POLKIT(8) polkit POLKIT(8)
NAME
polkit - Authorization Manager
OVERVIEW
polkit provides an authorization API intended to be used by privileged programs (“MECHANISMS”) offering
service to unprivileged programs (“SUBJECTS”) often through some form of inter-process communication
mechanism. In this scenario, the mechanism typically treats the subject as untrusted. For every request from a
subject, the mechanism needs to determine if the request is authorized or if it should refuse to service the
subject. Using the polkit APIs, a mechanism can offload this decision to a trusted party: The polkit
authority.
The polkit authority is implemented as an system daemon, polkitd(8), which itself has little privilege as it
is running as the polkitd system user. Mechanisms, subjects and authentication agents communicate with the
authority using the system message bus.
In addition to acting as an authority, polkit allows users to obtain temporary authorization through
authenticating either an administrative user or the owner of the session the client belongs to. This is useful
for scenarios where a mechanism needs to verify that the operator of the system really is the user or really
is an administrative user.
SYSTEM ARCHITECTURE
The system architecture of polkit is comprised of the Authority (implemented as a service on the system
message bus) and an Authentication Agent per user session (provided and started by the user's graphical
environment). Actions are defined by applications. Vendors, sites and system administrators can control
authorization policy through Authorization Rules.
[IMAGE][1]
+-------------------+
| Authentication |
| Agent |
+-------------------+
| libpolkit-agent-1 |
+-------------------+
^ +---------+
| | Subject |
+--------------+ +---------+
| ^
| |
User Session | |
=======================|========================|=============
System Context | |
| |
| +---+
V |
/------------\ |
| System Bus | |
\------------/ |
^ ^ V
| | +---------------------+
+--------------+ | | Mechanism |
| | +---------------------+
V +----> | libpolkit-gobject-1 |
+------------------+ +---------------------+
| polkitd(8) |
+------------------+
For convenience, the libpolkit-gobject-1 library wraps the polkit D-Bus API and is usable from any C/C++
program as well as higher-level languages supporting GObjectIntrospection[2] such as Javascript and Python. A
mechanism can also use the D-Bus API or the pkcheck(1) command to check authorizations. The libpolkit-agent-1
library provides an abstraction of the native authentication system, e.g. pam(8) and also facilities
registration and communication with the polkit D-Bus service.
See the developer documentation[3] for more information about writing polkit applications.
AUTHENTICATION AGENTS
An authentication agent is used to make the user of a session prove that the user of the session really is the
user (by authenticating as the user) or an administrative user (by authenticating as a administrator). In
order to integrate well with the rest of the user session (e.g. match the look and feel), authentication
agents are meant to be provided by the user session that the user uses. For example, an authentication agent
may look like this:
[IMAGE][4]
+----------------------------------------------------------+
| |
| [Icon] Authentication required |
| |
| Authentication is required to format INTEL |
| SSDSA2MH080G1GC (/dev/sda) |
| |
| Administrator |
| |
| Password: [__________________________________] |
| |
| [Cancel] [Authenticate] |
+----------------------------------------------------------+
If the system is configured without a root account it may prompt for a specific user designated as the
administrative user:
[IMAGE][5]
+----------------------------------------------------------+
| |
| [Icon] Authentication required |
| |
| Authentication is required to format INTEL |
| SSDSA2MH080G1GC (/dev/sda) |
| |
| [Icon] David Zeuthen |
| |
| Password: [__________________________________] |
| |
| [Cancel] [Authenticate] |
+----------------------------------------------------------+
Applications that do not run under a desktop environment (for example, if launched from a ssh(1) login) may
not have have an authentication agent associated with them. Such applications may use the
PolkitAgentTextListener type or the pkttyagent(1) helper so the user can authenticate using a textual
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policyconfig PUBLIC "-//freedesktop//DTD polkit Policy Configuration 1.0//EN"
"http://www.freedesktop.org/software/polkit/policyconfig-1.dtd">
The policyconfig element must be present exactly once. Elements that can be used inside policyconfig includes:
vendor
The name of the project or vendor that is supplying the actions in the XML document. Optional.
vendor_url
A URL to the project or vendor that is supplying the actions in the XML document. Optional.
icon_name
An icon representing the project or vendor that is supplying the actions in the XML document. The icon
name must adhere to the Freedesktop.org Icon Naming Specification[6]. Optional.
action
Declares an action. The action name is specified using the id attribute and can only contain the
characters [A-Z][a-z][0-9].- e.g. ASCII, digits, period and hyphen.
Elements that can be used inside action include:
description
A human readable description of the action, e.g. “Install unsigned software”.
message
A human readable message displayed to the user when asking for credentials when authentication is needed,
e.g. “Installing unsigned software requires authentication”.
defaults
This element is used to specify implicit authorizations for clients. Elements that can be used inside
defaults include:
allow_any
Implicit authorizations that apply to any client. Optional.
allow_inactive
Implicit authorizations that apply to clients in inactive sessions on local consoles. Optional.
allow_active
Implicit authorizations that apply to clients in active sessions on local consoles. Optional.
