The libvirt KVM/QEMU driver can manage any QEMU emulator from version 1.5.0 or later.
/usr/bin
for the presence of qemu, qemu-system-x86_64,
qemu-system-microblaze,
qemu-system-microblazeel,
qemu-system-mips,qemu-system-mipsel,
qemu-system-sparc,qemu-system-ppc. The results
of this can be seen from the capabilities XML output.
/usr/bin
for the presence of qemu-kvm and /dev/kvm device
node. If both are found, then KVM fully virtualized, hardware accelerated
guests will be available.
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 URI driver protocol is "qemu". Some example connection URIs for the libvirt driver are:
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)
Since 6.1.0 the QEMU driver has experimental support for operating in an embedded mode. In this scenario, rather than connecting to the libvirtd daemon, the QEMU driver runs in the client application process directly. To use this the client application must have registered & be running an instance of the event loop. To open the driver in embedded mode the app use the new URI path and specify a virtual root directory under which the driver will create content.
qemu:///embed?root=/some/dir
Broadly speaking the range of functionality is intended to be on a par with that seen when using the traditional system or session libvirt connections to QEMU. The features will of course differ depending on whether the application using the embedded driver is running privileged or unprivileged. For example PCI device assignment or TAP based networking are only available when running privileged. While the embedded mode is still classed as experimental some features may change their default settings between releases.
By default if the application uses any APIs associated with secondary drivers, these will result in a connection being opened to the corresponding driver in libvirtd. For example, this allows a virtual machine from the embedded QEMU to connect its NIC to a virtual network or connect its disk to a storage volume. Some of the secondary drivers will also be able to support running in embedded mode. Currently this is supported by the secrets driver, to allow for use of VMs with encrypted disks
Under the specified root directory the following locations will be used
/some/dir
|
+- log
| |
| +- qemu
| +- swtpm
|
+- etc
| |
| +- qemu
| +- pki
| |
| +- qemu
|
+- run
| |
| +- qemu
| +- swtpm
|
+- cache
| |
| +- qemu
|
+- lib
|
+- qemu
+- swtpm
Note that UNIX domain sockets used for QEMU virtual machines had a maximum filename length of 108 characters. Bear this in mind when picking a root directory to avoid risk of exhausting the filename space. The application is responsible for recursively purging the contents of this directory tree once they no longer require a connection, though it can also be left intact for reuse when opening a future connection.
To use the QEMU driver in embedded mode the application must register an event loop with libvirt. Many of the QEMU driver API calls will rely on the event loop processing data. With this in mind, applications must NEVER invoke API calls from the event loop thread itself, only other threads. Not following this rule will lead to deadlocks in the API. This restriction is intended to be lifted in a future release of libvirt, once QMP processing moves to a dedicated thread.
There are multiple layers to security in the QEMU driver, allowing for flexibility in the use of QEMU based virtual machines.
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 images in the user's home directory and being managed from the local desktop login session.
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.
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.
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 /etc/libvirt/qemu.conf configuration file via
the user=$USERNAME and group=$GROUPNAME
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.
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.
The directories /var/run/libvirt/qemu/,
/var/lib/libvirt/qemu/ and
/var/cache/libvirt/qemu/ 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
/etc/libvirt/qemu.conf, they will need to
manually set the ownership on these directories.
When attaching USB and PCI devices to a QEMU guest,
QEMU will need to access files in /dev/bus/usb
and /sys/bus/pci/devices 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.
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'.
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
chmod o+x /path/to/directory
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.
The libvirt maintainers strongly recommend against running QEMU as the root user/group. This should not be required in most supported usage scenarios, as libvirt will generally do the right thing to grant QEMU access to files it is permitted to use when it is running non-root.
In versions of libvirt prior to 6.0.0, even if QEMU was configured to run as the root user / group, libvirt would strip all process capabilities. This meant that QEMU could only read/write files owned by root, or with open permissions. In reality, stripping capabilities did not have any security benefit, as it was trivial to get commands to run in another context with full capabilities, for example, by creating a cronjob.
