The libvirt LXC driver manages "Linux Containers". At their simplest, containers can just be thought of as a collection of processes, separated from the main host processes via a set of resource namespaces and constrained via control groups resource tunables. The libvirt LXC driver has no dependency on the LXC userspace tools hosted on sourceforge.net. It directly utilizes the relevant kernel features to build the container environment. This allows for sharing of many libvirt technologies across both the QEMU/KVM and LXC drivers. In particular sVirt for mandatory access control, auditing of operations, integration with control groups and many other features.
In order to control the resource usage of processes inside containers, the
libvirt LXC driver requires that certain cgroups controllers are mounted on
the host OS. The minimum required controllers are 'cpuacct', 'memory' and
'devices', while recommended extra controllers are 'cpu', 'freezer' and
'blkio'. Libvirt will not mount the cgroups filesystem itself, leaving
this up to the init system to take care of. Systemd will do the right thing
in this respect, while for other init systems the cgconfig
init service will be required. For further information, consult the general
libvirt cgroups documentation.
In order to separate processes inside a container from those in the primary "host" OS environment, the libvirt LXC driver requires that certain kernel namespaces are compiled in. Libvirt currently requires the 'mount', 'ipc', 'pid', and 'uts' namespaces to be available. If separate network interfaces are desired, then the 'net' namespace is required. If the guest configuration declares a UID or GID mapping, the 'user' namespace will be enabled to apply these. A suitably configured UID/GID mapping is a pre-requisite to making containers secure, in the absence of sVirt confinement.
When the container "init" process is started, it will typically
not be given any command line arguments (eg the equivalent of
the bootloader args visible in /proc/cmdline). If
any arguments are desired, then must be explicitly set in the
container XML configuration via one or more initarg
elements. For example, to run systemd --unit emergency.service
would use the following XML
<os>
<type arch='x86_64'>exe</type>
<init>/bin/systemd</init>
<initarg>--unit</initarg>
<initarg>emergency.service</initarg>
</os>
When the container "init" process is started, it will be given several useful environment variables. The following standard environment variables are mandated by systemd container interface to be provided by all container technologies on Linux.
libvirt-lxc to identify libvirt as the creator/bin:/usr/binlinuxIn addition to the standard variables, the following libvirt specific environment variables are also provided
initarg config element.In the absence of any explicit configuration, the container will inherit the host OS filesystem mounts. A number of mount points will be made read only, or re-mounted with new instances to provide container specific data. The following special mounts are setup by libvirt
/dev a new "tmpfs" pre-populated with authorized device nodes/dev/pts a new private "devpts" instance for console devices/sys the host "sysfs" instance remounted read-only/proc a new instance of the "proc" filesystem/proc/sys the host "/proc/sys" bind-mounted read-only/sys/fs/selinux the host "selinux" instance remounted read-only/sys/fs/cgroup/NNNN the host cgroups controllers bind-mounted to
only expose the sub-tree associated with the container/proc/meminfo a FUSE backed file reflecting memory limits of the container
The container init process will be started with CAP_MKNOD
capability removed and blocked from re-acquiring it. As such it will
not be able to create any device nodes in /dev or anywhere
else in its filesystems. Libvirt itself will take care of pre-populating
the /dev filesystem with any devices that the container
is authorized to use. The current devices that will be made available
to all containers are
/dev/zero/dev/null/dev/full/dev/random/dev/urandom/dev/stdin symlinked to /proc/self/fd/0/dev/stdout symlinked to /proc/self/fd/1/dev/stderr symlinked to /proc/self/fd/2/dev/fd symlinked to /proc/self/fd/dev/ptmx symlinked to /dev/pts/ptmx/dev/console symlinked to /dev/pts/0
In addition, for every console defined in the guest configuration,
a symlink will be created from /dev/ttyN symlinked to
the corresponding /dev/pts/M pseudo TTY device. The
first console will be /dev/tty1, with further consoles
numbered incrementally from there.
Since /dev/ttyN and /dev/console are linked to the pts devices. The tty device of login program is pts device. the pam module securetty may prevent root user from logging in container. If you want root user to log in container successfully, add the pts device to the file /etc/securetty of container.
Further block or character devices will be made available to containers depending on their configuration.
The libvirt LXC driver is fairly flexible in how it can be configured, and as such does not enforce a requirement for strict security separation between a container and the host. This allows it to be used in scenarios where only resource control capabilities are important, and resource sharing is desired. Applications wishing to ensure secure isolation between a container and the host must ensure that they are writing a suitable configuration.
If the guest configuration does not list any network interfaces,
the network namespace will not be activated, and thus
the container will see all the host's network interfaces. This will
allow apps in the container to bind to/connect from TCP/UDP addresses
and ports from the host OS. It also allows applications to access
UNIX domain sockets associated with the host OS, which are in the
abstract namespace. If access to UNIX domains sockets in the abstract
namespace is not wanted, then applications should set the
<privnet/> flag in the
<features>....</features> element.
