Sandbox Permissions

One of Flatpak’s main goals is to increase the security of desktop systems by isolating applications from one another. This is achieved using sandboxing and means that, by default, applications that are run with Flatpak have extremely limited access to the host environment. This includes:

• No access to any host files except the runtime, the app, ~/.var/app/$FLATPAK_ID, and $XDG_RUNTIME_DIR/app/$FLATPAK_ID. Only the latter two being writable.

• No access to the network.

• No access to any device nodes (apart from /dev/null, etc).

• No access to processes outside the sandbox.

• Limited syscalls. For instance, apps can’t use nonstandard network socket types or ptrace other processes.

• Limited access to the session D-Bus instance - an app can only own its own name on the bus.

• No access to host services like X11, system D-Bus, or PulseAudio.

Most applications will need access to some of these resources in order to be useful. This is primarily done during the finishing build stage, which can be configured through the finish-args section of the manifest file (see Manifests).

Portals

Portals have already been mentioned in Grundlegende Konzepte. They are a framework for providing access to resources outside of the sandbox, including:

• Opening files with a native file chooser dialog

• Opening URIs

• Printing

• Showing notifications

• Taking screenshots

• Inhibiting the user session from ending, suspending, idling or getting switched away

• Getting network status information

In many cases, portals use a system component to implicitly ask the user for permission before granting access to a particular resource. For example, in the case of opening a file, the user’s selection of a file using the file chooser dialog is interpreted as implicitly granting the application access to whatever file is chosen.

This approach enables applications to avoid having to configure blanket access to large amounts of data or services and gives users control over what their applications have access to.

Interface toolkits like GTK3 and Qt5 implement transparent support for portals, meaning that applications don’t need to do any additional work to use them (it is worth checking which portals each toolkit supports). Applications that aren’t using a toolkit with support for portals can refer to the xdg-desktop-portal API documentation for information on how to use them.

Permissions guidelines

Bemerkung

Note, that these permissions are completely static and variable expansion or substitution (for example in --filesystem or --env) is not possible.

While application developers have control over the sandbox permissions they wish to configure, good practice is encouraged and can be enforced. For example, the Flathub hosting service places requirements on which permissions can be used, and software on the host may warn users if certain permissions are used.

The following guidelines describe which permissions can be freely used, which can be used on an as-needed basis, and which should be avoided.

Standard permissions

The following permissions provide access to basic resources that applications commonly require, and can therefore be freely used.

• --allow=bluetooth - Allow access to Bluetooth (AF_BLUETOOTH) sockets

• --device=dri - OpenGL rendering

• --share=ipc - Share IPC namespace with the host [1]

• --share=network - Access the network [2]

• --socket=cups - Talk to the CUPS printing system ($CUPS_SERVER or server defined in CUPS’s client.conf. Falls back to /var/run/cups/cups.sock)

• --socket=gpg-agent - Talk to the GPG agent (The socket in gpgconf --list-dir agent-socket)

• --socket=pcsc - Smart card access $PCSCLITE_CSOCK_NAME

• --socket=pulseaudio - Access to PulseAudio, includes sound input (mic), sound output/playback, MIDI and ALSA sound devices in /dev/snd

• --socket=ssh-auth- Allow access to $SSH_AUTH_SOCK

Bemerkung

Applications that do not support native Wayland should use only --socket=x11 and applications that do, should use --socket=fallback-x11 and --socket=wayland. The two configurations here will make the application work on both X11 and Wayland sessions of the desktop environment.

• --socket=wayland - Show windows with Wayland

• --socket=x11 - Show windows using X11

• --socket=fallback-x11 - Show windows using X11, if Wayland is not available, overrides x11 socket permission. Note that you must still use --socket=wayland for wayland permission

D-Bus access

DBus access is filtered by default. The default policy for the session bus only allows the application to own its own namespace named by $FLATPAK_ID, subnames of it and org.mpris.MediaPlayer2.$FLATPAK_ID for MPRIS. Furthermore, it is only allowed to talk to names matching those patterns, the bus itself org.freedesktop.DBus and portal APIs of the form org.freedesktop.portal.*.

Access to the entire bus with --socket=system-bus or --socket=session-bus stops the filtering and using them is a security risk. So they must be avoided, unless the application is a development tool.

flatpak run --log-session-bus $FLATPAK_ID can be used to find the specific D-Bus permissions needed. See Audit session or system bus traffic for more information.

