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HTML
629 lines
29 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<span class="active">QEMU / KVM</span>
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<a title="Driver for the Linux native container API" class="inactive" href="drvlxc.html">Linux Container</a>
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<a title="Driver for the OpenVZ container technology" class="inactive" href="drvopenvz.html">OpenVZ</a>
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<a title="Driver for the User Mode Linux technology" class="inactive" href="drvuml.html">UML</a>
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<a title="Driver for the storage management APIs" class="inactive" href="storage.html">Storage</a>
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<a title="Driver for VirtualBox" class="inactive" href="drvvbox.html">VirtualBox</a>
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<a title="Driver for OpenNebula" class="inactive" href="drvone.html">OpenNebula</a>
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<a title="Overview of all content on the website" class="inactive" href="sitemap.html">Sitemap</a>
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</li></ul>
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</div>
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<div id="content">
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<h1>QEMU/KVM hypervisor driver</h1>
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<ul><li>
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<a href="#prereq">Deployment pre-requisites</a>
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</li><li>
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<a href="#uris">Connections to QEMU driver</a>
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</li><li>
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<a href="#security">Driver security architecture</a>
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<ul><li>
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<a href="#securitydriver">Driver instances</a>
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</li><li>
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<a href="#securitydac">POSIX users/groups</a>
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</li><li>
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<a href="#securitycap">Linux process capabilities</a>
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</li><li>
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<a href="#securityselinux">SELinux basic confinement</a>
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</li><li>
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<a href="#securitysvirt">SELinux sVirt confinement</a>
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</li><li>
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<a href="#securityacl">Cgroups device ACLs</a>
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</li></ul>
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</li><li>
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<a href="#imex">Import and export of libvirt domain XML configs</a>
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<ul><li>
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<a href="#xmlimport">Converting from QEMU args to domain XML</a>
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</li><li>
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<a href="#xmlexport">Converting from domain XML to QEMU args</a>
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</li></ul>
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</li><li>
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<a href="#xmlconfig">Example domain XML config</a>
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</li></ul>
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<p>
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The libvirt QEMU driver can manage any QEMU emulator from version 0.8.1
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or later. It can also manage anything that provides the same QEMU command
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line syntax and monitor interaction. This includes KVM, and Xenner.
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</p>
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<h2>
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<a name="prereq" id="prereq">Deployment pre-requisites</a>
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</h2>
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<ul><li>
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<strong>QEMU emulators</strong>: The driver will probe <code>/usr/bin</code>
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for the presence of <code>qemu</code>, <code>qemu-system-x86_64</code>,
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<code>qemu-system-mips</code>,<code>qemu-system-mipsel</code>,
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<code>qemu-system-sparc</code>,<code>qemu-system-ppc</code>. The results
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of this can be seen from the capabilities XML output.
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</li><li>
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<strong>KVM hypervisor</strong>: The driver will probe <code>/usr/bin</code>
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for the presence of <code>qemu-kvm</code> and <code>/dev/kvm</code> device
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node. If both are found, then KVM fullyvirtualized, hardware accelerated
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guests will be available.
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</li><li>
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<strong>Xenner hypervisor</strong>: The driver will probe <code>/usr/bin</code>
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for the presence of <code>xenner</code> and <code>/dev/kvm</code> device
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node. If both are found, then Xen paravirtualized guests can be run using
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the KVM hardware acceleration.
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</li></ul>
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<h2>
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<a name="uris" id="uris">Connections to QEMU driver</a>
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</h2>
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<p>
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The libvirt QEMU driver is a multi-instance driver, providing a single
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system wide privileged driver (the "system" instance), and per-user
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unprivileged drivers (the "session" instance). The of the driver protocol
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is "qemu". Some example conection URIs for the libvirt driver are:
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</p>
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<pre>
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qemu:///session (local access to per-user instance)
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qemu+unix:///session (local access to per-user instance)
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qemu:///system (local access to system instance)
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qemu+unix:///system (local access to system instance)
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qemu://example.com/system (remote access, TLS/x509)
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qemu+tcp://example.com/system (remote access, SASl/Kerberos)
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qemu+ssh://root@example.com/system (remote access, SSH tunnelled)
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</pre>
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<h2>
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<a name="security" id="security">Driver security architecture</a>
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</h2>
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<p>
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There are multiple layers to security in the QEMU driver, allowing for
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flexibility in the use of QEMU based virtual machines.
