Docker security

The fundamental Docker security issue is that a "root" (uid 0) user inside container is equivalent to "root" outside, unless steps are taken to limit container permissions. We want to disallow containers from sending data outside the private Arvados network, prevent breakout from the container, and limit access if a breakout does occur. We don't allow end users to invoke Docker directly, so we can impose security measures both in the daemon configuration and the individual container invocation.

Some of the knobs we have include:

Setting the uid/gid of pid 1 in container

docker run --user

We can explicitly set the uid/gid of pid 1 inside the container so it is not uid 0. This overrides the USER directive of the image. One drawback is that some programs behave badly when the current uid cannot be found in /etc/passwd.

User id mapping

docker daemon --userns-remap

User ids inside container corresponds to a different host user id. Can map uid 0 inside the container to non-root user outside the container. Processes have two sets of capabilities; one set of capabilities apply when manipulating resources inside the user namespace, a second set of capabilities apply when manipulating resources in the parent namespace. This makes it possible for a process to be "root" inside the container but "non-root" outside the container.

http://man7.org/linux/man-pages/man7/user_namespaces.7.html

This may also be useful for working with bind-mounted directories. Mapping "root" to the host user "crunch" would mean that files written by "root" inside the container would actually be owned by "crunch" outside the container. Because user id mappings are 1:1, this would require always using the same uid inside the container (probably uid 0).

Dropping capabilities

docker run --drop-cap

Drop capabilities of root user inside the container ("man capabilities" for list). Dropping all capabilities effectively neuters the root user (for example, without CAP_DAC_OVERRIDE the root user is subject to the same file permission checks as regular users). Can be used to limit the scope of what "root" user can do inside the container.

Restrict container networking

docker run --net=none

Crunch v2 communicates via arv-mount, which means most containers don't need networking to read/write to Keep. Crunch v2 policy is that networking is disabled by default but can be enabled with the runtime constraint API: true (necessary for Arvados-aware containers). The Docker network bridge should be configured with firewall whitelist that limits communication to essential Arvados services (API server + Keep server).

Disable inter-container communication

docker daemon --icc=false

Our containers don't need to talk to each other.

Resource limits via cgroups

Slurm can set up a cgroup (control group) to dictate resource limits, and crunch-run can instruct Docker to put the container in the cgroup set up by slurm. Note, for this to work, we may need to invoke the Docker daemon with this option:

--exec-opt native.cgroupdriver=cgroupfs

Further research is required to see if slurm cgroup settings are sufficient to prevent overloading the node or denial-of-service, or if we need to set other limits (for example, a limit on the number of processes inside the container to prevent forkbomb attacks.)

Resource limits via ulimit

We can also set ulimits on daemon invocation (--default-ulimit) and on container invocation (--ulimit). ulimit has some overlap with cgroups but the difference seems to be that most ulimit settings apply per-process rather than to a group of processes.

seccomp

Seccomp filters system calls that can be made by programs inside the container; many system calls it filters can also be blocked by dropping capabilities.

https://docs.docker.com/engine/security/seccomp/

AppArmor

Apply additional fine-grained access controls to the container, e.g. deny access to specific paths. Can be used to limit access to /proc and /sys.

https://docs.docker.com/engine/security/apparmor/

SELinux

docker daemon --selinux-enabled

Enable SELinux support. I don't know exactly what that entails, but is probably similar to AppArmor in providing a mechanism for fine-grained access controls for containers. Can also be used to lock down crunch-run and arv-mount on the host system.