Ownership

Any ownership beyond what is granted by default ie. own namespace and org.mpris.MediaPlayer2.$FLAPTAK_ID is typically unnecessary although there can be exceptions.

Talk

Talk permissions can be freely used, although it is recommended to use the minimum required.

Filesystem access

As a general rule, static and permanent filesystem access should be limited as much as possible. This includes:

• Using portals as an alternative to blanket filesystem access, wherever possible.

• Using read-only access wherever possible, using the :ro option.

• Using XDG base directories to store application’s cache, config and state. Then no additional filesystem access would be required.

• Avoiding full home access and instead using XDG directories such as xdg-music or xdg-download etc.

The following permission options are available:

• :ro - read-only access

• :create - read/write access, and create the directory if it doesn’t exist

Additionally the following permissions are available:

host	Access to /home, /media, /opt, /run/media, /srv and everything provided by host-os, host-etc mounted in /run/host	Includes any subpaths
host-etc	Host’s /etc	Host’s /etc is mounted at /run/host/etc
host-os	Host’s /usr, /bin, /sbin, /lib{32, 64}, /etc/ld.so.cache, /etc/alternatives	Mounted at /run/host
home	Access the home directory	Except ~/.var/app
/some/dir	Access an arbitrary path except any reserved path	Includes any subpaths
~/some/dir	Arbitrary path relative to the home directory	Includes any subpaths
xdg-desktop	Access the XDG desktop directory	$XDG_DESKTOP_DIR or $HOME/Desktop
xdg-documents	Access the XDG documents directory	$XDG_DOCUMENTS_DIR or $HOME/Documents
xdg-download	Access the XDG download directory	$XDG_DOWNLOAD_DIR or $HOME/Downloads
xdg-music	Access the XDG music directory	$XDG_MUSIC_DIR or $HOME/Music
xdg-pictures	Access the XDG pictures directory	$XDG_PICTURES_DIR or $HOME/Pictures
xdg-public-share	Access the XDG public directory	$XDG_PUBLICSHARE_DIR or $HOME/Public
xdg-videos	Access the XDG videos directory	$XDG_VIDEOS_DIR or $HOME/Videos
xdg-templates	Access the XDG templates directory	$XDG_TEMPLATES_DIR or $HOME/Templates
xdg-config	Access the XDG config directory [3]	$XDG_CONFIG_HOME or $HOME/.config
xdg-cache	Access the XDG cache directory [3]	$XDG_CACHE_HOME or $HOME/.cache
xdg-data	Access the XDG data directory [3]	$XDG_DATA_HOME or $HOME/.local/share
xdg-run/path	Access subdirectories of the XDG runtime directory	$XDG_RUNTIME_DIR/path (/run/user/$UID/path)

Except host, host-etc, host-os paths can be added to all the above filesystem options. For example, --filesystem=xdg-documents/path.

Other filesystem access guidelines include:

• The --persist=DIR option can be used to map directories from the user’s home directory into the sandbox filesystem. This only works if the application has no home or a broader permission like host that includes home.

  For example, if an application hardcodes the directory ~/.foo, without any home access and no --persist the directory will be lost from the sandbox once exited due to the filesystem being set up as tmpfs by flatpak unless overriden. A --persist=.foo bind mounts ~/.foo inside the sandbox to ~/.var/app/$FLATPAK_ID/.foo on host thus allowing an app to persistently store data in ~/.var/app/$FLATPAK_ID/.foo which would otherwise be lost.

  A --persist=. will persist all directories.

  This does not support :create, :ro, :rw suffixes or special values like xdg-documents. However, the directory will be created by flatpak if it doesn’t already exist.

  This makes it possible to avoid configuring access to the entire home directory, and can be useful for applications that hardcode file paths in ~/.

• If an application uses $TMPDIR to contain lock files you may want to add a wrapper script that sets it to $XDG_RUNTIME_DIR/app/$FLATPAK_ID (tmpfs) or /var/tmp (persistent on host).

• Retaining and sharing configuration with non-Flatpak installations is to be avoided.