Each of the allow_any, allow_inactive and allow_active elements can contain the following values:
no
Not authorized.
yes
Authorized.
auth_self
Authentication by the owner of the session that the client originates from is required. Note that this
is not restrictive enough for most uses on multi-user systems; auth_admin* is generally recommended.
Used for annotating an action with a key/value pair. The key is specified using the the key attribute and
the value is specified using the value attribute. This element may appear zero or more times. See below
for known annotations.
vendor
Used for overriding the vendor on a per-action basis. Optional.
vendor_url
Used for overriding the vendor URL on a per-action basis. Optional.
icon_name
Used for overriding the icon name on a per-action basis. Optional.
For localization, description and message elements may occur multiple times with different xml:lang
attributes.
To list installed polkit actions, use the pkaction(1) command.
Known annotations
The org.freedesktop.policykit.exec.path annotation is used by the pkexec program shipped with polkit - see the
pkexec(1) man page for details.
The org.freedesktop.policykit.imply annotation (its value is a string containing a space separated list of
action identifiers) can be used to define meta actions. The way it works is that if a subject is authorized
for an action with this annotation, then it is also authorized for any action specified by the annotation. A
typical use of this annotation is when defining an UI shell with a single lock button that should unlock
multiple actions from distinct mechanisms.
The org.freedesktop.policykit.owner annotation can be used to define a set of users who can query whether a
client is authorized to perform this action. If this annotation is not specified then only root can query
whether a client running as a different user is authorized for an action. The value of this annotation is a
string containing a space separated list of PolkitIdentity entries, for example "unix-user:42
unix-user:colord". A typical use of this annotation is for a daemon process that runs as a system user rather
than root.
AUTHORIZATION RULES
polkitd reads .rules files from the /etc/polkit-1/rules.d and /usr/share/polkit-1/rules.d directories by
sorting the files in lexical order based on the basename on each file (if there's a tie, files in /etc are
processed before files in /usr). For example, for the following four files, the order is
· /etc/polkit-1/rules.d/10-auth.rules
· /usr/share/polkit-1/rules.d/10-auth.rules
· /etc/polkit-1/rules.d/15-auth.rules
· /usr/share/polkit-1/rules.d/20-auth.rules
Both directories are monitored so if a rules file is changed, added or removed, existing rules are purged and
all files are read and processed again. Rules files are written in the JavaScript[7] programming language and
interface with polkitd through the global polkit object (of type Polkit).
While the JavaScript interpreter used in particular versions of polkit may support non-standard features (such
as the let keyword), authorization rules must conform to ECMA-262 edition 5[8] (in other words, the JavaScript
The following methods are available on the polkit object:
void addRule(polkit.Result function(action, subject) {...});
void addAdminRule(string[] function(action, subject) {...});
void log(string message);
string spawn(string[] argv);
The addRule() method is used for adding a function that may be called whenever an authorization check for
action and subject is performed. Functions are called in the order they have been added until one of the
functions returns a value. Hence, to add an authorization rule that is processed before other rules, put it in
a file in /etc/polkit-1/rules.d with a name that sorts before other rules files, for example
00-early-checks.rules. Each function should return a value from polkit.Result
polkit.Result = {
NO : "no",
YES : "yes",
AUTH_SELF : "auth_self",
AUTH_SELF_KEEP : "auth_self_keep",
AUTH_ADMIN : "auth_admin",
AUTH_ADMIN_KEEP : "auth_admin_keep",
NOT_HANDLED : null
};
corresponding to the values that can be used as defaults. If the function returns polkit.Result.NOT_HANDLED,
null, undefined or does not return a value at all, the next user function is tried.
Keep in mind that if polkit.Result.AUTH_SELF_KEEP or polkit.Result.AUTH_ADMIN_KEEP is returned, authorization
checks for the same action identifier and subject will succeed (that is, return polkit.Result.YES) for the
next brief period (e.g. five minutes) even if the variables passed along with the check are different.
Therefore, if the result of an authorization rule depend on such variables, it should not use the "*_KEEP"
constants (if similar functionality is required, the authorization rule can easily implement temporary
authorizations using the Date[9] type for timestamps).
The addAdminRule() method is used for adding a function may be called whenever administrator authentication is
required. The function is used to specify what identies may be used for administrator authentication for the
authorization check identified by action and subject. Functions added are called in the order they have been
added until one of the functions returns a value. Each function should return an array of strings where each
string is of the form "unix-group:<group>", "unix-netgroup:<netgroup>" or "unix-user:<user>". If the function
returns null, undefined or does not return a value at all, the next function is tried.
There is no guarantee that a function registered with addRule() or addAdminRule() is ever called - for example
an early rules file could register a function that always return a value, hence ensuring that functions added
later are never called.
If user-provided code takes a long time to execute an exception will be thrown which normally results in the
function being terminated (the current limit is 15 seconds). This is used to catch runaway scripts.
