Skip to content

Install Singularity

Singularity is a tool that creates docker-like process containers but without giving extra privileges to unprivileged users. It is used by grid pilot jobs (which are submitted by per-VO grid workload management systems) to isolate user jobs from the pilot's files and processes and from other users' files and processes. It also supplies a chroot environment in order to run user jobs in different operating system images under one Linux kernel.

Kernels with a version 3.10.0-957 or newer include a feature that allows singularity to run completely unprivileged. This kernel version is the default for RHEL/CentOS/Scientific Linux 7.6 and is available as a security update for previous 7.x releases. Although the feature is available, it needs to be enabled to be usable (instructions below). This kernel version is not available for RHEL 6 and derivatives.

Without this kernel version, singularity must be installed and run with setuid-root executables. Singularity keeps the privileged code to a minimum in order to reduce the potential for vulnerabilities.

The OSG has installed singularity in OASIS, so many sites will eventually (after it is supported by VOs) not need to install singularity locally if they enable it to run unprivileged. Meanwhile an RPM installation can be configured to be unprivileged or privileged.

Kernel vs. Userspace Security

Enabling unprivileged user namespaces increases the risk to the kernel. However, the kernel is more widely reviewed than singularity and the additional capability given to users is more limited. OSG Security considers the non-setuid, kernel-based method to have a lower security risk.

This document is intended for system administrators that wish to install and/or configure singularity.

Before Starting

As with all OSG software installations, there are some one-time (per host) steps to prepare in advance:

In addition, this is highly recommended for image distribution:

Choosing Unprivileged vs Privileged Singularity

There are two sets of instructions on this page:

OSG VOs are working to support running singularity directly from OASIS, the OSG Software CVMFS distribution, when unprivileged singularity is enabled. At that point sites will not have to install the singularity RPM themselves. As of April 2019, no VO in the OSG is yet ready to do this, but OSG recommends that all RHEL 7.x installations enable support for unprivileged singularity and for now also install the RPM. Sites may also choose to configure their RHEL 7.x RPM installations to run unprivileged. RHEL 6.x installations have no option for unprivileged singularity so there the RPM has to be installed and left configured as privileged.

In addition to improved security, unprivileged singularity enables condor_ssh_to_job to enter a container namespace without itself needing privileges. Also, unprivileged singularity enables nesting containers within another container (when the outer container is started by singularity 3.x, which is currently in the osg-upcoming repository).

On the other hand, there are a few rare use cases that require singularity to run privileged:

  1. Using single-file container images. Some systems, especially High Performance Computing (HPC) systems, deal poorly with collections of small files. In this case, container images stored in a single file (as opposed to an unpacked directory) may be needed.

    However, known images from OSG VOs are directory-based, and we recommend disabling this feature on privileged installations in order to avoid potential kernel exploits.

  2. The overlay feature. The "overlay" feature of singularity uses overlayfs to add bind mounts where mount points don't exist in the underlying image.

    However, this feature doesn't work if the image is a directory distributed in CVMFS, singularity has an "underlay" feature that is equivalent which does work with CVMFS and does not require privileges, and the overlay feature has been a source of security vulnerabilities in the past. For these reasons, we recommend replacing overlay with underlay even on privileged installations.

  3. Allocating new pseudo-tty devices. Support for allocating pseudo-tty devices was accidentally left out of the user namespace support in the RHEL 7.6 kernel.

    However, this feature is only required for a small number of applications, and singularity 3.x works around the limitation for most of them without needing privileges.

Enabling Unprivileged Singularity

The instructions in this section are for enabling singularity to run unprivileged.

