I wanted to create some keyboard short cuts to change the brightness of my screen (similar to what I did here). On my Fedora install I didn’t have any gnome components installed, I was just using lightdm with icewm.

Change Screen Brightness with Sysfs

First I discovered that using sysfs, I can change the brightness of my screen. To do this, first check out the current level:

elatov@kmac:~$cat /sys/class/backlight/gmux_backlight/brightness
3700

you can also check out the maximum brightness level like so:

elatov@kmac:~$cat /sys/class/backlight/gmux_backlight/max_brightness
82311

Then increase the value by about 700 to increase the brightness and vice versa to decrease the brightness. So to increase the brightness you can run the following:

elatov@kmac:~$sudo su -c 'echo 4400 > /sys/class/backlight/gmux_backlight/brightness'

You can then confirm that the brightness changed (other than visually) by checking the value again:

elatov@kmac:~$cat /sys/class/backlight/gmux_backlight/brightness
4400

and to decrease the brightness you can do the following:

elatov@kmac:~$sudo su -c 'echo 3700 > /sys/class/backlight/gmux_backlight/brightness'

Using sudo to Change the Brightness Level

I put together a quick script to change the brightness and placed it under /usr/local/bin. Here is how the script looked like:

elatov@kmac:~$cat /usr/local/bin/ch_br
#!/bin/bash
cur_bri=$(/usr/bin/cat /sys/class/backlight/gmux_backlight/brightness)

if [ $1 == "up" ] ; then
    bri=$(($cur_bri+700))
    `echo $bri > /sys/class/backlight/gmux_backlight/brightness`
fi

if [ $1 == "down" ] ; then
    bri=$(($cur_bri-700))
    `echo $bri > /sys/class/backlight/gmux_backlight/brightness`
fi

Since we are editing a file owned by root we need to use sudo to elevate our privileges. I was going to use that script with a keyboard shortcut, so I needed to allow myself to execute that script with sudo without a password. I was already part of the wheel group, which allowed me to run any command with sudo. Each distribution has a different group for this, you can check what group you need to belong to by checking the contents of /etc/sudoers:

elatov@kmac:~$sudo grep wheel /etc/sudoers
## Allows people in group wheel to run all commands
%wheel  ALL=(ALL)   ALL

and of course I was part of the wheel group:

elatov@kmac:~$groups
elatov wheel

Lastly, you can check what you can run with sudo by running the following:

elatov@kmac:~$sudo -l
User elatov may run the following commands on this host:
    (ALL) ALL

So I could run any commands, but I would need to type my password to authenticate myself. To add a rule to sudoers too allow me to execute my script as root without typing my password, I did the following:

elatov@kmac:~$sudo vi /etc/sudoers.d/mac_brig

and I placed the following into the file:

Cmnd_Alias CMDS = /usr/local/bin/ch_br *
elatov ALL = NOPASSWD: CMDS

then checking my sudo list, I saw the following:

elatov@kmac:~$sudo -l
User elatov may run the following commands on this host:
    (ALL) ALL
    (root) NOPASSWD: /usr/local/bin/ch_br *

Now when I ran the following:

elatov@kmac:~$sudo /usr/local/bin/ch_br up

and it just increased the brightness without asking me for my password.

IceWM KeyBoard Shortcuts

I wanted to create keyboard shortcuts which would decrease the brightness upon typing “Command+F1” and would increase brightness by typing “Command+F2”. On Linux the Mac Command key shows up as the Windows Super Key. So adding the following to my ~/.icewm/keys file should’ve taken care of this:

elatov@kmac:~$grep ch_br .icewm/keys
key "Super+F1"  sudo /usr/local/bin/ch_br down
key "Super+F2"  sudo /usr/local/bin/ch_br up

But I quickly realized that running any command with sudo from the keys file didn’t work out. So I decided to look for an alternative to sudo and I found PolKit.

PolKit

From this ArchWiki page:

PolicyKit is an application-level toolkit for defining and handling the policy that allows unprivileged processes to speak to privileged processes: It is a framework for centralizing the decision making process with respect to granting access to privileged operations for unprivileged applications. PolicyKit is specifically targeting applications in rich desktop environments on multi-user UNIX-like operating systems. It does not imply or rely on any exotic kernel features.

