Security Options

Singularity 3.0 introduces many new security related options to the container runtime. This document will describe the new methods users have for specifying the security scope and context when running Singularity containers.

Linux Capabilities


It is extremely important to recognize that granting users Linux capabilities with the capability command group is usually identical to granting those users root level access on the host system. Most if not all capabilities will allow users to “break out” of the container and become root on the host. This feature is targeted toward special use cases (like cloud-native architectures) where an admin/developer might want to limit the attack surface within a container that normally runs as root. This is not a good option in multi-tenant HPC environments where an admin wants to grant a user special privileges within a container. For that and similar use cases, the fakeroot feature is a better option.

Singularity provides full support for granting and revoking Linux capabilities on a user or group basis. For example, let us suppose that an admin has decided to grant a user (named pinger) capabilities to open raw sockets so that they can use ping in a container where the binary is controlled via capabilities (i.e. a recent version of CentOS).

To do so, the admin would issue a command such as this:

$ sudo singularity capability add --user pinger CAP_NET_RAW

This means the user pinger has just been granted permissions (through Linux capabilities) to open raw sockets within Singularity containers.

The admin can check that this change is in effect with the capability list command.

$ sudo singularity capability list --user pinger

To take advantage of this new capability, the user pinger must also request the capability when executing a container with the --add-caps flag like so:

$ singularity exec --add-caps CAP_NET_RAW library://centos ping -c 1
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=128 time=18.3 ms

--- ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 18.320/18.320/18.320/0.000 ms

If the admin decides that it is no longer necessary to allow the user pinger to open raw sockets within Singularity containers, they can revoke the appropriate Linux capability like so:

$ sudo singularity capability drop --user pinger CAP_NET_RAW

The capability add and drop subcommands will also accept the case insensitive keyword all to grant or revoke all Linux capabilities to a user or group. Similarly, the --add-caps option will accept the all keyword. Of course appropriate caution should be exercised when using this keyword.