Usermode Migration
Summary
All granting of privileged operations to ordinary users should be handled by centrally-managed system policy. Usermode/userhelper should be fully replaced by polkit in the future.
Owner
- Name: Harald Hoyer
- Email: harald@redhat.com
- Name: Kay Sievers
- Email: kay@redhat.com
Current status
- Targeted release: Fedora 18
- Last updated: 2012-04-03
- Percentage of completion: 20%
Detailed Description
The usermode/userhelper program is a setuid-root wrapper around a couple of tools, to provide superuser privileges to ordinary users. It’s policy is controlled by text files in /etc.
Most system policy today is controlled by polkit, a well-established, fine-grained, possible network-transparent infrastructure to manage privileged operations by ordinary users. Polkit can be used by privileged process to decide if it should execute privileged operations on behalf of the requesting user. Polkit also provides a setuid-root helper program called pkexec, its callbacks to ask for authorizations are well-integrated into shell and graphical environments.
Benefit to Fedora
- centrally managed policy, network-transparency possible
- polkit auth can distinguish between multiple sessions: e.g. untrusted user reboot request reboot only allowed when only a single user session runs
- intercepting tools in sbin/ with tools in bin/ is considered bad practice; fewer dependencies on $PATH ordering
Scope
- document how to convert consolehelper to polkit:
- python: put pkexec in the wrapper shell
- 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 (ugly!)
- open tracker bug and file bugs against all individual packages
- convert all packages, where it makes sense to use polkit, to pkexec
- for the rest, drop usermode and recommend to use pkexec like sudo
How to convert
A fast and easy way to convert a former consolehelper program is the use of pkexec.
As an example, we convert system-config-date to PolicyKit:
# 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>
# cat /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_self_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
Make sure, you can call all the tools, which used to use usermode and be asked the appropriate password.
User Experience
anaconda audit-viewer authconfig-gtk backintime beesu bootconf-gui chkrootkit driftnet drobo-utils-gui eclipse-oprofile ejabberd fwfstab galternatives gnome-system-log gsmartcontrol hddtemp kdenetwork-kppp kismet liveusb-creator lshw-gui mock mtr-gtk netgo nmap-frontend ntfs-config policycoreutils-gui preupgrade pure-ftpd qtparted 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-nfs system-config-rootpassword system-config-users system-switch-displaymanager system-switch-java system-switch-mail system-switch-mail-gnome tuned vpnc-consoleuser wifi-radar wlassistant xawtv yumex zyx-liveinstaller
Contingency Plan
Even, if we cannot drop usermode, the changes in the packages do not have to be reverted.