PolicyKit has gone through a lot of changes and is now called PolKit. From the same wiki page:

In the development of PolicyKit, major changes were introduced around version 0.92. In order to make the distinction clear between the way the old and the new versions worked, the new ones are referred to as ‘polkit’ rather than PolicyKit. Searching for PolicyKit on the web will mostly point to outdated documentation and lead to confusion and frustration. The main distinction between PolicyKit and polkit is the abandonment of single-file configuration in favour of directory-based configuration, i.e. there is no PolicyKit.conf.

From this freedesktop page, here is a good architecture of PolKit:

polkit arch Using PolKit to Change Screen Brightness on a Mac BookPro Running Fedora 19

From the initial Arch wiki page, here is the structure of PolKit:

PolicyKit definitions can be divided into three kinds:

  • Actions are defined in XML .policy files located in /usr/share/polkit-1/actions. Each action has a set of default permissions attached to it (e.g. you need to identify as an administrator to use the GParted action). The defaults can be overruled but editing the actions files is NOT the correct way. The files installed under /usr/share/polkit-1/actions are not meant to be modified. Instead, you should modify the authorities under /etc/polkit-1/localauthority/50-local.d.
  • Authorities are defined in INI-like .pkla files. They are found in two places: 3rd party packages can use /var/lib/polkit-1 (though few if any do) and /etc/polkit-1 is for local configuration. The .pkla files designate a subset of users, refer to one (or more) of the actions specified in the actions files and determine with what restrictions these actions can be taken by that/those user(s). As an example, an authority file could overrule the default requirement for all users to authenticate as an admin when using GParted, determining that some specific user doesn’t need to. Or isn’t allowed to use GParted at all.
  • Admin identities are set in /etc/polkit-1/localauthority.conf.d. One of the basic points of using PolicyKit is determining whether or not a user needs to authenticate (possibly as an administrative user) or not in order to get permission to carry out the action. PolicyKit therefore has a specific configuration for deciding if the user trying to carry out an action is or is not an administrative user. Common definitions are ‘only root user’ or ‘all members of wheel’ (the Arch default).

PolKit Actions

From the Arch wiki page:

Each action is defined in an tag in a .policy file. The org.archlinux.pkexec.gparted.policy contains a single action and looks like this:

< ?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">
<policyconfig>

  <action id="org.archlinux.pkexec.gparted">
    <message>Authentication is required to run the GParted Partition Editor</message>
    <icon_name>gparted</icon_name>
    <defaults>
      <allow_any>auth_admin</allow_any>
      <allow_inactive>auth_admin</allow_inactive>
      <allow_active>auth_admin</allow_active>
    </defaults>
    <annotate key="org.freedesktop.policykit.exec.path">/usr/sbin/gparted</annotate>
    <annotate key="org.freedesktop.policykit.exec.allow_gui">true</annotate>
  </action>

</policyconfig>

The attribute id is the actual command sent to dbus, the message tag is the explanation to the user when authentification is required and the icon_name is sort of obvious. The default tag is where the permissions or lack thereof are located. It contains three settings: allow_any, allow_inactive, and allow_active. Inactive sessions are generally remote sessions (SSH, VNC, etc.) whereas active sessions are logged directly into the machine on a TTY or an X display. Allow_any is the setting encompassing both scenarios. For each of these settings the following options are available:

  • no: The user is not authorized to carry out the action. There is therefore no need for authentification.
  • yes: The user is authorized to carry out the action without any authentification.
  • auth_self: Authentication is required but the user need not be an administrative user.
  • auth_admin: Authentication as an administrative user is require.
  • auth_self_keep: The same as auth_self but, like sudo, the authorization lasts a few minutes.
  • auth_admin_keep: The same as auth_admin but, like sudo, the authorization lasts a few minutes.

You can list all the defined actions by running pkaction:

elatov@kmac:~$pkaction  | head
dk.yumex.backend.pkexec.run
net.reactivated.fprint.device.enroll
net.reactivated.fprint.device.setusername
net.reactivated.fprint.device.verify
org.blueman.bluez.config
org.blueman.dhcp.client
org.blueman.hal.manager
org.blueman.network.setup
org.fedoraproject.config.date.pkexec.run
org.fedoraproject.config.keyboard.pkexec.run

If you want more information about an action, you can run the following:

elatov@kmac:~$pkaction --action-id org.fedoraproject.config.date.pkexec.run --verbose
org.fedoraproject.config.date.pkexec.run:
  description:       Run System Config Date
  message:           Authentication is required to run system-config-date
  vendor:            System Config Date
  vendor_url:        http://fedorahosted.org/system-config-date
  icon:              system-config-date
  implicit any:      no
  implicit inactive: no
  implicit active:   auth_admin
  annotation:        org.freedesktop.policykit.exec.path -> /usr/share/system-config-date/system-config-date.py
  annotation:        org.freedesktop.policykit.exec.allow_gui -> true

So if a regular user ran the system-config-date command with pkexec they would have to authenticate as an admin identity.

