mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-08-29 03:51:17 +00:00
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
671 lines
25 KiB
ReStructuredText
671 lines
25 KiB
ReStructuredText
.. role:: since
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====================
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LXC container driver
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====================
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.. contents::
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The libvirt LXC driver manages "Linux Containers". At their simplest, containers
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can just be thought of as a collection of processes, separated from the main
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host processes via a set of resource namespaces and constrained via control
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groups resource tunables. The libvirt LXC driver has no dependency on the LXC
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userspace tools hosted on sourceforge.net. It directly utilizes the relevant
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kernel features to build the container environment. This allows for sharing of
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many libvirt technologies across both the QEMU/KVM and LXC drivers. In
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particular sVirt for mandatory access control, auditing of operations,
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integration with control groups and many other features.
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Control groups Requirements
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---------------------------
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In order to control the resource usage of processes inside containers, the
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libvirt LXC driver requires that certain cgroups controllers are mounted on the
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host OS. The minimum required controllers are 'cpuacct', 'memory' and 'devices',
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while recommended extra controllers are 'cpu', 'freezer' and 'blkio'. Libvirt
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will not mount the cgroups filesystem itself, leaving this up to the init system
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to take care of. Systemd will do the right thing in this respect, while for
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other init systems the ``cgconfig`` init service will be required. For further
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information, consult the general libvirt `cgroups
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documentation <cgroups.html>`__.
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Namespace requirements
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----------------------
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In order to separate processes inside a container from those in the primary
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"host" OS environment, the libvirt LXC driver requires that certain kernel
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namespaces are compiled in. Libvirt currently requires the 'mount', 'ipc',
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'pid', and 'uts' namespaces to be available. If separate network interfaces are
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desired, then the 'net' namespace is required. If the guest configuration
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declares a `UID or GID mapping <formatdomain.html#container-boot>`__, the
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'user' namespace will be enabled to apply these. **A suitably configured UID/GID
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mapping is a pre-requisite to making containers secure, in the absence of sVirt
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confinement.**
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Default container setup
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-----------------------
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Command line arguments
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~~~~~~~~~~~~~~~~~~~~~~
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When the container "init" process is started, it will typically not be given any
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command line arguments (eg the equivalent of the bootloader args visible in
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``/proc/cmdline``). If any arguments are desired, then must be explicitly set in
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the container XML configuration via one or more ``initarg`` elements. For
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example, to run ``systemd --unit emergency.service`` would use the following XML
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::
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<os>
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<type arch='x86_64'>exe</type>
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<init>/bin/systemd</init>
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<initarg>--unit</initarg>
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<initarg>emergency.service</initarg>
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</os>
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Environment variables
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~~~~~~~~~~~~~~~~~~~~~
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When the container "init" process is started, it will be given several useful
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environment variables. The following standard environment variables are mandated
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by `systemd container
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interface <https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface>`__
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to be provided by all container technologies on Linux.
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``container``
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The fixed string ``libvirt-lxc`` to identify libvirt as the creator
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``container_uuid``
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The UUID assigned to the container by libvirt
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``PATH``
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The fixed string ``/bin:/usr/bin``
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``TERM``
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The fixed string ``linux``
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``HOME``
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The fixed string ``/``
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In addition to the standard variables, the following libvirt specific
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environment variables are also provided
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``LIBVIRT_LXC_NAME``
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The name assigned to the container by libvirt
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``LIBVIRT_LXC_UUID``
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The UUID assigned to the container by libvirt
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``LIBVIRT_LXC_CMDLINE``
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The unparsed command line arguments specified in the container configuration.
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Use of this is discouraged, in favour of passing arguments directly to the
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container init process via the ``initarg`` config element.