Reserved Paths

The following paths and subpaths of them are reserved and asking access to them with --filesystem will have no effect:

/app, /bin, /dev, /etc, /lib, /lib32, /lib64, /proc, /run/flatpak, /run/host, /sbin, /usr

The entire /run is not allowed and all subpaths of /run except /run/flatpak, /run/host is allowed to be exposed via --filesystem. Additionally, if /var/run on host is a symlink to ../run, exposing it or a subpath of it, is not allowed.

Additionally the following directories from host need to be explicitly requested with --filesystem and are not available with home, host, host-os, host-etc by default:

• ~/.var/app - The app can access only its own directory in ~/.var/app/$FLATPAK_ID

• $XDG_DATA_HOME/flatpak (~/.local/share/flatpak)

• /boot

• /efi

• /root

• /sys

• /tmp

• /var - Note that by default /var/{cache, config, data, tmp} inside the sandbox are the same as ~/.var/app/$FLATPAK_ID/{cache, config, data, cache/tmp}. However an explicit --filesystem=/var will make only /var from host available and those will no longer be available.

• /var/lib/flatpak - /var does not give access to this.

Device access

You can provide the following device permissions:

dri	Direct Rendering Interface. Necessary for GL.
kvm	Kernel based Virtual Machine /dev/kvm
shm	Shared Memory in /dev/shm.
input	Input devices as exposed in /dev/input. This includes game controllers. Since Flatpak 1.15.6.
usb	Raw USB devices as exposed in /dev/bus/usb. Since Flatpak 1.15.11.
all	All devices, including all of the above except shm

Bemerkung

Using newer permissions like input or usb will have no effect on older Flatpak versions and will fail when used through Flatpak commandline.

While not ideal, --device=all can be used to access devices like webcams, CD/DVD drives etc.

USB portal

Since 1.5.11.

Sandboxed access to individual USB devices can be controlled by portals. Flatpak allows specifying enumerable USB devices to allow access.

Like --device=usb, this is just about accessing the raw USB device, that needs libusb (or equivalent). By using the portal, you can restrict which device can be requested (enumerable) and then request an explicit permission to access. For example, if you run a scanner driver, there is no reason for USB security devices to be accessible.

A list of valid use cases includes scanners (handled, for example by SANE), photo cameras (handled by libgphoto2), flashing devices, etc.

While this is portal dependent and xdg-desktop-portal is currently the only portal implementation, the overall permission flow is as follows:

• The Flatpak package specifies the devices it wishes to enumerate through finish-args.

• The application requests the portal to enumerate the available USB devices based on that list. If the list is empty it will enumerate all USB devices.

• When the application wants to access the device, it will make a request for the device it wants to access via the portal.

• The portal then requests permission from the user if not already granted.

• If the permission was granted, a file descriptor for the device is passed back to the application.

The application is then able to open the devices it is supposed to use while the others would be hidden.

Specifying the enumerable devices

You can specify devices on the flatpak command line, and by extension in the finish arguments for Flatpak Builder. Enumerable devices are specified with a query passed with --usb= while hidden devices are specified with a query passed with --nousb=. The hidden list takes precedence over the enumerable list, like an exception list. The goal is to be able to specify a broad range and then exclude the few devices that shall not be enumerated.

Queries are made out of rules. These rules are composable with +.

The rule all enumerates every USB device. There is no further rule allowed in the query.

The vnd and dev rules specify a USB vendor and a USB device ID respectively. A vendor can be specified alone, but a device rule always comes with a vendor rule as a device ID is only unique within a vendor. Vendor and device ID are specified with 4 digit hex numbers. For more information about the USB IDs, you can refer to the Linux USB ID repository

cls specifies the device USB class and subclass. Both class and subclass are two digit hex numbers separated by a colon :. You can use * to specify any subclass within the class.

Some examples of the syntax:

• vnd:1234: Devices from vendor 1234

• vnd:1234+dev:3456: Only device 3456 from vendor 1234.

• vnd:1234+cls:06:*: All the PTP devices from vendor 1234.

• cls:06:*: All the PTP devices.

This permission only allows to enumerate devices. To open them, permission must be requested from the portal. It is not possible to open a device that is not enumerable.

Bemerkung

The --device=usb permission is broader than what the USB portal is supposed to provide and allows unfettered access to any USB device on the bus.

In some situations you may need to specify a very long list of devices.

Device lists can be passed in one single argument, or through a file.