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</p>
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<h3>
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<a name="securitydriver" id="securitydriver">Driver instances</a>
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</h3>
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<p>
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As explained above there are two ways to access the QEMU driver
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in libvirt. The "qemu:///session" family of URIs connect to a
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libvirtd instance running as the same user/group ID as the client
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application. Thus the QEMU instances spawned from this driver will
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share the same privileges as the client application. The intended
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use case for this driver is desktop virtualization, with virtual
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machines storing their disk imags in the user's home directory and
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being managed from the local desktop login session.
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</p>
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<p>
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The "qemu:///system" family of URIs connect to a
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libvirtd instance running as the privileged system account 'root'.
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Thus the QEMU instances spawned from this driver may have much
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higher privileges than the client application managing them.
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The intended use case for this driver is server virtualization,
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where the virtual machines may need to be connected to host
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resources (block, PCI, USB, network devices) whose access requires
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elevated privileges.
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</p>
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<h3>
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<a name="securitydac" id="securitydac">POSIX users/groups</a>
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</h3>
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<p>
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In the "session" instance, the POSIX users/groups model restricts QEMU
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virtual machines (and libvirtd in general) to only have access to resources
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with the same user/group ID as the client application. There is no
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finer level of configuration possible for the "session" instances.
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</p>
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<p>
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In the "system" instance, libvirt releases from 0.7.0 onwards allow
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control over the user/group that the QEMU virtual machines are run
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as. A build of libvirt with no configuration parameters set will
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still run QEMU processes as root:root. It is possible to change
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this default by using the --with-qemu-user=$USERNAME and
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--with-qemu-group=$GROUPNAME arguments to 'configure' during
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build. It is strongly recommended that vendors build with both
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of these arguments set to 'qemu'. Regardless of this build time
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default, administrators can set a per-host default setting in
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the <code>/etc/libvirt/qemu.conf</code> configuration file via
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the <code>user=$USERNAME</code> and <code>group=$GROUPNAME</code>
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parameters. When a non-root user or group is configured, the
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libvirt QEMU driver will change uid/gid to match immediately
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before executing the QEMU binary for a virtual machine.
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</p>
|
|
<p>
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If QEMU virtual machines from the "system" instance are being
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run as non-root, there will be greater restrictions on what
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host resources the QEMU process will be able to access. The
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libvirtd daemon will attempt to manage permissions on resources
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to minimise the likelihood of unintentional security denials,
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|
but the administrator / application developer must be aware of
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some of the consequences / restrictions.
|
|
</p>
|
|
<ul><li>
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<p>
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The directories <code>/var/run/libvirt/qemu/</code>,
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<code>/var/lib/libvirt/qemu/</code> and
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<code>/var/cache/libvirt/qemu/</code> must all have their
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ownership set to match the user / group ID that QEMU
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guests will be run as. If the vendor has set a non-root
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user/group for the QEMU driver at build time, the
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permissions should be set automatically at install time.
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If a host administrator customizes user/group in
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<code>/etc/libvirt/qemu.conf</code>, they will need to
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manually set the ownership on these directories.
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</p>
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</li><li>
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<p>
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When attaching USB and PCI devices to a QEMU guest,
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QEMU will need to access files in <code>/dev/bus/usb</code>
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and <code>/sys/bus/pci/devices</code> respectively. The libvirtd daemon
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will automatically set the ownership on specific devices
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that are assigned to a guest at start time. There should
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not be any need for administrator changes in this respect.