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Filesystem mounts
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~~~~~~~~~~~~~~~~~
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In the absence of any explicit configuration, the container will inherit the
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host OS filesystem mounts. A number of mount points will be made read only, or
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re-mounted with new instances to provide container specific data. The following
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special mounts are setup by libvirt
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- ``/dev`` a new "tmpfs" pre-populated with authorized device nodes
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- ``/dev/pts`` a new private "devpts" instance for console devices
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- ``/sys`` the host "sysfs" instance remounted read-only
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- ``/proc`` a new instance of the "proc" filesystem
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- ``/proc/sys`` the host "/proc/sys" bind-mounted read-only
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- ``/sys/fs/selinux`` the host "selinux" instance remounted read-only
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- ``/sys/fs/cgroup/NNNN`` the host cgroups controllers bind-mounted to only
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expose the sub-tree associated with the container
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- ``/proc/meminfo`` a FUSE backed file reflecting memory limits of the
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container
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Device nodes
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~~~~~~~~~~~~
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The container init process will be started with ``CAP_MKNOD`` capability removed
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and blocked from re-acquiring it. As such it will not be able to create any
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device nodes in ``/dev`` or anywhere else in its filesystems. Libvirt itself
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will take care of pre-populating the ``/dev`` filesystem with any devices that
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the container is authorized to use. The current devices that will be made
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available to all containers are
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- ``/dev/zero``
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- ``/dev/null``
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- ``/dev/full``
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- ``/dev/random``
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- ``/dev/urandom``
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- ``/dev/stdin`` symlinked to ``/proc/self/fd/0``
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- ``/dev/stdout`` symlinked to ``/proc/self/fd/1``
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- ``/dev/stderr`` symlinked to ``/proc/self/fd/2``
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- ``/dev/fd`` symlinked to ``/proc/self/fd``
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- ``/dev/ptmx`` symlinked to ``/dev/pts/ptmx``
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- ``/dev/console`` symlinked to ``/dev/pts/0``
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In addition, for every console defined in the guest configuration, a symlink
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will be created from ``/dev/ttyN`` symlinked to the corresponding ``/dev/pts/M``
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pseudo TTY device. The first console will be ``/dev/tty1``, with further
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consoles numbered incrementally from there.
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Since /dev/ttyN and /dev/console are linked to the pts devices. The tty device
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of login program is pts device. The pam module securetty may prevent root user
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from logging in container. If you want root user to log in container
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successfully, add the pts device to the file /etc/securetty of container.
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Further block or character devices will be made available to containers
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depending on their configuration.
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Security considerations
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-----------------------
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The libvirt LXC driver is fairly flexible in how it can be configured, and as
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such does not enforce a requirement for strict security separation between a
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container and the host. This allows it to be used in scenarios where only
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resource control capabilities are important, and resource sharing is desired.
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Applications wishing to ensure secure isolation between a container and the host
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must ensure that they are writing a suitable configuration.
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Network isolation
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~~~~~~~~~~~~~~~~~
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If the guest configuration does not list any network interfaces, the ``network``
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namespace will not be activated, and thus the container will see all the host's
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network interfaces. This will allow apps in the container to bind to/connect
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from TCP/UDP addresses and ports from the host OS. It also allows applications
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to access UNIX domain sockets associated with the host OS, which are in the
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abstract namespace. If access to UNIX domains sockets in the abstract namespace
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is not wanted, then applications should set the ``<privnet/>`` flag in the
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``<features>....</features>`` element.
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Filesystem isolation
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~~~~~~~~~~~~~~~~~~~~
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If the guest configuration does not list any filesystems, then the container
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will be set up with a root filesystem that matches the host's root filesystem.
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As noted earlier, only a few locations such as ``/dev``, ``/proc`` and ``/sys``
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will be altered. This means that, in the absence of restrictions from sVirt, a
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process running as user/group N:M inside the container will be able to access
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almost exactly the same files as a process running as user/group N:M in the
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host.
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There are multiple options for restricting this. It is possible to simply map
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the existing root filesystem through to the container in read-only mode.
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Alternatively a completely separate root filesystem can be configured for the
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guest. In both cases, further sub-mounts can be applied to customize the content
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that is made visible. Note that in the absence of sVirt controls, it is still
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possible for the root user in a container to unmount any sub-mounts applied. The
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user namespace feature can also be used to restrict access to files based on the
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UID/GID mappings.