Thus since 6.0.0, if QEMU is running as root, it will keep all process capabilities. Behaviour when QEMU is running non-root is unchanged, it still has no capabilities.
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.
In the basic model, all QEMU virtual machines run under the confined
domain root:system_r:qemu_t. It is required that any
disk image assigned to a QEMU virtual machine is labelled with
system_u:object_r:virt_image_t. In a default deployment,
package vendors/distributor will typically ensure that the directory
/var/lib/libvirt/images 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 system_u:object_r:virt_image_t.
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
system_u:object_r:virt_image_t. In the case of
NFS, there is an alternative option, of enabling the virt_use_nfs
SELinux boolean.
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.
In the sVirt model, each QEMU virtual machine runs under its own
confined domain, which is based on system_u:system_r:svirt_t:s0
with a unique category appended, eg, system_u:system_r:svirt_t:s0:c34,c44.
The rules are setup such that a domain can only access files which are
labelled with the matching category level, eg
system_u:object_r:svirt_image_t:s0:c34,c44. This prevents one
QEMU process accessing any file resources that are prevent to another QEMU
process.
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 system_u:system_r:svirt_image_t:s0 allowing all guests
read/write access them, while disks marked as <readonly> will
get a generic label system_u:system_r:svirt_content_t:s0
which allows all guests read-only access.
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.
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 /etc/libvirt/qemu.conf
file can be used to change the setting to security_driver="none"
When using basic AppArmor protection for the libvirtd daemon and QEMU virtual machines, the intention is to protect the host OS from a compromised virtual machine process. There is no protection between guests.
The AppArmor sVirt protection for QEMU virtual machines builds on this basic level of protection, to also allow individual guests to be protected from each other.
In the sVirt model, if a profile is loaded for the libvirtd daemon,
then each qemu:///system QEMU virtual machine will have
a profile created for it when the virtual machine is started if one
does not already exist. This generated profile uses a profile name
based on the UUID of the QEMU virtual machine and contains rules
allowing access to only the files it needs to run, such as its disks,
pid file and log files. Just before the QEMU virtual machine is
started, the libvirtd daemon will change into this unique profile,
preventing the QEMU process from accessing any file resources that
are present in another QEMU process or the host machine.
The AppArmor sVirt implementation is flexible in that it allows an
administrator to customize the template file in
/etc/apparmor.d/libvirt/TEMPLATE for site-specific
access for all newly created QEMU virtual machines. Also, when a new
profile is generated, two files are created:
/etc/apparmor.d/libvirt/libvirt-<uuid> and
/etc/apparmor.d/libvirt/libvirt-<uuid>.files. The
former can be fine-tuned by the administrator to allow custom access
for this particular QEMU virtual machine, and the latter will be
updated appropriately when required file access changes, such as when
a disk is added. This flexibility allows for situations such as
having one virtual machine in complain mode with all others in
enforce mode.
While users can define their own AppArmor profile scheme, a typical
configuration will include a profile for /usr/sbin/libvirtd,
/usr/lib/libvirt/virt-aa-helper (a helper program which the
libvirtd daemon uses instead of manipulating AppArmor directly), and
an abstraction to be included by /etc/apparmor.d/libvirt/TEMPLATE
(typically /etc/apparmor.d/abstractions/libvirt-qemu).
An example profile scheme can be found in the examples/apparmor
directory of the source distribution.
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 profile name. If enabled at compile time, the sVirt
security model will be activated if AppArmor is available on the host
OS and a profile for the libvirtd daemon is loaded when libvirtd is
started. To disable sVirt, and revert to the basic level of AppArmor
protection (host protection only), the /etc/libvirt/qemu.conf
file can be used to change the setting to security_driver="none".
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.
The list of shared devices a guest is allowed access to is
/dev/null, /dev/full, /dev/zero, /dev/random, /dev/urandom, /dev/ptmx, /dev/kvm, /dev/rtc, /dev/hpet
In the event of unanticipated needs arising, this can be customized
via the /etc/libvirt/qemu.conf file.