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</p>
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</li><li>
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<p>
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Any files/devices used as guest disk images must be
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accessible to the user/group ID that QEMU guests are
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configured to run as. The libvirtd daemon will automatically
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set the ownership of the file/device path to the correct
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user/group ID. Applications / administrators must be aware
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though that the parent directory permissions may still
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deny access. The directories containing disk images
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must either have their ownership set to match the user/group
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configured for QEMU, or their UNIX file permissions must
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have the 'execute/search' bit enabled for 'others'.
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</p>
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<p>
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The simplest option is the latter one, of just enabling
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the 'execute/search' bit. For any directory to be used
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for storing disk images, this can be achieved by running
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the following command on the directory itself, and any
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parent directories
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</p>
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<pre>
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chmod o+x /path/to/directory
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</pre>
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<p>
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In particular note that if using the "system" instance
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and attempting to store disk images in a user home
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directory, the default permissions on $HOME are typically
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too restrictive to allow access.
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</p>
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</li></ul>
|
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<h3>
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<a name="securitycap" id="securitycap">Linux process capabilities</a>
|
|
</h3>
|
|
<p>
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|
The libvirt QEMU driver has a build time option allowing it to use
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the <a href="http://people.redhat.com/sgrubb/libcap-ng/index.html">libcap-ng</a>
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|
library to manage process capabilities. If this build option is
|
|
enabled, then the QEMU driver will use this to ensure that all
|
|
process capabilities are dropped before executing a QEMU virtual
|
|
machine. Process capabilities are what gives the 'root' account
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|
its high power, in particular the CAP_DAC_OVERRIDE capability
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|
is what allows a process running as 'root' to access files owned
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|
by any user.
|
|
</p>
|
|
<p>
|
|
If the QEMU driver is configured to run virtual machines as non-root,
|
|
then they will already loose all their process capabilities at time
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|
of startup. The Linux capability feature is thus aimed primarily at
|
|
the scenario where the QEMU processes are running as root. In this
|
|
case, before launching a QEMU virtual machine, libvirtd will use
|
|
libcap-ng APIs to drop all process capabilities. It is important
|
|
for administrators to note that this implies the QEMU process will
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|
<strong>only</strong> be able to access files owned by root, and
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|
not files owned by any other user.
|
|
</p>
|
|
<p>
|
|
Thus, if a vendor / distributor has configured their libvirt package
|
|
to run as 'qemu' by default, a number of changes will be required
|
|
before an administrator can change a host to run guests as root.
|
|
In particular it will be neccessary to change ownership on the
|
|
directories <code>/var/run/libvirt/qemu/</code>,
|
|
<code>/var/lib/libvirt/qemu/</code> and
|
|
<code>/var/cache/libvirt/qemu/</code> back to root, in addition
|
|
to changing the <code>/etc/libvirt/qemu.conf</code> settings.
|
|
</p>
|
|
<h3>
|
|
<a name="securityselinux" id="securityselinux">SELinux basic confinement</a>
|
|
</h3>
|
|
<p>
|
|
The basic SELinux protection for QEMU virtual machines is intended to
|
|
protect the host OS from a compromised virtual machine process. There
|
|
is no protection between guests.
|
|
</p>
|
|
<p>
|
|
In the basic model, all QEMU virtual machines run under the confined
|
|
domain <code>root:system_r:qemu_t</code>. It is required that any
|
|
disk image assigned to a QEMU virtual machine is labelled with
|
|
<code>system_u:object_r:virt_image_t</code>. In a default deployment,
|
|
package vendors/distributor will typically ensure that the directory
|
|
<code>/var/lib/libvirt/images</code> has this label, such that any
|
|
disk images created in this directory will automatically inherit the
|
|
correct labelling. If attempting to use disk images in another
|
|
location, the user/administrator must ensure the directory has be
|
|
given this requisite label. Likewise physical block devices must
|
|
be labelled <code>system_u:object_r:virt_image_t</code>.
