Running your programs in a jail with Firejail

Profile

In addition to command-line parameters, you can create your own configuration file for each program. In these security profiles, you can explicitly enable the --seccomp and --caps functions. Additionally, you can define in detail which directories Firejail will not pass through to the sandbox, or where access will be restricted to read-only, and you can define mountpoints to match. Finally, you can define limits for the sandbox, for example, by restricting the number of processes running in it.

All security profiles reside in the /etc/firejail directory. Firejail 0.9.18 comes with preconfigured profiles for Chromium, Dropbox, Evince, Firefox, Iceweasel, and Midori. The profile file then starts with the program name and has a file extension of .profile. The profile for firefox thus resides in the firefox.profile file.

After calling firejail firefox, Firejail automatically applies all the settings from the matching security profile. The security profiles themselves have a very simple setup, with a setting in each line. Table 1 shows the most important settings at a glance. For more information on the structure of the security profiles, check out the man page: man firejail-profile.

Table 1

Security Profiles

Statement

Meaning

blacklist /usr/bin

The sandbox hides the /usr/bin directory

read-only /usr/bin

The sandbox mounts the /usr/bin directory read-only

tmpfs /etc

Mounts a tempfs filesystem via the /etc directory (any files stored are discarded after terminating the sandbox)

bind /root/config/ssh, /etc/ssh

Mounts the /root/config/ssh directory as /etc/ssh in the sandbox

private /tmp/muell

Mounts the directory /tmp/muell as the home directory in the sandbox

caps

Enables a kernel-side security filter (see text)

seccomp

Blocks certain security-critical actions (see text)

rlimit-fsize 1024

A program in the sandbox can create files with a maximum size of 1024 bytes

rlimit-nofile 500

A program in the sandbox can open a maximum of 500 files at the same time

rlimit-nproc 1000

A maximum of 1,000 processes can be created in the sandbox

Insights

The firejail --list command lists all jails currently running and programs running in them. Using firejail --tree lets you quickly discover which program started which other processes and identify the user accounts they are running on (Figure 7). The --top parameter delivers a similar view to the popular top command.

Figure 7: Using firejail --tree tells the tool to display the processes from the sandboxes in a tree. In the sandbox at the bottom, a shell was opened first, and Firefox was called in it.

You can retroactively change into the sandbox. To do so, you first need to determine the process ID (PID) of the sandbox by typing firejail --list. In the list of all the sandboxes that is then shown, you will be able to identify the correct one by looking at the program name. At the start of the matching line, you will see the PID, such as 2652. Then, simply append this to the --join= parameter:

firejail --join=2652

Firejail then opens a shell running in the sandbox and subject to that sandbox's restrictions (Figure 8). To use the join function, however, you need a Linux kernel version 3.8 or newer.

Figure 8: Firefox running in a sandbox in which firejail --join=2652 opens a shell.

Conclusions

Firejail, which is licensed under the GPL v2, is easy to use and enhances security. Applications don't need any special preparation for life in the sandbox; you can even put Dropbox in jail [4]. That said, a sandbox is not a panacea. A web server remotely controlled by hackers might not be able to destroy the system on which it is running, but it still would be able to exploit the system, using it to attack other Internet sites. Firejail is thus simply another brick in the wall for hardening your Linux system; however, it is one that you can set up quickly and pretty effectively.

For more information on Firejail, visit the Firejail homepage [1]. In addition to a small collection of links with various tutorials, you will also find a how-to that explains how to use Firejail as your login shell.

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