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Sharing the host filesystem tree, also allows applications to access UNIX
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domains sockets associated with the host OS, which are in the filesystem
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namespaces. It should be noted that a number of init systems including at least
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``systemd`` and ``upstart`` have UNIX domain socket which are used to control
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their operation. Thus, if the directory/filesystem holding their UNIX domain
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socket is exposed to the container, it will be possible for a user in the
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container to invoke operations on the init service in the same way it could if
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outside the container. This also applies to other applications in the host which
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use UNIX domain sockets in the filesystem, such as DBus, Libvirtd, and many
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more. If this is not desired, then applications should either specify the
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UID/GID mapping in the configuration to enable user namespaces and thus block
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access to the UNIX domain socket based on permissions, or should ensure the
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relevant directories have a bind mount to hide them. This is particularly
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important for the ``/run`` or ``/var/run`` directories.
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User and group isolation
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~~~~~~~~~~~~~~~~~~~~~~~~
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If the guest configuration does not list any ID mapping, then the user and group
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IDs used inside the container will match those used outside the container. In
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addition, the capabilities associated with a process in the container will infer
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the same privileges they would for a process in the host. This has obvious
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implications for security, since a root user inside the container will be able
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to access any file owned by root that is visible to the container, and perform
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more or less any privileged kernel operation. In the absence of additional
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protection from sVirt, this means that the root user inside a container is
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effectively as powerful as the root user in the host. There is no security
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isolation of the root user.
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The ID mapping facility was introduced to allow for stricter control over the
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privileges of users inside the container. It allows apps to define rules such as
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"user ID 0 in the container maps to user ID 1000 in the host". In addition the
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privileges associated with capabilities are somewhat reduced so that they cannot
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be used to escape from the container environment. A full description of user
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namespaces is outside the scope of this document, however LWN has `a good
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write-up on the topic <https://lwn.net/Articles/532593/>`__. From the libvirt
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point of view, the key thing to remember is that defining an ID mapping for
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users and groups in the container XML configuration causes libvirt to activate
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the user namespace feature.
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Location of configuration files
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-------------------------------
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The LXC driver comes with sane default values. However, during its
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initialization it reads a configuration file which offers system administrator
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to override some of that default. The file is located under
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``/etc/libvirt/lxc.conf``
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Systemd Socket Activation Integration
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-------------------------------------
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The libvirt LXC driver provides the ability to pass across pre-opened file
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descriptors when starting LXC guests. This allows for libvirt LXC to support
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systemd's `socket activation
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capability <https://0pointer.de/blog/projects/socket-activated-containers.html>`__,
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where an incoming client connection in the host OS will trigger the startup of a
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container, which runs another copy of systemd which gets passed the server
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socket, and then activates the actual service handler in the container.
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Let us assume that you already have a LXC guest created, running a systemd
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instance as PID 1 inside the container, which has an SSHD service configured.
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The goal is to automatically activate the container when the first SSH
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connection is made. The first step is to create a couple of unit files for the
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host OS systemd instance. The ``/etc/systemd/system/mycontainer.service`` unit
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file specifies how systemd will start the libvirt LXC container
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::
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[Unit]
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Description=My little container
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[Service]
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ExecStart=/usr/bin/virsh -c lxc:///system start --pass-fds 3 mycontainer
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ExecStop=/usr/bin/virsh -c lxc:///system destroy mycontainer
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Type=oneshot
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RemainAfterExit=yes
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KillMode=none
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The ``--pass-fds 3`` argument specifies that the file descriptor number 3 that
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``virsh`` inherits from systemd, is to be passed into the container. Since
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``virsh`` will exit immediately after starting the container, the
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``RemainAfterExit`` and ``KillMode`` settings must be altered from their
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defaults.
