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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.

For operating system kernels older than the one released for Red Hat Enterprise Linux (RHEL) 7.4, singularity needs to use kernel capabilities that are only accessible to the root user, so it has to be installed with setuid-root executables. Securing setuid-root programs is difficult, but singularity keeps that privileged code to a minimum to keep the vulnerability low.

Beginning with the kernel released with RHEL 7.4, there is a technology preview feature to allow unprivileged bind mounts in user namespaces, which allows singularity to run as an unprivileged user. The OSG has installed singularity in OASIS, so you can avoid installing singularity at all.

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. However, Red Hat does not consider technology preview features to be ready for production, so security issues are not guaranteed to be addressed promptly.

The document is intended for system administrators who wish to either install singularity or enable it to be run as an unprivileged user.

Before Starting

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

Choosing Privileged vs Unprivileged Singularity

There are two separate sets of instructions on this page:

As of May 2018, no VO in the OSG is ready to use unprivileged, non-setuid Singularity out of OASIS in production. Only testing sites will need to follow these instructions; contact the VOs you support for more information.

Most sites will want to follow the privileged RPM install instructions until there is wider VO support.

Privileged Singularity

The instructions in this section are for installing singularity with setuid-root executables.

Installing Singularity via RPM

To install singularity as setuid, 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. The singularity packages are split into two parts, choose the command that corresponds to your situation:

    • If you are installing singularity on a worker node, where images do not need to be created or manipulated, install just the smaller part to limit the amount of setuid-root code that is installed:

      [email protected] # yum install singularity-runtime
    • If you want a full singularity installation, run the following command:

      [email protected] # yum install singularity


In most cases, only singularity-runtime is needed on the worker node; installing only this smaller package reduces risk of potential security exploits.

Configuring singularity

There are no required changes to the default configuration. If you want to see what options are available, see /etc/singularity/singularity.conf.

Limiting image types

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 May 2018) we are only aware of directory-based images being used by OSG VOs. Hence, it is a reasonable measure 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.


If you modify /etc/singularity/singularity.conf, carefully test any 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 renames configuration keys more frequently than typical OSG software.

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

Validating singularity

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

[email protected] $ singularity exec --contain --ipc --pid \
                --home $PWD:/srv \
                --bind /cvms \
                /cvmfs/ \
                ps -ef
WARNING: Container does not have an exec helper script, calling 'ps' directly
user         1     0  0 21:34 ?        00:00:00 ps -ef

Unprivileged Singularity

The instructions in this section are for enabling singularity with non-setuid executables, which is available in OASIS, the OSG Software CVMFS distribution.

Technology Preview

Unprivileged Singularity relies on the Red Hat technology preview of unprivileged user namespaces. Red Hat does not consider technology preview features to be ready for production, so security issues are not guaranteed to be addressed promptly.

Enabling Singularity via OASIS

If the operating system is an EL 7 variant and has been updated to the EL 7.4 kernel or later, you can skip installation altogether and instead do these steps to enable singularity to be run as an unprivileged user via CVMFS:

  1. Set the namespace.unpriv_enable=1 boot option. The easiest way to do this is to add it in /etc/sysconfig/grub to the end of the GRUB_CMDLINE_LINUX variable, before the ending double-quote.
  2. Update the grub configuration:

    [email protected] # grub2-mkconfig -o /boot/grub2/grub.cfg
  3. Enable user namespaces via sysctl:

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

    [email protected] # echo "user.max_net_namespaces = 0" \
        > /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 container systems, such as Docker. Disabling network namespaces may break other container solutions, or limit their capabilities (such as requiring the --net=host option in Docker).

  5. Reboot

  6. If you haven't yet installed cvmfs, do so.

Validating singularity

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

[email protected] $ /cvmfs/ \
                exec --contain --ipc --pid \
                --home $PWD:/srv \
                --bind /cvmfs \
                /cvmfs/ \
                ps -ef
WARNING: Container does not have an exec helper script, calling 'ps' directly
user         1     0  0 21:34 ?        00:00:00 ps -ef

Starting and Stopping Services

singularity has no services to start or stop.