Snaps first launched with the ability to ship desktop apps on Ubuntu 16.04, which is an X11 based platform. It was noted that while secure and containerized, the fact that many Snaps were using X11 this made them less secure than they could be. It was a reality of shipping Snaps for 16.04, but something we definitely want to fix for 18.04 using Unity8 and the Mir graphics stack. We can't just ignore all the apps that folks have built for 16.04 though, so we need a solution to run X11 applications on Unity8 securely.

To accomplish this we give each X11 application its own instance of the XMir server. This means that even evil X applications that use insecure features of (or find vulnerabilities in) the Xorg server, they're only compromising their individual instance of the Xserver and are unable to affect other applications. Sounds simple, right? Unfortunately there is a lot more to making an application experience seamless than just handling the graphic buffers and making sure it can display on screen.

The Mir server is designed to handle graphics buffers and their positions on the screen, it doesn't handle all the complexities of things like cut-and-paste and window menus. To help make X11 apps that use these features we're using some pieces of the libertine project which runs X11 apps in LXD containers. It has in it a set of helpers, like pasted, who handle these additional protocols. pasted watches the selected window and the X11 clip buffers to connect into Unity8's cut-and-paste mechanisms which behave very differently. For instance, Unity8 doesn't allow or snooping on clip buffers to steal passwords.

It is also important at this point to note that in Ubuntu Personal we aren't just snapping up applications, we are snapping everything. We expect to have snaps of Unity8, snaps of Network Manager and a snap of XMir. This means that XMir isn't even running in the same security context as Unity8. A vulnerability in XMir only compromises XMir and the files that it has access to. This means that a bug in an X11 application would have get into XMir and then work on the Mir protocol itself before getting to other applications or user session resources.

The final user experience? We hope that no one notices that their applications are X11 applications or Mir applications, users shouldn't have to care about display servers. What we've tried to create is a way for them to still have their favorite X11 applications, as hopefully they transition away from X11, while still being able to get the security benefits of a Mir based desktop.


posted Apr 5, 2017 | permanent link