Firejail

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Firejail is an easy to use SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces, seccomp-bpf and Linux capabilities.

Installation

Install either firejail, or the firejail-gitAUR package. A GUI application for use with Firejail is also available, firetools.

Note: For information about user_namespaces(7) support in Arch Linux kernels see Security#Sandboxing applications. Firejail can use it even if it is disabled.
Warning: While upstream is gradually adopting whitelists, (cf /etc/firejail/firefox.profile,) most of the supplied profiles still rely heavily on blacklists. This means that anything not explicitly forbidden by the profile will be accessible to the application. For example, if you have btrfs snapshots available in /mnt/btrfs, a jailed program may be forbidden from accessing $HOME/.ssh, but would still be able to access /mnt/btrfs/@some-snapshot/$HOME/.ssh. Make sure to audit your profiles, see #Testing profiles

Apparmor integration

Since 0.9.60-1, Firejail, has supported more direct integration with Apparmor through a generic apparmor profile. During installation, the profile, firejail-default, is placed in /etc/apparmor.d directory, and needs to be loaded into the kernel by running the following command as root:

# apparmor_parser -r /etc/apparmor.d/firejail-default

See firejail(1) § APPARMOR.

Local customizations of the apparmor profile are supported by editing the file /etc/apparmor.d/local/firejail-local

Configuration

Most users will not require any custom configuration and can proceed to #Usage.

Firejail uses profiles to set the security protections for each of the applications executed inside of it - you can find the default profiles in /etc/firejail/application.profile. Should you require custom profiles for applications not included, or wish to modify the defaults, you may place new rules or copies of the defaults in the ~/.config/firejail/ directory. You may have multiple custom profile files for a single application, and you may share the same profile file among several applications.

If firejail does not have a profile for a particular application, it uses its restrictive system-wide default profile. This can result in the application not functioning as desired, without first creating a custom, and less restrictive profile.

Refer to firejail-profile(5).

Usage

To execute an application using firejail's default protections for that application (the default profile), execute the following:

$ firejail <program name>

One-time additions to the default profile can be added as command line options (see the man page). For example, to execute okular with seccomp protection, execute the following:

$ firejail --seccomp okular

You may define multiple non-default profiles for a single program. Once you create your profile file, you can use it by executing:

$ firejail --profile=/absolute/path/to/profile <program name>

Using Firejail by default

To use Firejail by default for all applications for which it has profiles, run the firecfg tool as root.

# firecfg

This creates symbolic links in /usr/local/bin pointing to /usr/bin/firejail, for all programs for which firejail has default profiles.

Tip: A pacman hook can be used to automatically run firecfg on pacman operations:
/etc/pacman.d/hooks/firejail.hook
[Trigger]
Type = Path
Operation = Install
Operation = Upgrade
Operation = Remove
Target = usr/bin/*
Target = usr/local/bin/*
Target = usr/share/applications/*.desktop

[Action]
Description = Configure symlinks in /usr/local/bin based on firecfg.config...
When = PostTransaction
Depends = firejail
Exec = /bin/sh -c 'firecfg >/dev/null 2>&1'

To manually map individual applications execute:

# ln -s /usr/bin/firejail /usr/local/bin/<application>
Note:
  • /usr/local/bin should be set before /usr/bin in the PATH environment variable.
  • To run a symbolic program with custom Firejail setting, simple prefix firejail as seen in #Usage.
  • For a daemon, you will need to overwrite the systemd unit file for that daemon to call firejail, see systemd#Editing provided units.
  • firecfg only symlinks the programs listed on /usr/lib/firejail/firecfg.config. Certain cli programs are absent, such as: tar, curl, and git. These need to be symlinked manually. See Profiles not in firecfg #2507 for why they are not included.
  • Symbolic links to gzip and xz interfere with makepkg's ability to preload libfakeroot.so. See BBS#230913.

Use with hardened_malloc

hardened_mallocAUR is a hardened implementation of glibc's malloc() allocator, originally written for Android but extended for use on the desktop. While not integrated into glibc yet, it can be used selectively with LD_PRELOAD. The proper way to launch an application within firejail using hardened_malloc is demonstrated below. To make it permanent, you would need to create your own entry in /usr/local/bin for the desired application.

$ firejail --env=LD_PRELOAD='/usr/lib/libhardened_malloc.so' /usr/bin/firefox

The various environment variables and settings that can be used to tune hardened_malloc can be found on its github page.

Enable AppArmor support

There are a number of ways to enable AppArmor confinement on top of a Firejail security profile:

  • Pass the --apparmor flag to Firejail in the command line, e.g. $ firejail --apparmor firefox
  • Use a custom profile.
  • Enable Apparmor globally in /etc/firejail/globals.local and disable as needed through the use of ignore apparmor in /etc/firejail/<ProgramName>.local.