If the operating system is an EL 7 variant and has been updated to the EL 7.6 kernel or later, enable unprivileged singularity with the following steps:

  1. Enable user namespaces via sysctl:

    [email protected] # echo "user.max_user_namespaces = 15000" \
        > /etc/sysctl.d/90-max_user_namespaces.conf
    [email protected] # sysctl -p /etc/sysctl.d/90-max_user_namespaces.conf
  2. (Optional) Disable network namespaces:

    [email protected] # echo "user.max_net_namespaces = 0" \
        > /etc/sysctl.d/90-max_net_namespaces.conf
    [email protected] # sysctl -p /etc/sysctl.d/90-max_net_namespaces.conf

    OSG VOs do not need network namespaces with singularity, and disabling them reduces the risk profile of enabling user namespaces.

    Network namespaces are, however, utilized by other software, such as Docker. Disabling network namespaces may break other software, or limit its capabilities (such as requiring the --net=host option in Docker).

  3. If you haven't yet installed CVMFS, do so.

Validating Unprivileged Singularity

Once you have the host configured properly, log in as an ordinary unprivileged user and verify that singularity in OASIS works:

[email protected] $ /cvmfs/ \
                exec --contain --ipc --pid --bind /cvmfs \
                /cvmfs/ \
                ps -ef
WARNING: Container does not have an exec helper script, calling 'ps' directly
user         1     0  2 21:27 ?        00:00:00 shim-init
user         2     1  0 21:27 ?        00:00:00 ps -ef

Singularity via RPM

The instructions in this section are for the singularity RPM, which includes setuid-root executables. The setuid-root executables can however be disabled by configuration, details below.

Installing Singularity via RPM

To install the singularity RPM, make sure that your host is up to date before installing the required packages:

  1. Clean yum cache:

    [email protected] # yum clean all --enablerepo=*
  2. Update software:

    [email protected] # yum update

    This command will update all packages

  3. Install Singularity

    [email protected] # yum install singularity

Configuring Singularity

singularity includes an option called underlay that enables using bind mount points that do not exist in the container image. By default it is enabled, but only if the similar overlay option cannot be used, such as on RHEL6 where kernel support for overlayfs is missing or when running in unprivileged mode. On RHEL7 it is recommended to completely disable overlay, because it is more vulnerable to security problems than underlay and because it does not work correctly when container images are distributed by CVMFS.

Set this option in /etc/singularity/singularity.conf:

    enable overlay = no


If you modify /etc/singularity/singularity.conf, be careful with your upgrade procedures. RPM will not automatically merge your changes with new upstream configuration keys, which may cause a broken install or inadvertently change the site configuration. Singularity changes its default configuration file more frequently than typical OSG software.

Look for singularity.conf.rpmnew after upgrades and merge in any changes to the defaults.

Configuring the RPM to be Unprivileged

If you choose to run the RPM unprivileged, after enabling unprivileged singularity, change the line in /etc/singularity/singularity.conf that says allow setuid = yes to

    allow setuid = no

Note that the setuid-root executables stay installed, but they will exit very early if invoked when the configuration file disallows setuid, so the risk is very low. There are non-setuid equivalent executables that are used instead when setuid is disallowed.

Limiting Image Types

A side effect of disabling privileged singularity is that loopback mounts are disallowed. If the installation is privileged, also consider the following.

Images based on loopback devices carry an inherently higher exposure to unknown kernel exploits compared to directory-based images distributed via CVMFS. See this article for further discussion.

The loopback-based images are the default image type produced by singularity users and are common at sites with direct user logins. However (as of April 2019) we are only aware of directory-based images being used by OSG VOs. Hence, it is reasonable to disable the loopback-based images by setting the following option in /etc/singularity/singularity.conf:

    max loop devices = 0

While reasonable for some sites, this is not required as there are currently no public kernel exploits for this issue; any exploits are patched by Red Hat when they are discovered.

Validating Singularity RPM

After singularity is installed, as an ordinary user run the following command to verify it:

[email protected] $ singularity exec --contain --ipc --pid --bind /cvmfs \
                /cvmfs/ \
                ps -ef
WARNING: Container does not have an exec helper script, calling 'ps' directly
user         1     0  1 21:41 ?        00:00:00 shim-init
user         2     1  0 21:41 ?        00:00:00 ps -ef

Starting and Stopping Services

singularity has no services to start or stop.