pkexec - Online in the Cloud

PROGRAM:

NAME

pkexec - Execute a command as another user

SYNOPSIS

pkexec [--version] [--disable-internal-agent] [--help]

pkexec [--user username] PROGRAM [ARGUMENTS...]

DESCRIPTION

pkexec allows an authorized user to execute PROGRAM as another user. If username is not
specified, then the program will be executed as the administrative super user, root.

RETURN VALUE

Upon successful completion, the return value is the return value of PROGRAM. If the
calling process is not authorized or an authorization could not be obtained through
authentication or an error occured, pkexec exits with a return value of 127. If the
authorization could not be obtained because the user dismissed the authentication dialog,
pkexec exits with a return value of 126.

AUTHENTICATION AGENT

pkexec, like any other PolicyKit application, will use the authentication agent registered
for the calling process. However, if no authentication agent is available, then pkexec
will register its own textual authentication agent. This behavior can be turned off by
passing the --disable-internal-agent option.

SECURITY NOTES

Executing a program as another user is a privileged operation. By default the required
authorization (See the section called “REQUIRED AUTHORIZATIONS”) requires administrator
authentication. In addition, the authentication dialog presented to the user will display
the full path to the program to be executed so the user is aware of what will happen:

[IMAGE][1]

+----------------------------------------------------------+
| Authenticate [X] |
+----------------------------------------------------------+
| |
| [Icon] Authentication is needed to run `/bin/bash' |
| as the super user |
| |
| An application is attempting to perform an |
| action that requires privileges. Authentication |
| as the super user is required to perform this |
| action. |
| |
| Password for root: [_________________________] |
| |
| [V] Details: |
| Command: /bin/bash |
| Run As: Super User (root) |
| Action: org.freedesktop.policykit.exec |
| Vendor: The PolicyKit Project |
| |
| [Cancel] [Authenticate] |
+----------------------------------------------------------+

The environment that PROGRAM will run it, will be set to a minimal known and safe
environment in order to avoid injecting code through LD_LIBRARY_PATH or similar
mechanisms. In addition the PKEXEC_UID environment variable is set to the user id of the
process invoking pkexec. As a result, pkexec will not allow you to run X11 applications as
another user since the $DISPLAY and $XAUTHORITY environment variables are not set. These
two variables will be retained if the org.freedesktop.policykit.exec.allow_gui annotation
on an action is set to a nonempty value; this is discouraged, though, and should only be
used for legacy programs.

REQUIRED AUTHORIZATIONS

By default, the org.freedesktop.policykit.exec authorization is required unless an action
definition file is present for the program in question. To require another authorization,
it can be specified using the org.freedesktop.policykit.exec.path annotation on an action
(See the section called “EXAMPLE” for details).

EXAMPLE

To specify what kind of authorization is needed to execute the program
/usr/bin/pk-example-frobnicate as another user, simply write an action definition file
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>

<vendor>Examples for the PolicyKit Project</vendor>
<vendor_url>http://hal.freedesktop.org/docs/PolicyKit/</vendor_url>

<action id="org.freedesktop.policykit.example.pkexec.run-frobnicate">
<description>Run the PolicyKit example program Frobnicate</description>
<description xml:lang="da">Kør PolicyKit eksemplet Frobnicate</description>
<message>Authentication is required to run the PolicyKit example program Frobnicate (user=$(user), program=$(program), command_line=$(command_line))</message>
<message xml:lang="da">Autorisering er påkrævet for at afvikle PolicyKit eksemplet Frobnicate (user=$(user), program=$(program), command_line=$(command_line))</message>
<icon_name>audio-x-generic</icon_name>
<defaults>
<allow_any>no</allow_any>
<allow_inactive>no</allow_inactive>
<allow_active>auth_self_keep</allow_active>
</defaults>
<annotate key="org.freedesktop.policykit.exec.path">/usr/bin/pk-example-frobnicate</annotate>
</action>

</policyconfig>

and drop it in the /usr/share/polkit-1/actions directory under a suitable name (e.g.
matching the namespace of the action). Note that in addition to specifying the program,
the authentication message, description, icon and defaults can be specified. Note that
occurences of the strings $(user), $(program) and $(command_line) in the message will be
replaced with respectively the user (of the form "Real Name (username)" or just "username"
if there is no real name for the username), the binary to execute (a fully-qualified path,
e.g. "/usr/bin/pk-example-frobnicate") and the command-line, e.g. "pk-example-frobnicate
foo bar". For example, for the action defined above, the following authentication dialog
will be shown:

[IMAGE][2]

+----------------------------------------------------------+
| Authenticate [X] |
+----------------------------------------------------------+
| |
| [Icon] Authentication is required to run the PolicyKit |
| example program Frobnicate |
| |
| An application is attempting to perform an |
| action that requires privileges. Authentication |
| is required to perform this action. |
| |
| Password: [__________________________________] |
| |
| [V] Details: |
| Command: /usr/bin/pk-example-frobnicate |
| Run As: Super User (root) |
| Action: org.fd.pk.example.pkexec.run-frobnicate |
| Vendor: Examples for the PolicyKit Project |
| |
| [Cancel] [Authenticate] |
+----------------------------------------------------------+

If the user is using the da_DK locale, the dialog looks like this:

[IMAGE][3]

+----------------------------------------------------------+
| Autorisering [X] |
+----------------------------------------------------------+
| |
| [Icon] Autorisering er påkrævet for at afvikle |
| PolicyKit eksemplet Frobnicate |
| |
| Et program forsøger at udføre en handling der |
| kræver privilegier. Autorisering er påkrævet. |
| |
| Kodeord: [___________________________________] |
| |
| [V] Detaljer: |
| Bruger: Super User (root) |
| Program: /usr/bin/pk-example-frobnicate |
| Handling: org.fd.pk.example.pkexec.run-frobnicate |
| Vendor: Examples for the PolicyKit Project |
| |
| [Annullér] [Autorisering] |
+----------------------------------------------------------+

Note that pkexec does no validation of the ARGUMENTS passed to PROGRAM. In the normal case
(where administrator authentication is required every time pkexec is used), this is not a
problem since if the user is an administrator he might as well just run pkexec bash to get
root.

However, if an action is used for which the user can retain authorization (or if the user
is implicitly authorized), such as with pk-example-frobnicate above, this could be a
security hole. Therefore, as a rule of thumb, programs for which the default required
authorization is changed, should never implicitly trust user input (e.g. like any other
well-written suid program).

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