PolKit Authorities

Again from the same wiki page:

Authorizations that overrule the default settings are laid out in a set of directories as described above. For all purposes relating to personal configuration of a single system, only /etc/polkit-1/localauthority/50-local.d should be used. The authority files are read in alphabetical/numerical order, where later files take precedence, so that one configuration file can be relied upon to overrule another, e.g.

10_allow_all_users_group_members_to_automount_without_authentification.pkla
15_but_not_jack.pkla

The layout of the .pkla files is fairly self-explanatory:

[Ban users jack and jill from using gparted]
Identity=unix-user:jack;unix-user:jill
Action=org.archlinux.pkexec.gparted
ResultAny=no
ResultInactive=no
ResultActive=no

An authorization needs to be preceded by a heading enclosed in square brackets. The follows an identification with pairs of unix-user or unix-group and the name. Use semicolons to separate the pairs to include more than one user or group. The designating name of the action is the one from the action’s id attribute in /usr/share/polkit-1/actions. The three Result-settings mirror those from the action definition. Here we have overruled the default auth_admin setting and disallowed jack and jill from running gparted.

PolKit Admin Identities

From the wiki page:

Like the authorities files, configuration works by letting the last read file take precedence over earlier ones. The default configuration for admin identities is contained in the file 50-localauthority.conf so any changes to that configuration should be made by copying the file to, say, 60-localauthority.conf and editing that file.

/etc/polkit-1/localauthority.conf.d/50-localauthority.conf
# Configuration file for the PolicyKit Local Authority.
#
# DO NOT EDIT THIS FILE, it will be overwritten on update.
#
# See the pklocalauthority(8) man page for more information
# about configuring the Local Authority.
#

[Configuration]
AdminIdentities=unix-group:wheel

The only part to edit (once copied) is the right part of the equation: As whom should a user authenticate when asked to authenticate as an administrative user? If she herself is a member of the group designated as admins, she only need enter her own password. If some other user, e.g. root, is the only admin identity, she would need to enter in root’s password. The format of the user identification is the same as the one used in designating authorities. The Arch default is to make all members of the group wheel administrators.

PolKit in Different Linux Distributions

Different distributions use PolKit in different ways. On the Fedora laptop, I see that the rules are utilized rather that static authority files. For example we can see that there are no localauthority configurations files:

[root@kmac ~]# tree /etc/polkit-1/
/etc/polkit-1/
├── localauthority
│   ├── 10-vendor.d
│   ├── 20-org.d
│   ├── 30-site.d
│   ├── 50-local.d
│   └── 90-mandatory.d
├── localauthority.conf.d
└── rules.d
    ├── 49-polkit-pkla-compat.rules
    └── 50-default.rules

8 directories, 2 files

Notice only empty directories exist for the localauthority configurations. But if we check the default rules file, we see the following:

[root@kmac ~]# tail -3 /etc/polkit-1/rules.d/50-default.rules
polkit.addAdminRule(function(action, subject) {
    return ["unix-group:wheel"];
});

So anyone part of the wheel group has admin authority, similar to what sudoers does. On my Ubuntu laptop, I see the localauthority files used.

elatov@crbook:~$tree /etc/polkit-1/
/etc/polkit-1/
|-- localauthority
|   |-- 10-vendor.d
|   |-- 20-org.d
|   |-- 30-site.d
|   |-- 50-local.d
|   `-- 90-mandatory.d
|-- localauthority.conf.d
|   |-- 50-localauthority.conf
|   `-- 51-ubuntu-admin.conf
`-- nullbackend.conf.d
    `-- 50-nullbackend.conf

8 directories, 3 files

Now checking out the actual files:

elatov@crbook:~$tail -2 /etc/polkit-1/localauthority.conf.d/50-localauthority.conf
[Configuration]
AdminIdentities=unix-user:0

and here is the other file:

elatov@crbook:~$tail -2 /etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf
[Configuration]
AdminIdentities=unix-group:sudo;unix-group:admin

So in this case, anyone part of the sudo or admin groups can authenticate as them selves and will be considered admins. Or anyone that can provide the root password will also be considered an admin.

