Building Custom Profiles

Contents

• Introduction
• The Access Control Layer
• Building Security Profiles
• Testing Security Profiles
• Customizing Security Profiles
• A More Complex Example
• jailcheck
• Conclusion

Introduction

Firejail is a security system implemented using modern kernel technologies such as Linux namespaces and seccomp-bpf. On top of namespaces we place a thin software layer to control file access. By enforcing specific rules for each application, the access control layer proactively protects the system from external threats.

Security rules are stored in /etc/firejail directory in regular text files, one file for each application. We call these files security profiles. We recently passed the 1000 profile mark, covering various desktop programs people use every day. That’s about 10x more applications than SELinux and AppArmor combined!

This article is all about the access layer: how is built, how to create security profiles, how to test and how to customize them. I also describe jailcheck, a tool we introduced in version 0.9.66. The tool provides a high level view of the access system, warning the user in case of possible problems.

 

The Access Control Layer

C code, complicated at times, it already went trough several iterations. This is the code that provides security and privacy to our users. Contrary to popular opinion, seccomp plays a secondary role in the grand scheme of things.

For Firefox we start with a list of directories the browser needs to work correctly:

• .mozilla, .pki, .config/pulse, .cache/mozilla, .local/share, Downloads

Next, the list of directories we definitely don’t want the browser to access:

• .ssh, .gnupg …

This list is much longer. It is spread over several files such as this one in our git repository. There are literally hundreds of entries, everything from desktop configuration, to X11, compilers, and system tools.

Lastly, we build a list with directories we don’t want to expose for privacy reasons:

• Documents, Desktop, Pictures … and any other user directory

As we put all the lists together and load them in our file access engine, we end up with something like this:

 

User home directory in a sandboxed browser

 

The home directory is virtual. Downloads and the config files are brought in from the real file system, everything else is temporary and will be removed when the sandbox is closed.

We use this type of setup for networked applications, games, desktop tools such as calculators etc. For media players we bring in Music and Videos, for emails we bring in the email storage directory, and so on. Programs such as editors and console tools end up without the privacy list, and have access to most user directories (the security list is still included).

Firejail has no impact on the application speed, and it has a neglijable impact on user convenience. For example, the user won’t be able to download and upload files from random places in the file system, and web-based SSH tools won’t work in the browser. In our view, that’s a small price to pay for privacy and security.

 

Building Security Profiles

Security profiles are usually built starting from the template installed in /usr/share/doc/firejail/profile.template. Make a copy of the file and modify it. The details are in the template file, the sandbox commands are described in man firejail-profile.

You can also generate profiles automatically by running your application under firejail --build. We show an example in the video below, where we also take a look at the file system generated for the sandbox.

 

 

Testing Security Profiles

Here is a simple, practical method for testing file system access. It applies to any other mandatory access control system. Let’s start with this user question:

“I have the key I use to connect to GitHub in ~/.ssh directory, and another key I use to sign the release in ~/.gnupg. That’s my threat exposure. How do I test Firejail?”

 

Copy a PNG file in .ssh directory and try to open it in GIMP:

$ cp test.png ~/.ssh/.
$ firejail gimp ~/.ssh/test.png

 

Access test for .ssh directory

 

Copy the same file in .gnupg or any other directory you want to test. Do the same for other programs – for a media player use a test.mp4 file and so on.

Even better, go in File/Open menu in your application and soon you’ll get an idea what’s covered and what’s not. This is LibreOffice:

 

LibreOffice ~/.ssh test

 

For browsers, use the address bar to access your home directory:

 

Access denied!

 

Customizing Security Profiles

One funny complaint I got was “I want to put my pictures on Facebook and ~/Pictures is not there!”. Go in ~/.config/firejail directory and create a text file firefox.profile with the following content:

$ cat ~/.config/firejail/firefox.profile
whitelist ~/Pictures
include /etc/firejail/firefox.profile

 

Or you can just copy your pictures temporarily in ~/Downloads.