Verifying Firejail is being used

$ firejail --list

Creating custom profiles

Whitelists and blacklists

Blacklists are permissive:

  • Permit everything not explicitly forbidden: blacklist <location/file>
  • Permit file or location in any later blacklist: noblacklist <location/file>

Whitelists are restrictive:

  • Forbid everything not explicitly permitted: whitelist <location/file>
  • Forbid file or location in any later whitelist: nowhitelist <location/file>

Profile writing

The basic process is:

  1. Copy the default profile (which uses blacklists) to your work folder and give it a unique name
  2. Change the line include /etc/firejail/default.local to include /etc/firejail/ProfileName.local
  3. Gradually comment/uncomment the various options while checking at each stage that the application runs inside the new sandbox
  4. Desirable options not available in the copied default profile can be found by consulting the manual
  5. Build a whitelist of permitted locations. For portability, it may be advisable to place at least some of this list it in a .local file
  6. Test the profile for security holes, see #Testing profiles
  7. Once satisfied, copy your new profile to either /etc/firejail/ or ~/.config/firejail/

You may find the following to be useful:

  1. firejail --debug $OtherOptions $PathToProfile $Program > $PathToOutputFile Gives a detailed breakdown of the sandbox
  2. firejail --debug-caps gives a list of caps supported by the current Firejail software build. This is useful when building a caps whitelist.
  3. firejail --help for a full list of --debug options
  4. firemon PID monitors the running process. See firemon --help for details
  5. checksec may also be useful in testing which standard security features are being used
Note:
  • The idea is to be as restrictive as possible, while still maintaining usability. This may involve sacrificing potentially dangerous functionality and a change in cavalier work habits.
  • By default, seccomp filters work on a blacklist (which can be found in the manual). It is possible to use seccomp.keep to build a custom whitelist of filters for an application. [1].
  • The list of possible options for a firejail profile is extensive, and users should consult the firejail-profile(5) man page.

Persistent local customisation

The standard profile layout includes the capability to make persistent local customisations through the inclusion of .local files[2]. Basically, each officially supported profile contains the lines include /etc/firejail/ProgramName.local and include /etc/firejail/globals.local. Since the order of precedence is determined by which is read first, this makes for a very powerful way of making local customisations. For example, with reference this firejail question, to globally enable Apparmor and disable Internet connectivity, one could simply create/edit /etc/firejail/globals.local to include the lines

# enable Apparmor and disable Internet globally
net none
apparmor

Then, to allow, for example, "curl" to connect to the internet, yet still maintain its apparmor confinement, one would create/edit /etc/firejail/curl.local to include the lines.

# enable internet for curl
ignore net

Since curl.local is read before globals.local, ignore net overrides net none, and, as a bonus, the above changes would be persistent across future updates.

Testing profiles

Firejail's built in audit feature allows the user to find gaps in a security profile by replacing the program to be sandboxed with a test program. By default, firejail uses the faudit program distributed with Firejail. (Note: A custom test program supplied by the user can also be used.) Examples:

  1. Run the default audit program: $ firejail --audit transmission-gtk
  2. Run a custom audit program: $ firejail --audit=~/sandbox-test transmission-gtk

In the examples above, the sandbox configures the transmission-gtk profile and starts the test program. The real program, transmission-gtk, will not be started.

Note: The audit feature is not implemented for --x11 commands.

Firejail with Xephyr

Xephyr will allow you to sandbox Xorg. If you want to be able to resize windows, install a standalone window manager such as Openbox.

xephyr-screen WidthxHeight can be set in /etc/firejail/firejail.config where Width and Height are in pixels and based on your screen resolution.

To open the sandbox:

$ firejail --x11 --net=device openbox

device is your active network interface. Then right click and select your applications to run.

Note: If you use Unbound, dnsmasq, Pdnsd or any other local cache as your resolver on 127.0.0.1 for example, you would leave --net=device out of the command as your network should work automatically.

A great guide can be found on the Firejail Wordpress.

According to the guide:

The sandbox replaces the regular X11 server with Xpra or Xephyr server. This prevents X11 keyboard loggers and screenshot utilities from accessing the main X11 server.

Note that the statement:

The only way to disable the abstract socket @/tmp/.X11-unix/X0 is by using a network namespace. If for any reasons you cannot use a network namespace, the abstract socket will still be visible inside the sandbox. Hackers can attach keylogger and screenshot programs to this socket.

is incorrect, xserverrc can be edited to -nolisten local which disables the abstract sockets of X11 and helps isolate it.