|
|
</p>
|
|
<p>
|
|
Not all filesystems allow for labelling of individual files. In
|
|
particular NFS, VFat and NTFS have no support for labelling. In
|
|
these cases administrators must use the 'context' option when
|
|
mounting the filesystem to set the default label to
|
|
<code>system_u:object_r:virt_image_t</code>. In the case of
|
|
NFS, there is an alternative option, of enabling the <code>virt_use_nfs</code>
|
|
SELinux boolean.
|
|
</p>
|
|
<h3>
|
|
<a name="securitysvirt" id="securitysvirt">SELinux sVirt confinement</a>
|
|
</h3>
|
|
<p>
|
|
The SELinux sVirt protection for QEMU virtual machines builds to the
|
|
basic level of protection, to also allow individual guests to be
|
|
protected from each other.
|
|
</p>
|
|
<p>
|
|
In the sVirt model, each QEMU virtual machine runs under its own
|
|
confined domain, which is based on <code>system_u:system_r:svirt_t:s0</code>
|
|
with a unique category appended, eg, <code>system_u:system_r:svirt_t:s0:c34,c44</code>.
|
|
The rules are setup such that a domain can only access files which are
|
|
labelled with the matching category level, eg
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<code>system_u:object_r:svirt_image_t:s0:c34,c44</code>. This prevents one
|
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QEMU process accessing any file resources that are prevent to another QEMU
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process.
|
|
</p>
|
|
<p>
|
|
There are two ways of assigning labels to virtual machines under sVirt.
|
|
In the default setup, if sVirt is enabled, guests will get an automatically
|
|
assigned unique label each time they are booted. The libvirtd daemon will
|
|
also automatically relabel exclusive access disk images to match this
|
|
label. Disks that are marked as <shared> will get a generic
|
|
label <code>system_u:system_r:svirt_image_t:s0</code> allowing all guests
|
|
read/write access them, while disks marked as <readonly> will
|
|
get a generic label <code>system_u:system_r:svirt_content_t:s0</code>
|
|
which allows all guests read-only access.
|
|
</p>
|
|
<p>
|
|
With statically assigned labels, the application should include the
|
|
desired guest and file labels in the XML at time of creating the
|
|
guest with libvirt. In this scenario the application is responsible
|
|
for ensuring the disk images & similar resources are suitably
|
|
labelled to match, libvirtd will not attempt any relabelling.
|
|
</p>
|
|
<p>
|
|
If the sVirt security model is active, then the node capabilities
|
|
XML will include its details. If a virtual machine is currently
|
|
protected by the security model, then the guest XML will include
|
|
its assigned labels. If enabled at compile time, the sVirt security
|
|
model will always be activated if SELinux is available on the host
|
|
OS. To disable sVirt, and revert to the basic level of SELinux
|
|
protection (host protection only), the <code>/etc/libvirt/qemu.conf</code>
|
|
file can be used to change the setting to <code>security_driver="none"</code>
|
|
</p>
|
|
<h3>
|
|
<a name="securityacl" id="securityacl">Cgroups device ACLs</a>
|
|
</h3>
|
|
<p>
|
|
Recent Linux kernels have a capability known as "cgroups" which is used
|
|
for resource management. It is implemented via a number of "controllers",
|
|
each controller covering a specific task/functional area. One of the
|
|
available controllers is the "devices" controller, which is able to
|
|
setup whitelists of block/character devices that a cgroup should be
|
|
allowed to access. If the "devices" controller is mounted on a host,
|
|
then libvirt will automatically create a dedicated cgroup for each
|
|
QEMU virtual machine and setup the device whitelist so that the QEMU
|
|
process can only access shared devices, and explicitly disks images
|
|
backed by block devices.