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Next, the ``/etc/systemd/system/mycontainer.socket`` unit file is created to get
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the host systemd to listen on port 23 for TCP connections. When this unit file
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is activated by the first incoming connection, it will cause the
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``mycontainer.service`` unit to be activated with the FD corresponding to the
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listening TCP socket passed in as FD 3.
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::
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[Unit]
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Description=The SSH socket of my little container
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[Socket]
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ListenStream=23
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Port 23 was picked here so that the container doesn't conflict with the host's
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SSH which is on the normal port 22. That's it in terms of host side
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configuration.
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Inside the container, the ``/etc/systemd/system/sshd.socket`` unit file must be
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created
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::
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[Unit]
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Description=SSH Socket for Per-Connection Servers
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[Socket]
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ListenStream=23
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Accept=yes
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The ``ListenStream`` value listed in this unit file, must match the value used
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in the host file. When systemd in the container receives the pre-opened FD from
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libvirt during container startup, it looks at the ``ListenStream`` values to
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figure out which FD to give to which service. The actual service to start is
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defined by a correspondingly named ``/etc/systemd/system/sshd@.service``
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::
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[Unit]
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Description=SSH Per-Connection Server for %I
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[Service]
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ExecStart=-/usr/sbin/sshd -i
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StandardInput=socket
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Finally, make sure this SSH service is set to start on boot of the container, by
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running the following command inside the container:
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::
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# mkdir -p /etc/systemd/system/sockets.target.wants/
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# ln -s /etc/systemd/system/sshd.socket /etc/systemd/system/sockets.target.wants/
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This example shows how to activate the container based on an incoming SSH
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connection. If the container was also configured to have an httpd service, it
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may be desirable to activate it upon either an httpd or a sshd connection
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attempt. In this case, the ``mycontainer.socket`` file in the host would simply
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list multiple socket ports. Inside the container a separate ``xxxxx.socket``
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file would need to be created for each service, with a corresponding
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``ListenStream`` value set.
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Container security
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------------------
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sVirt SELinux
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~~~~~~~~~~~~~
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In the absence of the "user" namespace being used, containers cannot be
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considered secure against exploits of the host OS. The sVirt SELinux driver
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provides a way to secure containers even when the "user" namespace is not used.
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The cost is that writing a policy to allow execution of arbitrary OS is not
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practical. The SELinux sVirt policy is typically tailored to work with a simpler
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application confinement use case, as provided by the "libvirt-sandbox" project.
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Auditing
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~~~~~~~~
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The LXC driver is integrated with libvirt's auditing subsystem, which causes
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audit messages to be logged whenever there is an operation performed against a
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container which has impact on host resources. So for example, start/stop, device
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hotplug will all log audit messages providing details about what action occurred
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and any resources associated with it. There are the following 3 types of audit
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messages
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- ``VIRT_MACHINE_ID`` - details of the SELinux process and image security
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labels assigned to the container.
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- ``VIRT_CONTROL`` - details of an action / operation performed against a
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container. There are the following types of operation
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- ``op=start`` - a container has been started. Provides the machine name,
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uuid and PID of the ``libvirt_lxc`` controller process
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- ``op=init`` - the init PID of the container has been started. Provides the
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machine name, uuid and PID of the ``libvirt_lxc`` controller process and
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PID of the init process (in the host PID namespace)
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- ``op=stop`` - a container has been stopped. Provides the machine name,
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uuid
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- ``VIRT_RESOURCE`` - details of a host resource associated with a container
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action.
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Device access
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~~~~~~~~~~~~~
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All containers are launched with the CAP_MKNOD capability cleared and removed
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from the bounding set. Libvirt will ensure that the /dev filesystem is
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pre-populated with all devices that a container is allowed to use. In addition,
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the cgroup "device" controller is configured to block read/write/mknod from all
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devices except those that a container is authorized to use.
