POLKIT(8)polkitPOLKIT(8)NAMEpolkit - Authorization Manager
OVERVIEWpolkit 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) |
+------------------+
| org.freedesktop. |
| PolicyKit1 |<---------+
+------------------+ |
^ |
| +--------------------------------------+
| | /usr/share/polkit-1/actions/*.policy |
| +--------------------------------------+
|
+--------------------------------------+
| /etc/polkit-1/rules.d/*.rules |
| /usr/share/polkit-1/rules.d/*.rules |
+--------------------------------------+
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 interface.
DECLARING ACTIONS
A mechanism need to declare a set of actions in order to use polkit.
Actions correspond to operations that clients can request the mechanism
to carry out and are defined in XML files that the mechanism installs
into the /usr/share/polkit-1/actions directory.
polkit actions are namespaced and can only contain the characters
[A-Z][a-z][0-9].- e.g. ASCII, digits, period and hyphen. Each XML file
can contain more than one action but all actions need to be in the same
namespace and the file needs to be named after the namespace and have
the extension .policy.
The XML file must have the following doctype declaration
<?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.
auth_admin
Authentication by an administrative user is required.
auth_self_keep
Like auth_self but the authorization is kept for a brief period
(e.g. five minutes). The warning about auth_self above applies
likewise.
auth_admin_keep
Like auth_admin but the authorization is kept for a brief
period (e.g. five minutes).
annotate
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 interpreter used may change in future versions of
polkit).
Authorization rules are intended for two specific audiences
· System Administrators
· Special-purpose Operating Systems / Environments
and those audiences only. In particular, applications, mechanisms and
general-purpose operating systems must never include any authorization
rules.
The Polkit type
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 amount of time to complete and no other authorization
check can be handled while the helper is running. Note that the spawned
programs will run as the unprivileged polkitd system user.
The log() method writes the given message to the system logger prefixed
with the JavaScript filename and line number. Log entries are emitted
using the LOG_AUTHPRIV flag meaning that the log entries usually ends
up in the file /var/log/secure. The log() method is usually only used
when debugging rules. The Action and Subject types has suitable
toString() methods defined for easy logging, for example,
polkit.addRule(function(action, subject) {
if (action.id == "org.freedesktop.policykit.exec") {
polkit.log("action=" + action);
polkit.log("subject=" + subject);
}
});
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);
boolean isInNetGroup(string netGroupName);
The isInGroup() method can be used to check if the subject is in a
given group and isInNetGroup() can be used to check if the subject is
in a given netgroup.
Authorization Rules Examples
Allow all users in the admin group to perform user administration
without changing policy for other users:
polkit.addRule(function(action, subject) {
if (action.id == "org.freedesktop.accounts.user-administration" &&
subject.isInGroup("admin")) {
return polkit.Result.YES;
}
});
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
//
polkit.addRule(function(action, subject) {
if (action.id.indexOf("org.freedesktop.udisks2.") == 0 &&
action.lookup("drive.vendor") == "SEAGATE" &&
action.lookup("drive.model") == "ST3300657SS" &&
subject.isInGroup("engineers")) {
return polkit.Result.YES;
}
}
});
AUTHOR
Written by David Zeuthen davidz@redhat.com 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 ALSOpolkitd(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)
