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undocker - Running Docker containers from unprivileged users

There are many tools for running containers (e.g. LXC, Singularity, udocker, etc.) but docker is probably the most popular of them.

But some sysadmins do not want to let their users to run Docker containers because of security concerns.

The objective of undocker is to enable sysadmins to let the users run Docker containers using their user credentials (i.e. the containers will be processes owned by the user and not by root).

undocker inspects the parameters to docker, removes the dangerous parameters and adds some parameters that should be included depending on the user. It also enables to limit the docker images that each user may use.

Installation

From sources

In this case you need to install the runtime bashc, that can be obtained from here.

In this case you will need to install the dependencies of undocker.

$ git clone https://github.com/dealfonso/undocker
$ cd undocker
$ make
$ DESTDIR=/ make install

From packages

Get the appropriate package from the releases page and follow the instructions.

CentOS

$ yum install ./undocker.rpm

Ubuntu

$ dpkg -i ./undocker.deb
$ apt update
$ apt install -f

From ppa

$ apt-add-repository ppa:grycap/apps
$ apt update
$ apt install undocker

Features

Once undocker is installed, any user can call undocker to try to call docker. How the users can interact with undocker is configured in the file /etc/undocker.conf (that must be readonly for any user).

The configuration file describes the commandlines that the users are allowed to run. It is possible to specify specific permissions depending on linux groups or users.

Description of fine tune features

Let's use this configuration (taken from the distributed .conf file) as an example:

[command:run]
WHITELIST=--help,-i|--interactive,-t|--tty,-d|--detach,--rm,-e|--env list,-v|--volume list,-w|--workdir string,--name string
FORCED=-u $U_UID:$U_GID -v '$PWD:$PWD' -w '$PWD'
ALLOWEDIMAGES=alpine:.* ubuntu:.*

[command:run group:users]
RESTRICTEDIMAGES=ubuntu:14.04

[command:exec]
WHITELIST=--help,-i|--interactive,-t|--tty,-v|--volume list
FORCED=-u $U_UID:$U_GID -v '$PWD:$PWD' -w '$PWD'
UIDMUSTMATCH=true

allowed commands

If a section [command:<command name>] appears, then the command is allowed to be run by the users, with the restrictions that are defined in that sections.

i.e. if a section (e.g) [command:exec] does not appear in the configuration file, the users won't be able to issue undocker exec ... commands.

allowed parameters

Then the allowed flags must be defined in the WHITELIST variable. The flags may use a pipe separation to explain that the flags are equivalent (e.g. -t|--tty). If the flag has parameters, they must be included in the definition (e.g. -w|--wordir string).

The list of parameters can be obtained from the docker --help

In the case of [command:run], we are allowing to use a set of parameters (e.g. -i, -d), but not other that we consider dangerous (e.g. --privileged).

forced parameters

A variable FORCE is also enabled, that forces a set of parameters to appear in the final call. In our case, -u $U_UID:$U_GID -v '$PWD:$PWD' -w '$PWD'. That means that the docker call will include the credentials of the user that is calling undocker (and not root), and the container will have the current folder mapped into the container (and it will be the working folder in the container).

The parameters in variable FORCE appear just after those parameters used by the user. So they have more priority (that means that (e.g.) if the user included other -w option, the one valid is the forced one).

permitted images

It is possible to limit the images that the users are able to use, by using variables ALLOWEDIMAGES and RESTRICTEDIMAGES. These variables may include a list of space separated names of docker images (it is possible to use regular expressions).

In the case of [command:run] we are allowing any alpine standard image, along with any ubuntu standard image.

specific permissions for groups and users

It is possible to specify options for a group of users or a individual user, by including sections with the form [command:<command name> group:<group name>] or [command:<command name> user:<user name>].

Each section that begins with [command:<command name>] is accumulated to create the final permission set. The order of preference is [command:<command name>], [command:<command name> group:<group name>] and [command:<command name> user:<username>]. That means that the specific setting for an individual user have more preference.

The final WHITELIST is the accumulation of WHITELIST. The same applies for BLACKLIST, ALLOWEDIMAGES and RESTRICTEDIMAGES.

In our example, if a user belongs to group users, he will be able to use any alpine image and any ubuntu image, except from ubuntu:14.04.

preventing the usage from other's containers

Each command that accepts a container name as a parameter can be forced to be used only if the referred container was started by the user.

That is made by setting the variable UIDMUSTMATCH to true. In our example, a user can make a docker exec call to a container (e.g. docker exec -it <conainername> bash) if he started the container (i.e. the container has his credentials).

How undocker works

undocker is based in the existence of the application /usr/bin/undocker-rt. This application creates the safely constructed commandline and calls docker.

In order to make the final call to docker, needs to raise permissions to be able to make a final call to docker. It can be made in two ways: using sudo or a setuid application.

In any case, the application /usr/bin/undocker-rt must be readonly for any user except from root, to prevent users from unauthorized modifications.

SUDO mode (preferred).

A sudo configuration is needed. The distribution of undocker includes the proper configuration, that enables any user to sudo the application /usr/bin/undocker-rt.

The content of /etc/sudoers.d/undocker is the next:

ALL ALL=NOPASSWD:/usr/bin/undocker-rt

SETUID mode

An application with u+s permissions and owned by root has to be installed in /usr/bin/undocker-rts. That application simply calls /usr/bin/undocker-rt, but having "root" permissions. In this way, it is avoided the usage of sudo.

The permissions for file /usr/bin/undocker-rts look like this:

$ ls -l /usr/bin/undocker-rts
-rwsr-xr-x 1 root root 8880 jun  6 17:46 /usr/bin/undocker-rts

On container runtimes security

While most of other container runtimes are limited to run containers using the user's credentials by default (e.g. Singularity has that way of working, or LXC that uses user-namespace remapping, using subuid and subgid), Docker does not work like that.

While Docker can also remap users to make it more secure. Its default working mode is to run a daemon under root credentials and thus run the containers using that credentials.

root credentials are needed for running most of containers (otherwise they will probably be a simple chroot). And the other runtimes also use them:

  • LXC: runs the daemon as root, but starts give the containers other uid.
  • Singularity: uses a setuid application that gives root privileges to the singularity runtime (* singularity can work without that setuid mode, but they recognise that most of the functionality won't work).
  • udocker: the containers are not full featured containers, as it aims at being (more or less) a simple chroot on a Docker container filesystem to take profit from the installed applications.

How to secure Docker containers

One of my premises on Docker security is...

The best way to let the users run Docker containers is not to let the users run Docker containers

But I can rewrite the prase to

The best way to let the users run Docker containers is not to let the users run arbitrary Docker containers

And this is the key work: arbitraty.

A security aware sysadmin should not let arbitraty users to run arbitrary Docker containers under root credentials.

E.g.

Don't allow

$ docker run --privileged -it alpine ash

But allow

$ docker run -it alpine ash

or better

$ docker run -u $(id -u):$(id -g) -it alpine ash

And that is how undocker works. It analyzes the commands issued by the user and removes the prohibited ones (e.g. --privileged or --dev) and forces some other (e.g. -u $(id -u):$id -g)).