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[libvirt] FYI: Updated QEMU driver docs on security model



FYI, I just pushed the following patch to the repo which adds documentation
to the website for all the security model related aspects of libvirt's
QEMU driver. It should appear here shortly

  http://libvirt.org/drvqemu.html

Regards,
Daniel

diff --git a/docs/drvqemu.html.in b/docs/drvqemu.html.in
index e4c3d03..348eaaf 100644
--- a/docs/drvqemu.html.in
+++ b/docs/drvqemu.html.in
@@ -54,6 +54,292 @@
     qemu+ssh://root example com/system   (remote access, SSH tunnelled)
     </pre>
 
+    <h2><a name="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">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">POSIX DAC users/groups</a></h3>
+
+    <p>
+      In the "session" instance, the POSIX DAC 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 minise the likelihood of unintentionale 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 PCI and USB devices to a QEMU guest,
+          QEMU will need to access files in <code>/dev/bus/usb</code>
+          and <code>/sys/bus/devices</code>. 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 achived 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">Linux DAC 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">SELinux MAC 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">SELinux MAC 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 &lt;shared&gt; 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 &lt;readonly&gt; 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 &amp; 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 capabilties
+      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">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">Import and export of libvirt domain XML configs</a></h2>
 
     <p>The QEMU driver currently supports a single native


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