|
|
</p>
|
|
<p>
|
|
The list of shared devices a guest is allowed access to is
|
|
</p>
|
|
<pre>
|
|
/dev/null, /dev/full, /dev/zero,
|
|
/dev/random, /dev/urandom,
|
|
/dev/ptmx, /dev/kvm, /dev/kqemu,
|
|
/dev/rtc, /dev/hpet, /dev/net/tun
|
|
</pre>
|
|
<p>
|
|
In the event of unanticipated needs arising, this can be customized
|
|
via the <code>/etc/libvirt/qemu.conf</code> file.
|
|
To mount the cgroups device controller, the following command
|
|
should be run as root, prior to starting libvirtd
|
|
</p>
|
|
<pre>
|
|
mkdir /dev/cgroup
|
|
mount -t cgroup none /dev/cgroup -o devices
|
|
</pre>
|
|
<p>
|
|
libvirt will then place each virtual machine in a cgroup at
|
|
<code>/dev/cgroup/libvirt/qemu/$VMNAME/</code>
|
|
</p>
|
|
<h2>
|
|
<a name="imex" id="imex">Import and export of libvirt domain XML configs</a>
|
|
</h2>
|
|
<p>The QEMU driver currently supports a single native
|
|
config format known as <code>qemu-argv</code>. The data for this format
|
|
is expected to be a single line first a list of environment variables,
|
|
then the QEMu binary name, finally followed by the QEMU command line
|
|
arguments</p>
|
|
<h3>
|
|
<a name="xmlimport" id="xmlimport">Converting from QEMU args to domain XML</a>
|
|
</h3>
|
|
<p>
|
|
The <code>virsh domxml-from-native</code> provides a way to convert an
|
|
existing set of QEMU args into a guest description using libvirt Domain XML
|
|
that can then be used by libvirt.
|
|
</p>
|
|
<pre>$ cat > demo.args <<EOF
|
|
LC_ALL=C PATH=/bin HOME=/home/test USER=test \
|
|
LOGNAME=test /usr/bin/qemu -S -M pc -m 214 -smp 1 \
|
|
-nographic -monitor pty -no-acpi -boot c -hda \
|
|
/dev/HostVG/QEMUGuest1 -net none -serial none \
|
|
-parallel none -usb
|
|
EOF
|
|
$ virsh domxml-from-native qemu-argv demo.args
|
|
<domain type='qemu'>
|
|
<uuid>00000000-0000-0000-0000-000000000000</uuid>
|
|
<memory>219136</memory>
|
|
<currentMemory>219136</currentMemory>
|
|
<vcpu>1</vcpu>
|
|
<os>
|
|
<type arch='i686' machine='pc'>hvm</type>
|
|
<boot dev='hd'/>
|
|
</os>
|
|
<clock offset='utc'/>
|
|
<on_poweroff>destroy</on_poweroff>
|
|
<on_reboot>restart</on_reboot>
|
|
<on_crash>destroy</on_crash>
|
|
<devices>
|
|
<emulator>/usr/bin/qemu</emulator>
|
|
<disk type='block' device='disk'>
|
|
<source dev='/dev/HostVG/QEMUGuest1'/>
|
|
<target dev='hda' bus='ide'/>
|
|
</disk>
|
|
</devices>
|
|
</domain>
|
|
</pre>
|
|
<p>NB, don't include the literral \ in the args, put everything on one line</p>
|
|
<h3>
|
|
<a name="xmlexport" id="xmlexport">Converting from domain XML to QEMU args</a>
|
|
</h3>
|
|
<p>
|
|
The <code>virsh domxml-to-native</code> provides a way to convert a
|
|
guest description using libvirt Domain XML, into a set of QEMU args
|
|
that can be run manually.
