was doing a refactor of its own in the new 4.0 release. As with most refactors, this one caused some past features to disappear throughout the release candidates, but starting with 4.0-rc4, the release started to stabilize with a return of most of the features Qubes 3.2 users were used to. That's not to say everything is the same. In fact, a lot has changed both on the surface and under the hood.
Although Qubes goes over all of the significant changes in its Qubes 4 changelog, instead of rehashing every low-level change, I want to highlight just some of the surface changes in Qubes 4 and how they might impact you whether you've used Qubes in the past or are just now trying it out.
For the most part, the Qubes 4 installer looks and acts like the Qubes 3.2 installer with one big difference: Qubes 4 uses many different CPU virtualization features out of the box for better security, so it's now much more picky about CPUs that don't have those features enabled, and it will tell you so. At the beginning of the install process after you select your language, you will get a warning about any virtualization features you don't have enabled. In particular, the installer will warn you if you don't have IOMMU (also known as VT-d on Intel processors—a way to present virtualized memory to devices that need DMA within VMs) and SLAT (hardware-enforce memory virtualization). If you skip the warnings and finish the install anyway, you will find you have problems starting up VMs.
In the case of IOMMU, you can work around this problem by changing the virtualization mode for the sys-net and sys-usb VMs (the only ones by default that have PCI devices assigned to them) from being HVM (Hardware VM) to PV (ParaVirtualized) from the Qubes dom0 terminal:
$ qvm-prefs sys-net virt_mode pv
$ qvm-prefs sys-usb virt_mode pv
This will remove the reliance on IOMMU support, but it also means you lose the protection IOMMU gives you—malicious DMA-enabled devices you plug in might be able to access RAM outside the VM! (I discuss the differences between HVM and PV VMs in the next section.)
It's no surprise that the default templates are all updated in Qubes 4—software updates are always expected in a new distribution release. Qubes 4 now ships with Fedora 26 and Debian 9 templates out of the box. The dom0 VM that manages the desktop also has a much newer 4.14.13 kernel and Xen 4.8, so you are more likely to have better hardware support overall (this newer Xen release fixes some suspend issues on newer hardware, like the Purism Librem 13v2, for instance).
Another big difference in Qubes 4 is the default VM type it uses. Qubes relies on Xen for its virtualization platform and provides three main virtualization modes for VMs:
PV (ParaVirtualized): this is the traditional Xen VM type that requires a Xen-enabled kernel to work. Because of the hooks into the OS, it is very efficient; however, this also means you can't run an OS that doesn't have Xen enabled (such as Windows or Linux distributions without a Xen kernel).
HVM (Hardware VM): this VM type uses full hardware virtualization features in the CPU, so you don't need special Xen support. This means you can run Windows VMs or any other OS whether or not it has a Xen kernel, and it also provides much stronger security because you have hardware-level isolation of each VM from other VMs.
PVH (PV Hybrid mode): this is a special PV mode that takes advantage of hardware virtualization features while still using a pavavirtualized kernel.
In the past, Qubes would use PV for all VMs by default, but starting with Qubes 4, almost all of the VMs will default to PVH mode. Although initially the plan was to default all VMs to HVM mode, now the default for most VMs is PVH mode to help protect VMs from Meltdown with HVM mode being reserved for VMs that have PCI devices (like sys-net and sys-usb).
Another major change in Qubes 4 relates to the GUI VM manager. In past releases, this program provided a graphical way for you to start, stop and pause VMs. It also allowed you to change all your VM settings, firewall rules and even which applications appeared in the VM's menu. It also provided a GUI way to back up and restore VMs. With Qubes 4, a lot has changed. The ultimate goal with Qubes 4 is to replace the VM manager with standalone tools that replicate most of the original functionality.
One of the first parts of the VM manager to be replaced is the ability to manage devices (the microphone and USB devices including storage devices). In the past, you would insert a USB thumb drive and then right-click on a VM in the VM manager to attach it to that VM, but now there is an ever-present icon in the desktop panel (Figure 1) you can click that lets you assign the microphone and any USB devices to VMs directly. Beside that icon is another Qubes icon you can click that lets you shut down VMs and access their preferences.
Figure 1. Device Management from the Panel
For quite a few release candidates, those were the only functions you could perform through the GUI. Everything else required you to fall back to the command line. Starting with the Qubes 4.0-rc4 release though, a new GUI tool called the Qube Manager has appeared that attempts to replicate most of the functionality of the previous tool including backup and restore (Figure 2). The main features the new tool is missing are those features that were moved out into the panel. It seems like the ultimate goal is to move all of the features out into standalone tools, and this GUI tool is more of a stopgap to deal with the users who had relied on it in the past.
Figure 2. New Qube Manager
The final obvious surface change you will find in Qubes 4 is in backup and restore. With the creation of the Qube Manager, you now can back up your VM's GUI again, just like with Qubes 3.2. The general backup process is the same as in the past, but starting with Qubes 4, all backups are encrypted instead of making it optional.
Restoring backups also largely behaves like in past releases. One change, however, is when restoring Qubes 3.2 VMs. Some previous release candidates couldn't restore 3.2 VMs at all. Although you now can restore Qubes 3.2 VMs in Qubes 4, there are a few changes. First, old dom0 backups won't show up to restore, so you'll need to move over those files manually. Second, old template VMs don't contain some of the new tools Qubes 4 templates have, so although you can restore them, they may not integrate well with Qubes 4 without some work. This means when you restore VMs that depend on old templates, you will want to change them to point to the new Qubes 4 templates. At that point, they should start up as usual.
As I mentioned at the beginning of this article, these are only some of the more obvious surface changes in Qubes 4. Like with most refactors, even more has changed behind the scenes as well. If you are curious about some the underlying technology changes, check out the Qubes 4 release notes and follow the links related to specific features.
Kyle Rankin is a Tech Editor and columnist at Linux Journal and the Chief Security Officer at Purism. He is the author of Linux Hardening in Hostile Networks, DevOps Troubleshooting, The Official Ubuntu Server Book, Knoppix Hacks, Knoppix Pocket Reference, Linux Multimedia Hacks and Ubuntu Hacks, and also a contributor to a number of other O'Reilly books. Rankin speaks frequently on security and open-source software including at BsidesLV, O'Reilly Security Conference, OSCON, SCALE, CactusCon, Linux World Expo and Penguicon. You can follow him at @kylerankin.
