Revision as of 11:09, 8 November 2012

Usermode Migration

Summary

Access control of privileged operations for ordinary users should be handled exclusively by a centrally managed authority.

Usermode/consolehelper should be phased out and be replaced entirely by polkit.

Owner

Name: Harald Hoyer
Email: harald@redhat.com

Name: Kay Sievers
Email: kay@redhat.com

Name: Bill Nottingham
Email: notting@redhat.com

Current status

Targeted release: Fedora 19
Last updated: 2012-10-16
Percentage of completion: 20%

Detailed Description

The usermode/consolehelper program is a setuid-root wrapper around a couple of system tools, providing superuser privileges to ordinary users. Its policy is controlled by text files in /etc.

These days, most privileged system operations are already controlled by polkit, a well-established, fine-grained, (possibly) network-transparent service for managing privileged operations by ordinary users. Enterprise environments need to be able to centrally define access control policy for the organization, and automatically apply it to all connected workstations.

polkit can be used by privileged processes to decide if it should execute privileged operations on behalf of the requesting user. For directly executed tools, polkit provides a setuid-root helper program called ‘’pkexec’’.The hooks to ask the user for authorizations are well-integrated into text environments, and native in all major graphical environments.
The concept of a console user (that usermode/consolehelper implements) is no longer a sufficient concept to derive privileges from. OTOH polkit authorizations can properly distinguish between multiple active sessions and seats: e.g. an untrusted user’s reboot request is only granted if only a single user session runs at that time.

polkit(8) manual page

Benefit to Fedora

Consistency of system configuration.
Centralization of policy.
Cleaner system integration; no implicit interception of tools residing in sbin/ with symlinks in bin/, which is less dependent on $PATH ordering.
No difference regarding the hookup between tools installed in bin/ or sbin/.

Scope

Document how to convert consolehelper to polkit:
- python: put a pkexec invocation in the wrapping shell script
- C tools: re-exec with pkexec in C code
- C tools: move original to /usr/lib/<pkg>/<tool>, and wrap /usr/bin/<tool> with a pkexec shell script (ugly!)
File bugs against all individual packages, and add them to tracker bug 502765
Convert all packages where it makes sense to use polkit to pkexec.
If all packages are successfully converted, userhelper may be removed

How to Convert

A quick and easy way to convert a former consolehelper program is the use of pkexec.

As an example we convert system-config-date to polkit:

# ls -l /usr/bin/system-config-date
lrwxrwxrwx 1 root root 13  5. Feb 02:34 /usr/bin/system-config-date -> consolehelper

# rm /usr/bin/system-config-date
# cat /etc/security/console.apps/system-config-date
. config-util
PROGRAM=/usr/share/system-config-date/system-config-date.py
SESSION=true

OK, running /usr/bin/system-config-date would have executed /usr/share/system-config-date/system-config-date.py, so we create /usr/bin/system-config-date like the following:

# cat /usr/bin/system-config-date
#!/bin/sh
exec /usr/bin/pkexec /usr/share/system-config-date/system-config-date.py

This will not export the DISPLAY variable, so we have to add a policy file, although starting a GUI as root is not encouraged. The important part is: <annotate key="org.freedesktop.policykit.exec.allow_gui">true</annotate>

/usr/share/polkit-1/actions/org.fedoraproject.config.date.policy:

<?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>System Config Date</vendor>
 <vendor_url>http://fedorahosted.org/system-config-date</vendor_url>

 <action id="org.fedoraproject.config.date.pkexec.run">
    <_description>Run System Config Date</_description>
    <_message>Authentication is required to run system-config-date</_message>
    <icon_name>system-config-date</icon_name>
    <defaults>
     <allow_any>no</allow_any>
     <allow_inactive>no</allow_inactive>
     <allow_active>auth_admin_keep</allow_active>
    </defaults>
    <annotate key="org.freedesktop.policykit.exec.path">/usr/share/system-config-date/system-config-date.py</annotate>
    <annotate key="org.freedesktop.policykit.exec.allow_gui">true</annotate>
 </action>
</policyconfig>

How To Test

# yum remove usermode usermode-gtk

should succeed for an installation with all Fedora packages installed.

# repoquery --whatrequires usermode --whatrequires usermode-gtk
usermode-gtk-....

should not output a single package, except the usermode-gtk package.

Make sure you can call all the tools which used to use usermode and are asked the appropriate authentication.

User Experience

The user should experience no noticeable changes.

Dependencies

anaconda
audit-viewer
authconfig-gtk
backintime-gnome
backintime-kde
beesu
bootconf-gui
chkrootkit
driftnet
drobo-utils-gui
eclipse-oprofile
ejabberd
fwfstab
galternatives
gsmartcontrol
hddtemp
kdenetwork-kppp
kismet
liveusb-creator
livna-config-display
lshw-gui
mock
mtr-gtk
netgo
nmap-frontend
ntfs-config
policycoreutils-gui
preupgrade
pure-ftpd
qtparted
realcrypt
revisor-cli
rhn-setup
rhn-setup-gnome
sabayon
setools-gui
setuptool
smart-gui
subscription-manager-gnome
synaptic
system-config-audit
system-config-bind
system-config-boot
system-config-date
system-config-httpd
system-config-kdump
system-config-keyboard
system-config-language
system-config-lvm
system-config-network
system-config-network-tui
system-config-nfs
system-config-rootpassword
system-config-users
system-switch-displaymanager
system-switch-java
system-switch-mail
system-switch-mail-gnome
tuned
usermode-gtk
vpnc-consoleuser
wifi-radar
wlassistant
xawtv
yumex
zyx-liveinstaller

Contingency Plan

Even if we cannot drop usermode for F18 (because not all packages have been converted) the changes in the packages do not have to be reverted.

Documentation

Release Notes

The following packages now use the polkit policy configuration instead of the usermode/consolehelper configuration. Please migrate any policy, which you have created for those packages. Documentation about polkit can be found on http://www.freedesktop.org/software/polkit/docs/latest/

 <list of packages>

Comments and Discussion

See Talk:Features/UsermodeMigration

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