If the guest configuration does not list any filesystems, then
the container will be set up with a root filesystem that matches
the host's root filesystem. As noted earlier, only a few locations
such as /dev, /proc and /sys
will be altered. This means that, in the absence of restrictions
from sVirt, a process running as user/group N:M inside the container
will be able to access almost exactly the same files as a process
running as user/group N:M in the host.
There are multiple options for restricting this. It is possible to simply map the existing root filesystem through to the container in read-only mode. Alternatively a completely separate root filesystem can be configured for the guest. In both cases, further sub-mounts can be applied to customize the content that is made visible. Note that in the absence of sVirt controls, it is still possible for the root user in a container to unmount any sub-mounts applied. The user namespace feature can also be used to restrict access to files based on the UID/GID mappings.
Sharing the host filesystem tree, also allows applications to access
UNIX domains sockets associated with the host OS, which are in the
filesystem namespaces. It should be noted that a number of init
systems including at least systemd and upstart
have UNIX domain socket which are used to control their operation.
Thus, if the directory/filesystem holding their UNIX domain socket is
exposed to the container, it will be possible for a user in the container
to invoke operations on the init service in the same way it could if
outside the container. This also applies to other applications in the
host which use UNIX domain sockets in the filesystem, such as DBus,
Libvirtd, and many more. If this is not desired, then applications
should either specify the UID/GID mapping in the configuration to
enable user namespaces and thus block access to the UNIX domain socket
based on permissions, or should ensure the relevant directories have
a bind mount to hide them. This is particularly important for the
/run or /var/run directories.
If the guest configuration does not list any ID mapping, then the user and group IDs used inside the container will match those used outside the container. In addition, the capabilities associated with a process in the container will infer the same privileges they would for a process in the host. This has obvious implications for security, since a root user inside the container will be able to access any file owned by root that is visible to the container, and perform more or less any privileged kernel operation. In the absence of additional protection from sVirt, this means that the root user inside a container is effectively as powerful as the root user in the host. There is no security isolation of the root user.
The ID mapping facility was introduced to allow for stricter control over the privileges of users inside the container. It allows apps to define rules such as "user ID 0 in the container maps to user ID 1000 in the host". In addition the privileges associated with capabilities are somewhat reduced so that they cannot be used to escape from the container environment. A full description of user namespaces is outside the scope of this document, however LWN has a good write-up on the topic. From the libvirt point of view, the key thing to remember is that defining an ID mapping for users and groups in the container XML configuration causes libvirt to activate the user namespace feature.
The libvirt LXC driver provides the ability to pass across pre-opened file descriptors when starting LXC guests. This allows for libvirt LXC to support systemd's socket activation capability, where an incoming client connection in the host OS will trigger the startup of a container, which runs another copy of systemd which gets passed the server socket, and then activates the actual service handler in the container.
Let us assume that you already have a LXC guest created, running
a systemd instance as PID 1 inside the container, which has an
SSHD service configured. The goal is to automatically activate
the container when the first SSH connection is made. The first
step is to create a couple of unit files for the host OS systemd
instance. The /etc/systemd/system/mycontainer.service
unit file specifies how systemd will start the libvirt LXC container
[Unit] Description=My little container [Service] ExecStart=/usr/bin/virsh -c lxc:/// start --pass-fds 3 mycontainer ExecStop=/usr/bin/virsh -c lxc:/// destroy mycontainer Type=oneshot RemainAfterExit=yes KillMode=none
The --pass-fds 3 argument specifies that the file
descriptor number 3 that virsh inherits from systemd,
is to be passed into the container. Since virsh will
exit immediately after starting the container, the RemainAfterExit
and KillMode settings must be altered from their defaults.
Next, the /etc/systemd/system/mycontainer.socket unit
file is created to get the host systemd to listen on port 23 for
TCP connections. When this unit file is activated by the first
incoming connection, it will cause the mycontainer.service
unit to be activated with the FD corresponding to the listening TCP
socket passed in as FD 3.
[Unit] Description=The SSH socket of my little container [Socket] ListenStream=23
Port 23 was picked here so that the container doesn't conflict with the host's SSH which is on the normal port 22. That's it in terms of host side configuration.
Inside the container, the /etc/systemd/system/sshd.socket
unit file must be created
[Unit] Description=SSH Socket for Per-Connection Servers [Socket] ListenStream=23 Accept=yes
The ListenStream value listed in this unit file, must
match the value used in the host file. When systemd in the container
receives the pre-opened FD from libvirt during container startup, it
looks at the ListenStream values to figure out which
FD to give to which service. The actual service to start is defined
by a correspondingly named /etc/systemd/system/sshd@.service
[Unit] Description=SSH Per-Connection Server for %I [Service] ExecStart=-/usr/sbin/sshd -i StandardInput=socket
Finally, make sure this SSH service is set to start on boot of the container, by running the following command inside the container:
# mkdir -p /etc/systemd/system/sockets.target.wants/ # ln -s /etc/systemd/system/sshd.socket /etc/systemd/system/sockets.target.wants/
This example shows how to activate the container based on an incoming
SSH connection. If the container was also configured to have an httpd
service, it may be desirable to activate it upon either an httpd or a
sshd connection attempt. In this case, the mycontainer.socket
file in the host would simply list multiple socket ports. Inside the
container a separate xxxxx.socket file would need to be
created for each service, with a corresponding ListenStream
value set.
