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<h1>QEMU/KVM hypervisor driver</h1>
<ul><li>
<a href="#prereq">Deployment pre-requisites</a>
</li><li>
<a href="#uris">Connections to QEMU driver</a>
</li><li>
<a href="#security">Driver security architecture</a>
<ul><li>
<a href="#securitydriver">Driver instances</a>
</li><li>
<a href="#securitydac">POSIX users/groups</a>
</li><li>
<a href="#securitycap">Linux process capabilities</a>
</li><li>
<a href="#securityselinux">SELinux basic confinement</a>
</li><li>
<a href="#securitysvirt">SELinux sVirt confinement</a>
</li><li>
<a href="#securityacl">Cgroups device ACLs</a>
</li></ul>
</li><li>
<a href="#imex">Import and export of libvirt domain XML configs</a>
<ul><li>
<a href="#xmlimport">Converting from QEMU args to domain XML</a>
</li><li>
<a href="#xmlexport">Converting from domain XML to QEMU args</a>
</li></ul>
</li><li>
<a href="#xmlconfig">Example domain XML config</a>
</li></ul>
<p>
The libvirt QEMU driver can manage any QEMU emulator from version 0.8.1
or later. It can also manage anything that provides the same QEMU command
line syntax and monitor interaction. This includes KVM, and Xenner.
</p>
<h2>
<a name="prereq" id="prereq">Deployment pre-requisites</a>
</h2>
<ul><li>
<strong>QEMU emulators</strong>: The driver will probe <code>/usr/bin</code>
for the presence of <code>qemu</code>, <code>qemu-system-x86_64</code>,
<code>qemu-system-mips</code>,<code>qemu-system-mipsel</code>,
<code>qemu-system-sparc</code>,<code>qemu-system-ppc</code>. The results
of this can be seen from the capabilities XML output.
</li><li>
<strong>KVM hypervisor</strong>: The driver will probe <code>/usr/bin</code>
for the presence of <code>qemu-kvm</code> and <code>/dev/kvm</code> device
node. If both are found, then KVM fullyvirtualized, hardware accelerated
guests will be available.
</li><li>
<strong>Xenner hypervisor</strong>: The driver will probe <code>/usr/bin</code>
for the presence of <code>xenner</code> and <code>/dev/kvm</code> device
node. If both are found, then Xen paravirtualized guests can be run using
the KVM hardware acceleration.
</li></ul>
<h2>
<a name="uris" id="uris">Connections to QEMU driver</a>
</h2>
<p>
The libvirt QEMU driver is a multi-instance driver, providing a single
system wide privileged driver (the "system" instance), and per-user
unprivileged drivers (the "session" instance). The of the driver protocol
is "qemu". Some example conection URIs for the libvirt driver are:
</p>
<pre>
qemu:///session (local access to per-user instance)
qemu+unix:///session (local access to per-user instance)
qemu:///system (local access to system instance)
qemu+unix:///system (local access to system instance)
qemu://example.com/system (remote access, TLS/x509)
qemu+tcp://example.com/system (remote access, SASl/Kerberos)
qemu+ssh://root@example.com/system (remote access, SSH tunnelled)
</pre>
<h2>
<a name="security" id="security">Driver security architecture</a>
</h2>
<p>
There are multiple layers to security in the QEMU driver, allowing for
flexibility in the use of QEMU based virtual machines.
</p>
<h3>
<a name="securitydriver" id="securitydriver">Driver instances</a>
</h3>
<p>
As explained above there are two ways to access the QEMU driver
in libvirt. The "qemu:///session" family of URIs connect to a
libvirtd instance running as the same user/group ID as the client
application. Thus the QEMU instances spawned from this driver will
share the same privileges as the client application. The intended
use case for this driver is desktop virtualization, with virtual
machines storing their disk imags in the user's home directory and
being managed from the local desktop login session.
</p>
<p>
The "qemu:///system" family of URIs connect to a
libvirtd instance running as the privileged system account 'root'.
Thus the QEMU instances spawned from this driver may have much
higher privileges than the client application managing them.
The intended use case for this driver is server virtualization,
where the virtual machines may need to be connected to host
resources (block, PCI, USB, network devices) whose access requires
elevated privileges.
</p>
<h3>
<a name="securitydac" id="securitydac">POSIX users/groups</a>
</h3>
<p>
In the "session" instance, the POSIX users/groups model restricts QEMU
virtual machines (and libvirtd in general) to only have access to resources
with the same user/group ID as the client application. There is no
finer level of configuration possible for the "session" instances.
</p>
<p>
In the "system" instance, libvirt releases from 0.7.0 onwards allow
control over the user/group that the QEMU virtual machines are run
as. A build of libvirt with no configuration parameters set will
still run QEMU processes as root:root. It is possible to change
this default by using the --with-qemu-user=$USERNAME and
--with-qemu-group=$GROUPNAME arguments to 'configure' during
build. It is strongly recommended that vendors build with both
of these arguments set to 'qemu'. Regardless of this build time
default, administrators can set a per-host default setting in
the <code>/etc/libvirt/qemu.conf</code> configuration file via
the <code>user=$USERNAME</code> and <code>group=$GROUPNAME</code>
parameters. When a non-root user or group is configured, the
libvirt QEMU driver will change uid/gid to match immediately
before executing the QEMU binary for a virtual machine.
</p>
<p>
If QEMU virtual machines from the "system" instance are being
run as non-root, there will be greater restrictions on what
host resources the QEMU process will be able to access. The
libvirtd daemon will attempt to manage permissions on resources
to minimise the likelihood of unintentional security denials,
but the administrator / application developer must be aware of
some of the consequences / restrictions.
</p>
<ul><li>
<p>
The directories <code>/var/run/libvirt/qemu/</code>,
<code>/var/lib/libvirt/qemu/</code> and
<code>/var/cache/libvirt/qemu/</code> must all have their
ownership set to match the user / group ID that QEMU
guests will be run as. If the vendor has set a non-root
user/group for the QEMU driver at build time, the
permissions should be set automatically at install time.
If a host administrator customizes user/group in
<code>/etc/libvirt/qemu.conf</code>, they will need to
manually set the ownership on these directories.
</p>
</li><li>
<p>
When attaching USB and PCI devices to a QEMU guest,
QEMU will need to access files in <code>/dev/bus/usb</code>
and <code>/sys/bus/pci/devices</code> respectively. The libvirtd daemon
will automatically set the ownership on specific devices
that are assigned to a guest at start time. There should
not be any need for administrator changes in this respect.
</p>
</li><li>
<p>
Any files/devices used as guest disk images must be
accessible to the user/group ID that QEMU guests are
configured to run as. The libvirtd daemon will automatically
set the ownership of the file/device path to the correct
user/group ID. Applications / administrators must be aware
though that the parent directory permissions may still
deny access. The directories containing disk images
must either have their ownership set to match the user/group
configured for QEMU, or their UNIX file permissions must
have the 'execute/search' bit enabled for 'others'.
</p>
<p>
The simplest option is the latter one, of just enabling
the 'execute/search' bit. For any directory to be used
for storing disk images, this can be achieved by running
the following command on the directory itself, and any
parent directories
</p>
<pre>
chmod o+x /path/to/directory
</pre>
<p>
In particular note that if using the "system" instance
and attempting to store disk images in a user home
directory, the default permissions on $HOME are typically
too restrictive to allow access.
</p>
</li></ul>
<h3>
<a name="securitycap" id="securitycap">Linux process capabilities</a>
</h3>
<p>
The libvirt QEMU driver has a build time option allowing it to use
the <a href="http://people.redhat.com/sgrubb/libcap-ng/index.html">libcap-ng</a>
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
its high power, in particular the CAP_DAC_OVERRIDE capability
is what allows a process running as 'root' to access files owned
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
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
<strong>only</strong> be able to access files owned by root, and
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
<code>system_u:object_r:svirt_image_t:s0:c34,c44</code>. This prevents one
QEMU process accessing any file resources that are prevent to another QEMU
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>
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