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Adjust privileges allowed to the libvirt management daemon and QEMU processes to improve security and features


Current status

  • Targeted release: Fedora 12
  • Last updated: 2009-07-22
  • Percentage of completion: 95%

Detailed Description

The libvirtd daemon and QEMU driver has two modes of operation:

  1. A single system instance per machine, that runs with root privileges, launches QEMU instances as root, can use TAP device networking for QEMU, and has full storage and network management capabilities
  2. Fully unprivileged instances, which run as the same UID as the user accessing the API, but have a significantly reduced level of functionality.

The goal of this feature are to reduce the privileges of QEMU processes run by the system instance to improve security.

Specifically all QEMU processes will run as an unprivileged user/group called 'qemu'. libvirtd will be responsible for setting up file permissions to allow them to work

Benefit to Fedora

Reducing the privileges of the libvirt system instance will improve the security of a critical piece of infrastructure. Increasing the functionality of the session instance, will allow more widespread usage. By reducing the scenarios in which the system instance is needed, it will also improve security, since the session instance has far less privileges. Running everything as the same user account will also allow for better desktop session integration, particularly for the sound daemon, and facilitate usage of user home directories for disk image storage.


  • cap-ng: get this new library to be added to Fedora [DONE]
  • libvirt QEMU: ability to spawn QEMU instances as a non-root user / group ID (requires chown'ing of resources) [DONE]
  • libvirt QEMU: ability to drop capabilities before exec'ing QEMU instances. [DONE]
  • qemu: add a kvm or qemu user and group ID, and use to set /dev/kvm group ownership [DONE]
  • qemu: make /dev/kvm mod 666 by default to allow any user access to hardware acceleration [DONE]

How To Test

XXX this is too simplistic. When capabilities are in use, user ID is not good enough check of actual privileges,cf /proc/$PID/status CapXXX fields

  • Verify that when using 'qemu:///system', no QEMU processes run as root
  • Verify that the 'libvirtd' daemon started from init is not running as root
  • Using qemu:///session provision a new guest, and verify that it is able to use hardware acceleration
  • Verify that when running virt-manager for first time as a new user, it defaults to qemu:///session

User Experience

All virtual machines run by virt-manager on a local desktop install will be running under their user account. All virt-manager machiens run on a server install will running as an reduced privilege system account.


Scope extends to at last

  • libvirt
  • qemu
  • virt-manager
  • python-virtinst

Contingency Plan

This functionality is incrementally building on existing functionality. No existing functionality will be lost, so if problems are encountered, new features can be dropped or postponed to later Fedora releases.


Documentation will magically come into existance as the features are developed in the upstream apps

Release Notes

In the Fedora 12 release a couple of changes are introduced for QEMU/KVM virtual machines to improve host security in the event of a flaw in the QEMU binary.

- /dev/kvm is now world readable & writable. This allows all users on a machine to use hardware acceleration out of the box. This means more people can use the fully unprivileged 'qemu:///session' instance of libvirt, instead of qemu:///system.
- QEMU processes spawned by the privileged libvirtd daemon (qemu:///system), are now run under an unprivileged account, user 'qemu', group 'qemu'
- libvirtd will change ownership of any disks assigned to a virtual machine at startup, to user 'qemu', group 'qemu', except for readonly/shared disks.

Comments and Discussion