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.
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
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
$ sudo singularity capability list --user pinger CAP_NET_RAW
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 1 126.96.36.199 PING 188.8.131.52 (184.108.40.206) 56(84) bytes of data. 64 bytes from 220.127.116.11: icmp_seq=1 ttl=128 time=18.3 ms --- 18.104.22.168 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
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
capability add and
drop subcommands will also accept the case
all to grant or revoke all Linux capabilities to a user
or group. Similarly, the
--add-caps option will accept the
Of course appropriate caution should be exercised when using this keyword.