One popular customization is removing network access for media players such as VLC:

$ cat ~/.config/firejail/vlc.profile
net none
include /etc/firejail/vlc.profile

 

Another interesting one is shutting down the sound in Warzone2100:

$ cat ~/.config/firejail/warzone2100.profile
nosound
include /etc/firejail/warzone2100.profile

 

 

A More Complex Example

Audacity, the open source audio editor, has been in the news recently. The program was acquired by Muse Group, a multi-national company involved in data harvesting. Among other changes, starting with version 3.0.3 they introduced telemetry, a mechanism for phoning home with various pieces of data taken from user computers. Here is the problem:

Every telemetry program stores on hard disk an universally unique identifier (UUID). This is a huge random number used to track program install instances. This way they can differentiate users on a local network behind a network address translation (NAT) firewall.

Now, let’s say you start your computer, telemetry phones home, your IP address is recorded. Later, you start your VPN software and connect to Internet with a different IP. Telemetry phones home again, same UUID but a different IP – instant de-anonymization!

We’ve had "net none" in the default Audacity security profile distributed with Firejail software for a very long time. This disables network access. But since it’s the intention that counts, segregate Audacity in its own private home directory:

Create a new directory in your home, ~/myaudio, and a new profile in ~/.config/firejail:

$ cd ~
$ mkdir myaudio

$ cat ~/.config/firejail/audacity.profile
private ~/myaudio
net none
include /etc/firejail/audacity.profile

 

I added "net none" just in case you have an old version of Firejail. Move the files you want to edit in ~/myaudio, and start the program (firejail audacity):

 

Segregated home directory for Audacity

 

jailcheck

jailcheck is a small program we use to simplify the testing. The program will attach itself to all running sandboxes and try to access .ssh and .gnupg directories. You can add more directories on the command line, see man jailcheck.

$ sudo jailcheck
2014:netblue::firejail /usr/bin/gimp
   Virtual dirs: /tmp, /var/tmp, /dev, /usr/share,
   Warning: I can run programs in /home/netblue
   Networking: disabled

2055:netblue::firejail /usr/bin/ssh -X netblue@x.y.z.net
   Virtual dirs: /var/tmp, /dev, /usr/share, /run/user/1000,
   Warning: I can read ~/.ssh
   Networking: enabled

2186:netblue:libreoffice:firejail --appimage /opt/LibreOffice-fresh.appimage
   Virtual dirs: /tmp, /var/tmp, /dev,
   Networking: enabled

26090:netblue::/usr/bin/firejail /opt/firefox/firefox
   Virtual dirs: /home/netblue, /tmp, /var/tmp, /dev, /etc, /usr/share,
/run/user/1000,
   Networking: enabled

 

As I write this article I have Firefox, LibreOffice and GIMP open. I also have an SSH connection going out. As expected, .ssh directory is visible in SSH session. Nobody has access to .gnupg.

jailcheck also flags the presence of several system tools such as sudo, mount and strace, compilers such as gcc and clang, and networking tools such as dig, ncat, nmap and tcpdump, since these provide the perpetrators with a huge opportunity for mischief.

For each sandbox we list the most important virtual directories in use. These directories are build in temporary file systems by bringing in either real files and directories, or by copying files from the real file system. We deploy this type of mandatory access control in all sandboxes, with some differences here and there, depending on the application.

Another test is noexec test. In GIMP example above, the home directory allows GIMP to start programs stored locally. Usually these are application plugins the user installed himself, but they can also be aggressive malware programs inserted in the sandbox using a zero-day app exploit. If you don’t have external plugins installed, it is a good idea to make the home directory noexec:

$ cat ~/.config/firejail/gimp.profile
noexec ${HOME}
include /etc/firejail/gimp.profile

 

 

Conclusion

To give credit where credit is due, I’ve first seen people blocking access to .ssh and .gnupg in 2012 in a Firefox AppArmor profile in Ubuntu. In Debian 10 the block file is /etc/appamor.d/abstractions/private-files-strict. To test AppArmor or SELinux, copy a file in .ssh directory and try to open it in your application. Apparently, stealing SSH credentials is quite a sport in business environments. That’s all for now, have fun!