When using --usb-list, the queries are separated by a semi-colon ;, with queries for hidden devices (i.e. those that would be passed with --nousb) prefixed with !.

When using --usb-list-file, the filename of the file containing USB queries is passed line by line. Like with --usb-list queries for hidden devices are prefixed with !. Empty lines and lines starting with a # are ignored. When used with flatpak override or flatpak build-finish the file is no longer needed afterwards as the list is persisted internally.

dconf access

As of xdg-desktop-portal 1.1.0 and glib 2.60.5 (in the runtime) you do not need direct DConf access in most cases.

As of now this glib version is included in org.freedesktop.Platform//19.08 and org.gnome.Platform//3.34 and newer.

If an application existed prior to these runtimes you can tell Flatpak (>= 1.3.4) to migrate the DConf settings on the host into the sandbox by adding --metadata=X-DConf=migrate-path=/org/example/foo/ to finish-args. The path must be similar to your app-id or it will not be allowed (case is ignored and _ and - are treated equal).

If you are targeting older runtimes or require direct DConf access for other reasons you can use these permissions:

--filesystem=xdg-run/dconf
--filesystem=~/.config/dconf:ro
--talk-name=ca.desrt.dconf
--env=DCONF_USER_CONFIG_DIR=.config/dconf

With those permissions glib will continue using dconf directly.

If you use a newer runtime where dconf is no longer built and still need it you will have to build the dconf GIO module and set --env=GIO_EXTRA_MODULES=/app/lib/gio/modules/.

gvfs access

As of gvfs 1.48, the gvfs daemons and applications use an on-disk socket to communicate, rather than an abstract socket so that the gvfs infrastructure still works when network support is disabled in the application’s sandbox.

A number of different options need to be passed depending on the application’s use of gvfs.

--talk-name=org.gtk.vfs.* is necessary to talk to the gvfs daemons over D-Bus and list mounts using the GIO APIs.

--filesystem=xdg-run/gvfsd is necessary to use the GIO APIs to list and access non-native files using the GIO APIs, using URLs rather than FUSE paths.

--filesystem=xdg-run/gvfs is necessary to give access to the FUSE mounts non-GIO and legacy applications can use. This is what will make native files appear under /run/user/`id -u`/gvfs/.

Typical GNOME and GTK applications should use:

--talk-name=org.gtk.vfs.*
--filesystem=xdg-run/gvfsd

Typical non-GNOME and non-GTK applications should use:

--filesystem=xdg-run/gvfs

No application should be using --talk-name=org.gtk.vfs in its manifest, as there are no D-Bus services named org.gtk.vfs.

External drive access

External drives are mounted by the host system using systemd, udev, udisk fstab etc. and each of them can have different defaults. Flatpak has no control over how and where they get mounted. The following filesystem permissions should work in most cases:

--filesystem=/media
--filesystem=/run/media
--filesystem=/mnt

If --filesystem=host is used /media, /run/media is shared automatically if they exist.

Note that these should not have subpaths in them unless the value of the subpath can be consistently pre-determined. Block device naming depends on the kernel/fstab configuration and cannot be pre-determined.

Footnotes

[1]

This is not necessarily required, but without it the X11 shared memory extension will not work, which is very bad for X11 performance.

[2]

Giving network access also grants access to all host services listening on abstract Unix sockets (due to how network namespaces work), and these have no permission checks. This unfortunately affects e.g. the X server and the session bus which listens to abstract sockets by default. A secure distribution should disable these and just use regular sockets.

[3] (1,2,3)

xdg-{cache, config, data} bind mounts the paths from host to the per-app sandbox directory. Inside the sandbox $XDG_CACHE_HOME, $XDG_CONFIG_HOME and $XDG_DATA_HOME is set to $HOME/.var/app/$FLATPAK_ID/{cache, config, data} respectively. So for example, xdg-data/applications ie. $XDG_DATA_HOME/applications on host is bind mounted to $HOME/.var/app/$FLATPAK_ID/data/applications (inside the sandbox this is $XDG_DATA_HOME/applications). Additionally it’ll have two mount points - one expanded to $XDG_DATA_HOME/applications from the host and another to the sandbox’s $XDG_DATA_HOME/applications ie. $HOME/.var/app/$FLATPAK_ID/data/applications.