pkexec — Execute a command as another user
pkexec allows an authorized user to execute
PROGRAM as another user. If
PROGRAM is not specified, the default
shell will be run. If
not specified, then the program will be executed as the
administrative super user, root.
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. If the authorization could not
be obtained because the user dismissed the authentication
dialog, pkexec exits with a return value of
pkexec, like any other polkit application,
will use the authentication agent registered for the calling
process or session. However, if no authentication agent is
available, then pkexec will register its own
textual authentication agent. This behavior can be turned off by
Executing a program as another user is a privileged operation. By default the action to check for (see the section called “ACTION AND 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 by default allow you to run
X11 applications as another user since
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.
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) 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).
By default, the org.freedesktop.policykit.exec action is used. To use another action, use the org.freedesktop.policykit.exec.path annotation on an action with the value set to the full path of the program. In addition to specifying the program, the authentication message, description, icon and defaults can be specified. If the org.freedesktop.policykit.exec.argv1 annotation is present, the action will only be picked if the first argument to the program matches the value of the annotation.
Note that authentication messages may reference variables (see
the section called “VARIABLES”), for example
$(user) will be expanded to the value of the
To avoid modifying existing software to prefix their command-line invocations with pkexec, it's possible to use pkexec in a she-bang wrapper like this:
#!/usr/bin/pkexec /usr/bin/python import os import sys print "Hello, I'm running as uid %d"%(os.getuid()) for n in range(len(sys.argv)): print "arg[%d]=`%s'"%(n, sys.argv[n])
If this script is installed into
then the following annotations
[...] <annotate key="org.freedesktop.policykit.exec.path">/usr/bin/python</annotate> <annotate key="org.freedesktop.policykit.exec.argv1">/usr/bin/my-pk-test</annotate> [...]
can be used to select the appropriate polkit action. Be careful
to get the latter annotation right, otherwise it will match any
pkexec invocation of
The following variables are set by pkexec. They can be used in authorization rules and messages shown in authentication dialogs:
Fully qualified path to the program to be executed. Example: “/bin/cat”
The requested command-line (do not use this for any security checks, it is not secure). Example: “cat /srv/xyz/foobar”
The user name of the user to execute the program as. Example: “davidz”
The full name of the user to execute the program as. Example: “David Zeuthen”
A representation of the user to execute the program as that is suitable for display in an authentication dialog. Is typically set to a combination of the user name and the full name. Example: “David Zeuthen (davidz)”
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.