Sandboxing a browser

Openbox can be configured to start a certain browser at startup. program.profile is the respective profile contained in /etc/firejail, and --startup "command" is the command line used to start the program. For example, to start Chromium in the sandbox:

$ firejail --x11 --profile=/etc/firejail/chromium.profile openbox --startup "chromium"

Tips and tricks

Paths containing spaces

If you need to reference, whitelist, or blacklist a directory within a custom profile, such as with palemoonAUR, you must do so using the absolute path, without encapsulation or escapes:

/home/user/.moonchild productions

Private mode

Firejail also includes a one time private mode, in which no mounts are made in the chroots to your home directory. In doing this, you can execute applications without performing any changes to disk. For example, to execute okular in private mode, do the following:

$ firejail --seccomp --private okular

Experimental improved tools

Some of the Firejail developers recognized issues with the tools it ships with and made their own, improved versions of them.

Warning: As stated in some of the projects descriptions, these tools might be experimental.

Troubleshooting

Firejail can be hard to debug. The symptoms of a misconfigured or otherwise unfitting setup range from random segmentation faults and hangs in the applications to simple error messages.

Some applications are harder to sandbox than others. For example web browsers and Electron applications tend to need more troubleshooting than others since there is much that can go wrong. It is crucial to check the FAQ and open issues first since debugging can take quite some time.

Tip: Also see the upstream wiki, especially its page on debugging Firejail.

Remove Firejail symbolic links

To remove Firejail created symbolic links (e.g. reset to default):

# firecfg --clean

Verify if any leftovers of Desktop entries are still overruled by Firejail.

Desktop files

Some GUI application launchers (.desktop files) are coded using absolute paths to an executable, which circumvents firejail's symlink method of ensuring that it is being used. The firecfg tool includes an option to over-ride this on a per-user basis by copying the .desktop files from /usr/share/applications/*.desktop to ~/.local/share/applications/ and replacing the absolute paths with simple file names.

$ firecfg --fix

There may be cases for which you need to manually modify the Exec= line of the .desktop file in ~/.local/share/applications/ to explicitly call Firejail.

PulseAudio

Note: Using PulseAudio version 9.0 or later should fix this issue.

If Firejail causes PulseAudio issues with sandboxed applications [3], the following command may be used:

$ firecfg --fix-sound

This commands creates a custom ~/.config/pulse/client.conf file for the current user with enable-shm = no and possible other workarounds.

hidepid

If the system uses the hidepid kernel parameter, Firemon can only be run as root. This, among other things, will cause problems with the Firetools GUI incorrectly reporting "Capabilities", "Protocols" and the status of "Seccomp"[4].

Proprietary Nvidia drivers

Some users report problems when using Firejail and proprietary graphic drivers from NVIDIA (e.g. [5], [6] or [7]). This can often be solved by disabling the noroot Firejail option in the application's profile file.

--net options and Linux kernel >=4.20.0

There is a bug on firejail 0.5.96 with linux >= 4.20.0, see [8] and [9]

Example error message:

$ firejail --noprofile --net=eth0 ls
Parent pid 8521, child pid 8522
Error send: arp.c:182 arp_check: Invalid argument
Error: proc 8521 cannot sync with peer: unexpected EOF
Peer 8522 unexpectedly exited with status 1

Warning: Cannot confine the application using AppArmor

Merge-arrows-2.pngThis article or section is a candidate for merging with #Enable AppArmor support.Merge-arrows-2.png

Notes: The requirement to enable AppArmor should be made explicit in the previous text. And note that Manjaro references are irrelevant. (Discuss in Talk:Firejail#)

For some applications (e.g. Firefox [10][dead link 2021-05-11 ⓘ]) starting with Firejail may result in warnings like:

Warning: Cannot confine the application using AppArmor.
Maybe firejail-default AppArmor profile is not loaded into the kernel.
As root, run "aa-enforce firejail-default" to load it.

When running the suggested command you might see:

ERROR: Cache read/write disabled: interface file missing. (Kernel needs AppArmor 2.4 compatibility patch.)

This means that AppArmor is not enabled as a kernel parameter, so you have to set it according to AppArmor#Installation.

/usr/bin/patch: **** Can't open patch file

This means the PKGBUILD uses patch with the -i argument so a whitelist for $SRCDEST in /etc/makepkg.conf is needed.

Create the override patch.local with the value of your $SRCDEST:

whitelist /path/to/makepkg/sources

Changing the PKGBUILD to use stdin also works:

patch -p1 < ../file.patch

Graphical applications hang on start with AMDGPU

Some graphical applications, eg Firefox and mpv, will hang on start when using AMDGPU with Mesa >= 19.3.4. See [11]. The issue is fixed upstream, so firejail-gitAUR should work. Alternatively, for all affected applications, add seccomp !kcmp to their profiles in etc/firejail. If they already have a seccomp statement, you can concatenate them as a comma-separated list, eg seccomp !chroot,!kcmp.

Daemonizing/backgrounded processes hang

There is a known issue which prevents processes from daemonizing. There is currently no solution to this except not using Firejail to sandbox the affected application. Because it is a bug within Firejail, no configuration can solve this issue. Fortunately the applications mentioned in the issue usually do not have a large attack surface, so the risks of running them without a sandbox are comparatively low.

See also