Black and White

Oz also further bolsters your protection from exposed system calls through using seccomp. For non-Linux die-hards, system calls are simply the way that an application requests a service from the OS kernel.

These can be exploited by malicious people, whereby a program is run by a normal, unprivileged user, who then spawns a root shell, giving the user full access to the system. The Semtex exploit of 2013 is an excellent example of this.

Oz assigns policies on a per-application basis. Programs are killed if they violate the policy.

If a system call within an attacker's payload (such as one that would unnecessarily require root-level privileges) is not explicitly permitted, it will not be allowed.

Oz supports both blacklist and white-list policies. For all of Subgraph's own supported apps, Oz maintains a white list, which blocks all system calls except those which are explicitly allowed.

Oz has a generic blacklist that automatically blocks more dangerous or unusual system calls, located in /var/lib/oz/cells.d/generic-blacklist.seccomp.

The latest version of Subgraph includes a new Go seccomp-bpf library developed by the ThoughtWorks Tiger team to help create much more efficient policies for sandboxed apps. The Subgraph handbook's appendix maintains a complete list of system calls in the OS, which you can use to create your own policies if you wish.

Subpar

Subgraph's sandboxing features can cause problems for certain applications. PDFs, for instance, which are notoriously vulnerable to exploits, are opened by the built-in Evince document reader, which cannot access the Internet or any other file besides the PDF it is currently reading.

If Evince is already open, you can click on Oz's icon, which resembles a stylized zebra, at the top right of the screen and then Add File to open one or more files. Subgraph allows you to make them read-only. You can use the Oz menu to add files inside the Tor Browser in the same way (Figure 7). This is useful if you actually want to upload files. You do not need to do this for OnionShare as the process is handled automatically when you choose to share a file from within Nautilus.

Figure 7: Click on the Oz menu to add files to applications for viewing and/or upload.

Oz also makes a special exception for the Tor Browser sandbox when it comes to downloads to prevent all files being lost each time you close the browser. This is done through a shared directory located in ~/Downloads/TorBrowser. Any files saved there while surfing will be accessible after you close down the browser.

Final Subgraph

Overall, Subgraph is an excellent proof of concept. While the developers stress it's not yet ready for privacy applications, it certainly deserves a ringing endorsement for its careful selection of privacy-related apps, Oz's ingenious sandboxing, and the crafty CoyIM.

David Ahmad was kind enough to speak to Linux Magazine about some of the upcoming features in Subgraph. Chief amongst these is an Oz sandboxed version of the Chromium Browser which can visit regular 'clearnet' websites.

Any readers who are impatient to give this new setting a try can follow the steps on the Subgraph website [8] to install the browser manually. Watch this space for further developments.

While you should rely on the above two privacy distros for now, watch this space for further developments from the Subgraph project.