+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. +
+ +