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Example configurations
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----------------------
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Example config version 1
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~~~~~~~~~~~~~~~~~~~~~~~~
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::
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<domain type='lxc'>
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<name>vm1</name>
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<memory>500000</memory>
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<os>
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<type>exe</type>
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<init>/bin/sh</init>
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</os>
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<vcpu>1</vcpu>
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<clock offset='utc'/>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<devices>
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<emulator>/usr/libexec/libvirt_lxc</emulator>
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<interface type='network'>
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<source network='default'/>
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</interface>
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<console type='pty' />
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</devices>
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</domain>
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In the <emulator> element, be sure you specify the correct path to libvirt_lxc,
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if it does not live in /usr/libexec on your system.
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The next example assumes there is a private root filesystem (perhaps
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hand-crafted using busybox, or installed from media, debootstrap, whatever)
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under /opt/vm-1-root:
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::
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<domain type='lxc'>
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<name>vm1</name>
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<memory>32768</memory>
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<os>
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<type>exe</type>
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<init>/init</init>
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</os>
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<vcpu>1</vcpu>
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<clock offset='utc'/>
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<on_poweroff>destroy</on_poweroff>
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<on_reboot>restart</on_reboot>
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<on_crash>destroy</on_crash>
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<devices>
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<emulator>/usr/libexec/libvirt_lxc</emulator>
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<filesystem type='mount'>
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<source dir='/opt/vm-1-root'/>
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<target dir='/'/>
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</filesystem>
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<interface type='network'>
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<source network='default'/>
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</interface>
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<console type='pty' />
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</devices>
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</domain>
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Altering the available capabilities
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-----------------------------------
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By default the libvirt LXC driver drops some capabilities among which CAP_MKNOD.
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However :since:`since 1.2.6` libvirt can be told to keep or drop some
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capabilities using a domain configuration like the following:
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::
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...
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<features>
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<capabilities policy='default'>
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<mknod state='on'/>
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<sys_chroot state='off'/>
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</capabilities>
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</features>
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...
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The capabilities children elements are named after the capabilities as defined
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in ``man 7 capabilities``. An ``off`` state tells libvirt to drop the
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capability, while an ``on`` state will force to keep the capability even though
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this one is dropped by default.
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The ``policy`` attribute can be one of ``default``, ``allow`` or ``deny``. It
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defines the default rules for capabilities: either keep the default behavior
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that is dropping a few selected capabilities, or keep all capabilities or drop
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all capabilities. The interest of ``allow`` and ``deny`` is that they guarantee
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that all capabilities will be kept (or removed) even if new ones are added
|
|
later.
|
|
|
|
The following example, drops all capabilities but CAP_MKNOD:
|
|
|
|
::
|
|
|
|
...
|
|
<features>
|
|
<capabilities policy='deny'>
|
|
<mknod state='on'/>
|
|
</capabilities>
|
|
</features>
|
|
...
|
|
|
|
Note that allowing capabilities that are normally dropped by default can
|
|
seriously affect the security of the container and the host.
|
|
|
|
Inherit namespaces
|
|
------------------
|
|
|
|
Libvirt allows you to inherit the namespace from container/process just like lxc
|
|
tools or docker provides to share the network namespace. The following can be
|
|
used to share required namespaces. If we want to share only one then the other
|
|
namespaces can be ignored. The netns option is specific to sharenet. It can be
|
|
used in cases we want to use existing network namespace rather than creating new
|
|
network namespace for the container. In this case privnet option will be
|
|
ignored.
|
|
|
|
::
|
|
|
|
<domain type='lxc' xmlns:lxc='http://libvirt.org/schemas/domain/lxc/1.0'>
|
|
...
|
|
<lxc:namespace>
|
|
<lxc:sharenet type='netns' value='red'/>
|
|
<lxc:shareuts type='name' value='container1'/>
|
|
<lxc:shareipc type='pid' value='12345'/>
|
|
</lxc:namespace>
|
|
</domain>
|
|
|
|
The use of namespace passthrough requires libvirt >= 1.2.19
|
|
|
|
Container usage / management
|
|
----------------------------
|
|
|
|
As with any libvirt virtualization driver, LXC containers can be managed via a
|
|
wide variety of libvirt based tools. At the lowest level the ``virsh`` command
|
|
can be used to perform many tasks, by passing the ``-c lxc:///system`` argument.