In the absence of the "user" namespace being used, containers cannot be considered secure against exploits of the host OS. The sVirt SELinux driver provides a way to secure containers even when the "user" namespace is not used. The cost is that writing a policy to allow execution of arbitrary OS is not practical. The SELinux sVirt policy is typically tailored to work with an simpler application confinement use case, as provided by the "libvirt-sandbox" project.
The LXC driver is integrated with libvirt's auditing subsystem, which causes audit messages to be logged whenever there is an operation performed against a container which has impact on host resources. So for example, start/stop, device hotplug will all log audit messages providing details about what action occurred and any resources associated with it. There are the following 3 types of audit messages
VIRT_MACHINE_ID - details of the SELinux process and
image security labels assigned to the container.VIRT_CONTROL - details of an action / operation
performed against a container. There are the following types of
operation
op=start - a container has been started. Provides
the machine name, uuid and PID of the libvirt_lxc
controller processop=init - the init PID of the container has been
started. Provides the machine name, uuid and PID of the
libvirt_lxc controller process and PID of the
init process (in the host PID namespace)op=stop - a container has been stopped. Provides
the machine name, uuidVIRT_RESOURCE - details of a host resource
associated with a container action.All containers are launched with the CAP_MKNOD capability cleared and removed from the bounding set. Libvirt will ensure that the /dev filesystem is pre-populated with all devices that a container is allowed to use. In addition, the cgroup "device" controller is configured to block read/write/mknod from all devices except those that a container is authorized to use.
<domain type='lxc'>
<name>vm1</name>
<memory>500000</memory>
<os>
<type>exe</type>
<init>/bin/sh</init>
</os>
<vcpu>1</vcpu>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>destroy</on_crash>
<devices>
<emulator>/usr/libexec/libvirt_lxc</emulator>
<interface type='network'>
<source network='default'/>
</interface>
<console type='pty' />
</devices>
</domain>
In the <emulator> element, be sure you specify the correct path to libvirt_lxc, if it does not live in /usr/libexec on your system.
The next example assumes there is a private root filesystem (perhaps hand-crafted using busybox, or installed from media, debootstrap, whatever) under /opt/vm-1-root:
<domain type='lxc'>
<name>vm1</name>
<memory>32768</memory>
<os>
<type>exe</type>
<init>/init</init>
</os>
<vcpu>1</vcpu>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>destroy</on_crash>
<devices>
<emulator>/usr/libexec/libvirt_lxc</emulator>
<filesystem type='mount'>
<source dir='/opt/vm-1-root'/>
<target dir='/'/>
</filesystem>
<interface type='network'>
<source network='default'/>
</interface>
<console type='pty' />
</devices>
</domain>
By default the libvirt LXC driver drops some capabilities among which CAP_MKNOD. However since 1.2.6 libvirt can be told to keep or drop some capabilities using a domain configuration like the following:
...
<features>
<capabilities policy='default'>
<mknod state='on'/>
<sys_chroot state='off'/>
</capabilities>
</features>
...
The capabilities children elements are named after the capabilities as defined in
man 7 capabilities. An off state tells libvirt to drop the
capability, while an on state will force to keep the capability even though
this one is dropped by default.
The policy attribute can be one of default, allow
or deny. It defines the default rules for capabilities: either keep the
default behavior that is dropping a few selected capabilities, or keep all capabilities
or drop all capabilities. The interest of allow and deny is that
they guarantee 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.
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:/// argument. As an
alternative to repeating the URI with every command, the LIBVIRT_DEFAULT_URI
environment variable can be set to lxc:///. The
examples that follow outline some common operations with virsh
and LXC. For further details about usage of virsh consult its
manual page.
The virsh define command takes an XML configuration
document and loads it into libvirt, saving the configuration on disk
# virsh -c lxc:/// define myguest.xml
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:/// dumpxml myguest
The virsh start command can be used to start a
container from a previously defined persistent configuration
# virsh -c lxc:/// 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:/// create myguest.xml
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:/// 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:/// destroy myguest
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:/// reboot myguest
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:/// undefine myguest
The virsh console command can be used to connect
to the text console associated with a container.
# virsh -c lxc:/// 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:/// console myguest --devname console1
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:/// lxc-enter-namespace myguest -- /bin/ls -al /dev
The virt-top command can be used to monitor the
activity and resource utilization of all containers on a
host
# virt-top -c lxc:///
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:/// 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: