KVM/QEMU hypervisor driver
- Project Links
- Deployment pre-requisites
- Connections to QEMU driver
- Driver security architecture
- Import and export of libvirt domain XML configs
- Pass-through of arbitrary qemu commands
- Example domain XML config
The libvirt KVM/QEMU driver can manage any QEMU emulator from version 0.8.1 or later. It can also manage Xenner, which provides the same QEMU command line syntax and monitor interaction.
Project Links
Deployment pre-requisites
- 
        QEMU emulators: The driver will probe /usr/binfor the presence ofqemu,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.
- 
        KVM hypervisor: The driver will probe /usr/binfor the presence ofqemu-kvmand/dev/kvmdevice node. If both are found, then KVM fullyvirtualized, hardware accelerated guests will be available.
- 
        Xenner hypervisor: The driver will probe /usr/binfor the presence ofxennerand/dev/kvmdevice node. If both are found, then Xen paravirtualized guests can be run using the KVM hardware acceleration.
Connections to QEMU driver
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)
Driver security architecture
There are multiple layers to security in the QEMU driver, allowing for flexibility in the use of QEMU based virtual machines.
Driver instances
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.
POSIX users/groups
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/usband/sys/bus/pci/devicesrespectively. 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. 
Linux process capabilities
The libvirt QEMU driver has a build time option allowing it to use the libcap-ng 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.
If the QEMU driver is configured to run virtual machines as non-root, then they will already lose 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 only be able to access files owned by root, and not files owned by any other user.
      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 necessary to change ownership on the
      directories /var/run/libvirt/qemu/,
      /var/lib/libvirt/qemu/ and
      /var/cache/libvirt/qemu/ back to root, in addition
      to changing the /etc/libvirt/qemu.conf settings.
    
SELinux basic confinement
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.
    
SELinux sVirt confinement
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"
    
AppArmor sVirt confinement
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".
    
Cgroups device ACLs
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.
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/kqemu, /dev/rtc, /dev/hpet, /dev/net/tun
      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/
    
Import and export of libvirt domain XML configs
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
Converting from QEMU args to domain XML
      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.
    
$ 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>
        NB, don't include the literal \ in the args, put everything on one line
Converting from domain XML to QEMU args
      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.
    
$ 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
        Pass-through of arbitrary qemu commands
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).
    
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:arg
- Add an additional command-line argument to the qemu
          process when starting the domain, given by the value of the
          attribute value.
- qemu:env
- Add an additional environment variable to the qemu
          process when starting the domain, given with the name-value
          pair recorded in the attributes nameand optionalvalue.
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>
        Example domain XML config
QEMU emulated guest on x86_64
<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>
        KVM hardware accelerated guest on i686
<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>
        