pkexec Man page

PKEXEC(1) pkexec PKEXEC(1)

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


Examples for the PolicyKit Project
http://hal.freedesktop.org/docs/PolicyKit/


Run the PolicyKit example program Frobnicate
Kør PolicyKit eksemplet Frobnicate
Authentication is required to run the PolicyKit example program Frobnicate (user=$(user), program=$(program), command_line=$(command_line))
Autorisering er påkrævet for at afvikle PolicyKit eksemplet Frobnicate (user=$(user), program=$(program), command_line=$(command_line))
audio-x-generic

no
no
auth_self_keep

/usr/bin/pk-example-frobnicate

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).

AUTHOR

Written by David Zeuthen 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
http://lists.freedesktop.org/mailman/listinfo/polkit-devel on how to
subscribe.

SEE ALSO

polkit(8), pkaction, pkcheck, pkttyagent

NOTES
1. /usr/share/gtk-doc/html/polkit-1/pkexec-bash.png

2. /usr/share/gtk-doc/html/polkit-1/pkexec-frobnicate.png

3. /usr/share/gtk-doc/html/polkit-1/pkexec-frobnicate-da.png

polkit May 2009 PKEXEC(1)