To mount the cgroups device controller, the following command
should be run as root, prior to starting libvirtd
mkdir /dev/cgroup mount -t cgroup none /dev/cgroup -o devices
libvirt will then place each virtual machine in a cgroup at
/dev/cgroup/libvirt/qemu/$VMNAME/
The QEMU driver currently supports a single native
config format known as qemu-argv. 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
Note: this operation is deleted as of 5.5.0 and will return an error.
The virsh domxml-from-native 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.
Please note that this command is intended to be used to convert
existing qemu guests previously started from the command line to
be managed through libvirt. It should not be used a method of
creating new guests from scratch. New guests should be created
using an application calling the libvirt APIs (see
the libvirt applications page for some
examples) or by manually crafting XML to pass to virsh.
The virsh domxml-to-native provides a way to convert a
guest description using libvirt Domain XML, into a set of QEMU args
that can be run manually. Note that currently the command line formatted
by libvirt is no longer suited for manually running qemu as the
configuration expects various resources and open file descriptors passed
to the process which are usually prepared by libvirtd.
Libvirt provides an XML namespace and an optional
library libvirt-qemu.so for dealing specifically
with qemu. When used correctly, these extensions allow testing
specific qemu features that have not yet been ported to the
generic libvirt XML and API interfaces. However, they
are unsupported, in that the library is not guaranteed to
have a stable API, abusing the library or XML may result in
inconsistent state the crashes libvirtd, and upgrading either
qemu-kvm or libvirtd may break behavior of a domain that was
relying on a qemu-specific pass-through. If you find yourself
needing to use them to access a particular qemu feature, then
please post an RFE to the libvirt mailing list to get that
feature incorporated into the stable libvirt XML and API
interfaces.
The library provides two
API: virDomainQemuMonitorCommand, for sending an
arbitrary monitor command (in either HMP or QMP format) to a
qemu guest (Since 0.8.3),
and virDomainQemuAttach, for registering a qemu
domain that was manually started so that it can then be managed
by libvirtd (Since 0.9.4,
removed as of 5.5.0).
Additionally, the following XML additions allow fine-tuning of
the command line given to qemu when starting a domain
(Since 0.8.3). In order to use the
XML additions, it is necessary to issue an XML namespace request
(the special xmlns:name attribute) that
pulls in http://libvirt.org/schemas/domain/qemu/1.0;
typically, the namespace is given the name
of qemu. With the namespace in place, it is then
possible to add an element <qemu:commandline>
under driver, with the following sub-elements
repeated as often as needed:
qemu:argvalue.
qemu:envname
and optional value.Example:
<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
<name>QEMU-fedora-i686</name>
<memory>219200</memory>
<os>
<type arch='i686' machine='pc'>hvm</type>
</os>
<devices>
<emulator>/usr/bin/qemu-system-x86_64</emulator>
</devices>
<qemu:commandline>
<qemu:arg value='-newarg'/>
<qemu:env name='QEMU_ENV' value='VAL'/>
</qemu:commandline>
</domain>
In some cases e.g. when developing a new feature or for testing it may
be required to control a given qemu feature (or qemu capability) to test
it before it's complete or disable it for debugging purposes.
Since 5.5.0 it's possible to use the same
special qemu namespace as above
(http://libvirt.org/schemas/domain/qemu/1.0) and use
<qemu:capabilities> element to add
(<qemu:add capability="capname"/>) or remove
(<qemu:del capability="capname"/>) capability bits.
The naming of the feature bits is the same libvirt uses in the status
XML. Note that this feature is meant for experiments only and should
_not_ be used in production.
Example:
<domain type='qemu' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
<name>testvm</name>
[...]
<qemu:capabilities>
<qemu:add capability='blockdev'/>
<qemu:del capability='drive'/>
</qemu:capabilities>
</domain>
<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>
<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>