|
|
</p>
|
|
<pre>$ cat > demo.xml <<EOF
|
|
<domain type='qemu'>
|
|
<name>QEMUGuest1</name>
|
|
<uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
|
|
<memory>219200</memory>
|
|
<currentMemory>219200</currentMemory>
|
|
<vcpu>1</vcpu>
|
|
<os>
|
|
<type arch='i686' machine='pc'>hvm</type>
|
|
<boot dev='hd'/>
|
|
</os>
|
|
<clock offset='utc'/>
|
|
<on_poweroff>destroy</on_poweroff>
|
|
<on_reboot>restart</on_reboot>
|
|
<on_crash>destroy</on_crash>
|
|
<devices>
|
|
<emulator>/usr/bin/qemu</emulator>
|
|
<disk type='block' device='disk'>
|
|
<source dev='/dev/HostVG/QEMUGuest1'/>
|
|
<target dev='hda' bus='ide'/>
|
|
</disk>
|
|
</devices>
|
|
</domain>
|
|
EOF
|
|
$ virsh domxml-to-native qemu-argv demo.xml
|
|
LC_ALL=C PATH=/usr/bin:/bin HOME=/home/test \
|
|
USER=test LOGNAME=test /usr/bin/qemu -S -M pc \
|
|
-no-kqemu -m 214 -smp 1 -name QEMUGuest1 -nographic \
|
|
-monitor pty -no-acpi -boot c -drive \
|
|
file=/dev/HostVG/QEMUGuest1,if=ide,index=0 -net none \
|
|
-serial none -parallel none -usb
|
|
</pre>
|
|
<h2>
|
|
<a name="xmlconfig" id="xmlconfig">Example domain XML config</a>
|
|
</h2>
|
|
<h3>QEMU emulated guest on x86_64</h3>
|
|
<pre><domain type='qemu'>
|
|
<name>QEmu-fedora-i686</name>
|
|
<uuid>c7a5fdbd-cdaf-9455-926a-d65c16db1809</uuid>
|
|
<memory>219200</memory>
|
|
<currentMemory>219200</currentMemory>
|
|
<vcpu>2</vcpu>
|
|
<os>
|
|
<type arch='i686' machine='pc'>hvm</type>
|
|
<boot dev='cdrom'/>
|
|
</os>
|
|
<devices>
|
|
<emulator>/usr/bin/qemu-system-x86_64</emulator>
|
|
<disk type='file' device='cdrom'>
|
|
<source file='/home/user/boot.iso'/>
|
|
<target dev='hdc'/>
|
|
<readonly/>
|
|
</disk>
|
|
<disk type='file' device='disk'>
|
|
<source file='/home/user/fedora.img'/>
|
|
<target dev='hda'/>
|
|
</disk>
|
|
<interface type='network'>
|
|
<source network='default'/>
|
|
</interface>
|
|
<graphics type='vnc' port='-1'/>
|
|
</devices>
|
|
</domain></pre>
|
|
<h3>KVM hardware accelerated guest on i686</h3>
|
|
<pre><domain type='kvm'>
|
|
<name>demo2</name>
|
|
<uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
|
|
<memory>131072</memory>
|
|
<vcpu>1</vcpu>
|
|
<os>
|
|
<type arch="i686">hvm</type>
|
|
</os>
|
|
<clock sync="localtime"/>
|
|
<devices>
|
|
<emulator>/usr/bin/qemu-kvm</emulator>
|
|
<disk type='file' device='disk'>
|
|
<source file='/var/lib/libvirt/images/demo2.img'/>
|
|
<target dev='hda'/>
|
|
</disk>
|
|
<interface type='network'>
|
|
<source network='default'/>
|
|
<mac address='24:42:53:21:52:45'/>
|
|
</interface>
|
|
<graphics type='vnc' port='-1' keymap='de'/>
|
|
</devices>
|
|
</domain></pre>
|
|
<h3>Xen paravirtualized guests with hardware acceleration</h3>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<p id="sponsor">
|
|
Sponsored by:<br /><a href="http://et.redhat.com/"><img src="et.png" alt="Project sponsored by Red Hat Emerging Technology" /></a></p>
|
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</div>
|
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</body>
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