|PolicyKit Reference Manual|
pkexec — Execute a command as another user
pkexec allows an authorized user to
PROGRAM as another
username is not specified,
then the program will be executed as the administrative super
Upon successful completion, the return value is the return value
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.
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:
The environment that
PROGRAM will run
it, will be set to a minimal known and safe environment in order
to avoid injecting code
LD_LIBRARY_PATH or similar
mechanisms. In addition the
environment variable is set to the user id of the process
invoking pkexec. As a
result, pkexec will not allow you to run
e.g. X11 applications as another user since
$DISPLAY environment variable is not set.
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).
To specify what kind of authorization is needed to execute the
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</message> <message xml:lang="da">Autorisering er påkrævet for at afvikle PolicyKit eksemplet Frobnicate</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
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. For example, for the action defined above, the following
authentication dialog will be shown:
If the user is using the
da_DK locale, the
dialog looks like this:
Note that pkexec does no validation of
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
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).
Written by David Zeuthen
a lot of help from many others.
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.