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) |
+------------------+
| org.freedesktop. |
| PolicyKit1 |<---------+
+------------------+ |
^ |
| +--------------------------------------------+
| | /etc/polkit-1/actions/*.policy |
| | /run/polkit-1/actions/*.policy |
| | /usr/local/share/polkit-1/actions/*.policy |
| | /usr/share/polkit-1/actions/*.policy |
| +--------------------------------------------+
|
+--------------------------------------------+
| /etc/polkit-1/rules.d/*.rules |
| /run/polkit-1/rules.d/*.rules |
| /usr/local/share/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 for 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 an
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 an ssh(1) login) may not 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 needs 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 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 following directories in this
order:
• /etc/polkit-1/rules.d
• /run/polkit-1/rules.d
• /usr/local/share/polkit-1/rules.d
• /usr/share/polkit-1/rules.d
These directories are processed in lexical order based on the basename
of each file. If there's a tie, files in directories earlier in the
list are processed first. For example, for the following four files,
the order is:
• /etc/polkit-1/rules.d/10-auth.rules
• /run/polkit-1/rules.d/10-auth.rules
• /usr/local/share/polkit-1/rules.d/10-auth.rules
• /usr/share/polkit-1/rules.d/10-auth.rules
All of these 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 that may be
called whenever administrator authentication is required. The function
is used to specify what identities 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 returns a value, hence ensuring that
functions added later are never called.
If user-provided code takes a long time to execute, no exception will
be thrown and the script will be killed right away (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.
string system_unit
The systemd unit that the subject's process is part of (if any).
Note that this can only match on system units, as user units can be
created with any name without privileges (unlike system units which
require root to create). A process running in a user unit will
return the user session unit in this attribute (e.g.:
user-1000.service).
boolean local
Set to true only if seat is local.
boolean no_new_privileges
Set only if system_unit is not empty, and set to true only if the
referenced systemd service unit has the NoNewPrivileges= setting
enabled. This ensures that the process cannot gain any new
privileges via executing setuid binaries.
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 success (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;
}
}
});
Allow all processes running as part of the admin.service systemd system
unit to perform user administration, as long as they cannot gain new
privileges:
polkit.addRule(function(action, subject) {
if (action.id == "org.freedesktop.accounts.user-administration" &&
subject.system_unit == "admin.service" &&
subject.no_new_privileges) {
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
https://gitlab.freedesktop.org/polkit/polkit/-/issues/ 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-
US/docs/Web/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 February 2021 POLKIT(8)
polkit \- Authorization Manager