The flatpak security model – part 2: Who needs sandboxing anyway?

The ability to run an application sandboxed is a very important  feature of flatpak. However, its is not the only reason you might want to use flatpak. In fact, since currently very few applications work in a fully sandboxed environment, most of the apps you’d run are not sandboxed.

In the previous part we learned that by default the application sandbox is very limiting. If we want to run a normal application we need to open things up a bit.

Every flatpak application contains a manifest, called metadata. This file describes the details of the application, like its identity (app-id) and what runtime it uses. It also lists the permissions that the application requires.

By default, once installed, an application gets all the permissions that it requested. However, you can override the permissions each time you call flatpak run or globally on a per-application basis by using flatpak override (see manpages for flatpak-run and flatpak-override for details). The handling of application permissions are currently somewhat hidden in the interface, but the long term plan is to show permissions during installation and make it easier to override them.

So, what kind of permissions are there?

First apps need to be able to produce output and get input. To do this we have permissions that allow access to PulseAudio for sound and X11 and/or Wayland for graphical output and input. The way this works is that we just mount the unix domain socket for the corresponding service into the sandbox.

It should be noted that X11 is not very safe when used like this, you can easily use the X11 protocol to do lots of malicious things. PulseAudio is also not very secure, but work is in progress on making it better. Wayland however was designed from the start to isolate clients from each other, so it is pretty secure in a sandbox.

But, secure or not, almost all Linux desktop applications currently in existence use X11, so it is important that we are able to use it.

Another way for application to integrate with the system is to use DBus. Flatpak has a filtering dbus proxy, which lets it define rules for what the application is allowed to do on the bus. By default an application is allowed to own its app-id and subnames of it (i.e. org.gnome.gedit and org.gnome.gedit.*) on the session bus. This means other clients can talk to the application, but it can only talk to the bus itself, not any other clients.

Its interesting to note this connection between the app-id and the dbus name. In fact, valid flatpak app-ids are defined to be the same form as valid dbus names, and when applications export files to the host (such as desktop files, icons and dbus service files), we only allow exporting files that start with the app-id. This ties very neatly into modern desktop app activation were the desktop and dbus service files also have to be named by the dbus name. This rule ensures that applications can’t accidentally conflict with each other, but also that applications can’t attack the system by exporting a file that would be triggered by the user outside the sandbox.

There are also permissions for filesystem access. Flatpak always uses a filesystem namespace, because /usr and /app are never from the host, but other directories from the host can be exposed to the sandbox. The permission here is quite fine grained, starting with access to all host files, to your home-directory only or to individual directories. The directories can also be exposed read-only.

The default sandbox only has a loopback network interface and thius has no connection to the network, but if you grant network access then the app will get full network access. There are no partial access for network access however. For instance one would like to be able to set up a per-application firewall configuration. Unfortunately, it is quite complex and risky to set up networking so we can’t expose it in a safe way for unprivileged use.

There are also a few more specialized permissions, like various levels of hardware device access and some other details. See man flatpak-metadata for the available settings.

All this lets us open up exactly what is needed for each application, which means we can run current Linux desktop applications without modifications. However, the long term goal is to introduce features so that applications can run without opening the sandbox. We’ll get to this plan in the next part.

Until then, happy flatpaking.