The spawn() method spawns an external helper identified by the argument vector argv and waits for it to
terminate. If an error occurs or the helper doesn't exit normally with exit code 0, an exception is thrown. If
the helper does not exit within 10 seconds it is killed. Otherwise, the program's standard output is returned
as a string. The spawn() method should be used sparingly as helpers may take a very long or indeterminate
}
});
will produce the following when the user runs 'pkexec -u bateman bash -i' from a shell:
May 24 14:28:50 thinkpad polkitd[32217]: /etc/polkit-1/rules.d/10-test.rules:3: action=[Action id='org.freedesktop.policykit.exec' command_line='/usr/bin/bash -i' program='/usr/bin/bash' user='bateman' user.gecos='Patrick Bateman' user.display='Patrick Bateman (bateman)']
May 24 14:28:50 thinkpad polkitd[32217]: /etc/polkit-1/rules.d/10-test.rules:4: subject=[Subject pid=1352 user='davidz' groups=davidz,wheel, seat='seat0' session='1' local=true active=true]
The Action type
The action parameter passed to user functions is an object with information about the action being checked. It
is of type Action and has the following attribute:
string id
The action identifier, for example org.freedesktop.policykit.exec.
The following methods are available on the Action type:
string lookup(string key);
The lookup() method is used to lookup the polkit variables passed from the mechanism. For example, the
pkexec(1) mechanism sets the variable program which can be obtained in Javascript using the expression
action.lookup("program"). If there is no value for the given key, then undefined is returned.
Consult the documentation for each mechanism for what variables are available for each action.
The Subject type
The subject parameter passed to user functions is an object with information about the process being checked.
It is of type Subject and has the following attributes
int pid
The process id.
string user
The user name.
string[] groups
Array of groups that user user belongs to.
string seat
The seat that the subject is associated with - blank if not on a local seat.
string session
The session that the subject is associated with.
boolean local
Set to true only if seat is local.
boolean active
Set to true only if the session is active.
The following methods are available on the Subject type:
boolean isInGroup(string groupName);
}
});
Define administrative users to be the users in the wheel group:
polkit.addAdminRule(function(action, subject) {
return ["unix-group:wheel"];
});
Forbid users in group children to change hostname configuration (that is, any action with an identifier
starting with org.freedesktop.hostname1.) and allow anyone else to do it after authenticating as themselves:
polkit.addRule(function(action, subject) {
if (action.id.indexOf("org.freedesktop.hostname1.") == 0) {
if (subject.isInGroup("children")) {
return polkit.Result.NO;
} else {
return polkit.Result.AUTH_SELF_KEEP;
}
}
});
Run an external helper to determine if the current user may reboot the system:
polkit.addRule(function(action, subject) {
if (action.id.indexOf("org.freedesktop.login1.reboot") == 0) {
try {
// user-may-reboot exits with succeess (exit code 0)
// only if the passed username is authorized
polkit.spawn(["/opt/company/bin/user-may-reboot",
subject.user]);
return polkit.Result.YES;
} catch (error) {
// Nope, but do allow admin authentication
return polkit.Result.AUTH_ADMIN;
}
}
});
The following example shows how the authorization decision can depend on variables passed by the pkexec(1)
mechanism:
polkit.addRule(function(action, subject) {
if (action.id == "org.freedesktop.policykit.exec" &&
action.lookup("program") == "/usr/bin/cat") {
return polkit.Result.AUTH_ADMIN;
}
});
The following example shows another use of variables passed from the mechanism. In this case, the mechanism is
UDisks[10] which defines a set of actions and variables[11] that is used to match on:
// Allow users in group 'engineers' to perform any operation on
// some drives without having to authenticate
Written by David Zeuthen <[email protected]> with a lot of help from many others.
BUGS
Please send bug reports to either the distribution or the polkit-devel mailing list, see the link
http://lists.freedesktop.org/mailman/listinfo/polkit-devel on how to subscribe.
SEE ALSO
polkitd(8), pkaction(1), pkcheck(1), pkexec(1), pkttyagent(1)
NOTES
1. /usr/share/gtk-doc/html/polkit-1/polkit-architecture.png
2. GObjectIntrospection
https://live.gnome.org/GObjectIntrospection
3. developer documentation
http://www.freedesktop.org/software/polkit/docs/latest/
4. /usr/share/gtk-doc/html/polkit-1/polkit-authentication-agent-example.png
5. /usr/share/gtk-doc/html/polkit-1/polkit-authentication-agent-example-wheel.png
6. Freedesktop.org Icon Naming Specification
http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
7. JavaScript
http://en.wikipedia.org/wiki/JavaScript
8. ECMA-262 edition 5
http://en.wikipedia.org/wiki/ECMAScript#ECMAScript.2C_5th_Edition
9. Date
https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Date
10. UDisks
http://udisks.freedesktop.org/docs/latest/udisks.8.html
11. actions and variables
http://udisks.freedesktop.org/docs/latest/udisks-polkit-actions.html
polkit January 2009 POLKIT(8)
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