Adding a PolKit Action

Here is what I did to add an action to PolKit. First I created the file, like so:

elatov@kmac:~$sudo vi /usr/share/polkit-1/actions/org.fedora.pkexec.chbr.policy

and I added the following to the file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">
<policyconfig>

  <action id="org.fedora.pkexec.chbr">
    <message>Authentication is required to Change Brightness</message>
    <icon_name>battery</icon_name>
    <defaults>
      <allow_any>no</allow_any>
      <allow_inactive>no</allow_inactive>
      <allow_active>yes</allow_active>
    </defaults>
    <annotate key="org.freedesktop.policykit.exec.path">/usr/local/bin/ch_br</annotate>
  </action>

</policyconfig>

I then confirmed the action was added:

elatov@kmac:~$pkaction  | grep chbr
org.fedora.pkexec.chbr

and I was also able to get all the information regarding that action:

elatov@kmac:~$pkaction  --action-id org.fedora.pkexec.chbr --verbose
org.fedora.pkexec.chbr:
  description:
  message:           Authentication is required to Change Brightness
  vendor:
  vendor_url:
  icon:              battery
  implicit any:      no
  implicit inactive: no
  implicit active:   yes
  annotation:        org.freedesktop.policykit.exec.path -> /usr/local/bin/ch_br

I was then able to run the following:

elatov@kmac:~$pkexec /usr/local/bin/ch_br up

and the brightness changed without asking me for a password. If I wanted, I could change this line from the policy:

<allow_active>yes</allow_active>

to this

<allow_active>auth_admin</allow_active>

and since I was part of the wheel group, I would be able to authenticate as myself and be authorized to run the command. Here is how that would look like:

elatov@kmac:~$pkexec /usr/local/bin/ch_br up
==== AUTHENTICATING FOR org.fedora.pkexec.chbr ===
Authentication is required to Change Brightness
Authenticating as: KE (elatov)
Password:
==== AUTHENTICATION COMPLETE ===

But if did that, I wouldn’t be able to use my keyboard short cuts.

Change IceWM KeyBoard Shortcuts to Use pkexec Rather Than sudo

This was pretty simple, here is how my config looked like after the change:

elatov@kmac:~$grep ch_br .icewm/keys
key "Super+F1"  pkexec /usr/local/bin/ch_br down
key "Super+F2"  pkexec /usr/local/bin/ch_br up

After restarting IceWM, I was able to modify the brightness level with “Command+F1” and “Command+F2” without any issues.

PolKit Vs Sudo

There is an ongoing battle amongst Linux users who say that sudo is more secure. From the Arch Wiki page:

PolicyKit operates on top of the existing permissions systems in linux - group membership, administrator status - it does not replace them. The example above prohibited the user jack from using the GParted action, but it does not preclude him running GParted by some means that do not respect PolicyKit, e.g. the command line. Therefore it’s probably better to use PolicyKit to expand access to priviledged services for unpriviledged users, rather than to try using it to curtail the rights of (semi-)privileged users. For security purposes, the sudoers file is still the way to go.

From this wikipedia page:

Whilst contested by its developer, polkit is criticized for contravening the Unix philosophy of doing one task and doing it well as it has a primary task of restricting root processes as an ‘authority’ and a secondary but easily mistaken for its primary task of allowing users to obtain temporary privileges ‘authorization’.

Polkit and its predecessor have stirred up much debate in being criticized for generalizing security functions (actions) and contravening the more code equals more bugs philosophy. The idea being to allow greater peer review of the code running as root, but this requires more code to run as root in itself. So whereby smaller and less popular projects may gain security initially from using polkit. Its continued usage actually limits the potential of an application to be as secure as possible when compared to privilege separated processes utilizing privilege dropping (OpenSSH, truecrypts core service). Therefore cross pollination of reviewed to be secure code itself adaptable to smaller projects specific requirements produces more secure programs but with the added risk of being more likely to be left un-maintaned (unlikely for code running as root which should be as short as possible and need little maintenance in any case).

I do agree that sudoers is easier to use and configure, but once you grasp all the PolKit concepts it’s not that bad. I think PolKit is not as old/established as sudo but it looks like a lot more of different OS aspects are starting to support/include it. Also check out “Assigning Privileges with sudo and PolicyKit” for a great comparison of the two.

Mac Book Pro Keyboard Backlight Brightness with Sysfs

You can use a similar method as above to create keyboard shorts to change brightness of the keyboard backlight. Here is the manual command to change it on the fly:

$ sudo su -c 'echo 8 > /sys/class/leds/smc::kbd_backlight/brightness'

Published by Karim Elatov

26 August 2013

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