Provided by: polkitd_124-2ubuntu1_amd64 
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 |<---------+
+------------------+ |
^ |
| +--------------------------------------+
| | /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 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 /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 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, 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. If the duktape JavaScript backend is compiled in,
instead of mozjs, no exception will be thrown—the script will be killed right away (same
timeout).
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