|
|
As an alternative to repeating the URI with every command, the
|
|
``LIBVIRT_DEFAULT_URI`` environment variable can be set to ``lxc:///system``.
|
|
The examples that follow outline some common operations with virsh and LXC. For
|
|
further details about usage of virsh consult its manual page.
|
|
|
|
Defining (saving) container configuration
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh define`` command takes an XML configuration document and loads it
|
|
into libvirt, saving the configuration on disk
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system define myguest.xml
|
|
|
|
Viewing container configuration
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh dumpxml`` command can be used to view the current XML configuration
|
|
of a container. By default the XML output reflects the current state of the
|
|
container. If the container is running, it is possible to explicitly request the
|
|
persistent configuration, instead of the current live configuration using the
|
|
``--inactive`` flag
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system dumpxml myguest
|
|
|
|
Starting containers
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh start`` command can be used to start a container from a previously
|
|
defined persistent configuration
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system start myguest
|
|
|
|
It is also possible to start so called "transient" containers, which do not
|
|
require a persistent configuration to be saved by libvirt, using the
|
|
``virsh create`` command.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system create myguest.xml
|
|
|
|
Stopping containers
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh shutdown`` command can be used to request a graceful shutdown of the
|
|
container. By default this command will first attempt to send a message to the
|
|
init process via the ``/dev/initctl`` device node. If no such device node
|
|
exists, then it will send SIGTERM to PID 1 inside the container.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system shutdown myguest
|
|
|
|
If the container does not respond to the graceful shutdown request, it can be
|
|
forcibly stopped using the ``virsh destroy``
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system destroy myguest
|
|
|
|
Rebooting a container
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh reboot`` command can be used to request a graceful shutdown of the
|
|
container. By default this command will first attempt to send a message to the
|
|
init process via the ``/dev/initctl`` device node. If no such device node
|
|
exists, then it will send SIGHUP to PID 1 inside the container.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system reboot myguest
|
|
|
|
Undefining (deleting) a container configuration
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh undefine`` command can be used to delete the persistent
|
|
configuration of a container. If the guest is currently running, this will turn
|
|
it into a "transient" guest.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system undefine myguest
|
|
|
|
Connecting to a container console
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh console`` command can be used to connect to the text console
|
|
associated with a container.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system console myguest
|
|
|
|
If the container has been configured with multiple console devices, then the
|
|
``--devname`` argument can be used to choose the console to connect to. In LXC,
|
|
multiple consoles will be named as 'console0', 'console1', 'console2', etc.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system console myguest --devname console1
|
|
|
|
Running commands in a container
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh lxc-enter-namespace`` command can be used to enter the namespaces
|
|
and security context of a container and then execute an arbitrary command.
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system lxc-enter-namespace myguest -- /bin/ls -al /dev
|
|
|
|
Monitoring container utilization
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virt-top`` command can be used to monitor the activity and resource
|
|
utilization of all containers on a host
|
|
|
|
::
|
|
|
|
# virt-top -c lxc:///system
|
|
|
|
Converting LXC container configuration
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The ``virsh domxml-from-native`` command can be used to convert most of the LXC
|
|
container configuration into a domain XML fragment
|
|
|
|
::
|
|
|
|
# virsh -c lxc:///system domxml-from-native lxc-tools /var/lib/lxc/myguest/config
|
|
|
|
This conversion has some limitations due to the fact that the domxml-from-native
|
|
command output has to be independent of the host. Here are a few things to take
|
|
care of before converting:
|
|
|
|
- Replace the fstab file referenced by lxc.mount by the corresponding
|
|
lxc.mount.entry lines.
|
|
- Replace all relative sizes of tmpfs mount entries to absolute sizes. Also
|
|
make sure that tmpfs entries all have a size option (default is 50%).
|
|
- Define lxc.cgroup.memory.limit_in_bytes to properly limit the memory
|
|
available to the container. The conversion will use 64MiB as the default.
|