User Namespaces & Fakeroot

User namespaces are an isolation feature that allow processes to run with different user identifiers and/or privileges inside that namespace than are permitted outside. A user may have a uid of 1001 on a system outside of a user namespace, but run programs with a different uid with different privileges inside the namespace.

User namespaces are used with containers to make it possible to setup a container without privileged operations, and so that a normal user can act as root inside a container to perform administrative tasks, without being root on the host outside.

Singularity uses user namespaces in 3 situations:

  • When the setuid workflow is disabled or Singularity was installed without root.

  • When a container is run with the --userns option.

  • When --fakeroot is used to impersonate a root user when building or running a container.

User Namespace Requirements

To allow unprivileged creation of user namespaces a kernel >=3.8 is required, with >=3.18 being recommended due to security fixes for user namespaces (3.18 also adds OverlayFS support which is used by Singularity).

Additionally, some Linux distributions require that unprivileged user namespace creation is enabled using a sysctl or kernel command line parameter. Please consult your distribution documentation or vendor to confirm the steps necessary to ‘enable unprivileged user namespace creation’.

Debian

sudo sh -c 'echo kernel.unprivileged_userns_clone=1 \
    >/etc/sysctl.d/90-unprivileged_userns.conf'
sudo sysctl -p /etc/sysctl.d /etc/sysctl.d/90-unprivileged_userns.conf

RHEL/CentOS 7

From 7.4, kernel support is included but must be enabled with:

sudo sh -c 'echo user.max_user_namespaces=15000 \
    >/etc/sysctl.d/90-max_net_namespaces.conf'
sudo sysctl -p /etc/sysctl.d /etc/sysctl.d/90-max_net_namespaces.conf

Unprivileged Installations

As detailed in the non-setuid installation section, Singularity can be compiled or configured with the allow setuid = no option in singularity.conf to not perform privileged operations using the starter-setuid binary.

When singularity does not use setuid all container execution will use a user namespace. In this mode of operation, some features are not available, and there are impacts to the security/integrity guarantees when running SIF container images:

  • All containers must be run from sandbox directories. SIF images are extracted to a sandbox directory on the fly, preventing verification at runtime, and potentially allowing external modification of the container at runtime.

  • Filesystem image, and SIF-embedded persistent overlays cannot be used.

  • Encrypted containers cannot be used. Singularity mounts encrypted containers directly through the kernel, so that encrypted content is not extracted to disk. This requires the setuid workflow.

  • Fakeroot functionality will rely on external setuid root newuidmap and newgidmap binaries which may be provided by the distribution.

–userns option

The --userns option to singularity run/exec/shell will start a container using a user namespace, avoiding the setuid privileged workflow for container setup even if Singularity was compiled and configured to use setuid by default.

The same limitations apply as in an unprivileged installation.

Fakeroot feature

Fakeroot (or commonly referred as rootless mode) allows an unprivileged user to run a container as a “fake root” user by leveraging user namespaces with user namespace UID/GID mapping.

User namespace UID/GID mapping allows a user to act as a different UID/GID in the container than they are on the host. A user can access a configured range of UIDs/GIDs in the container, which map back to (generally) unprivileged user UIDs/GIDs on the host. This allows a user to be root (uid 0) in a container, install packages etc., but have no privilege on the host.

Requirements

In addition to user namespace support, Singularity must manipulate subuid and subgid maps for the user namepsace it creates. By default this happens transparently in the setuid workflow. With unprivileged installations of Singularity or where allow setuid = no is set in singularity.conf, Singularity attempts to use external setuid binaries newuidmap and newgidmap, so you need to install those binaries on your system.

Note

CentOS/RHEL 7 doesn’t provide a package for newuidmap and newgidmap, so you will need to compile/install shadow-utils by yourself.

Singularity expects to find these binaries in one of those standard paths: /bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin

Basics

Fakeroot relies on /etc/subuid and /etc/subgid files to find configured mappings from real user and group IDs, to a range of otherwise vacant IDs for each user on the host system that can be remapped in the usernamespace. A user must have an entry in these system configuration files to use the fakeroot feature. Singularity provides a config fakeroot command to assist in managing these files, but it is important to understand how they work.

For user foo an entry in /etc/subuid might be:

foo:100000:65536

where foo is the username, 100000 is the start of the UID range that can be used by foo in a user namespace uid mapping, and 65536 number of UIDs available for mapping.

Same for /etc/subgid:

foo:100000:65536

Note

Some distributions add users to these files on installation, or when useradd, adduser, etc. utilities are used to manage local users.

The glibc nss name service switch mechanism does not currently support managing subuid and subgid mappings with external directory services such as LDAP. You must manage or provision mapping files direct to systems where fakeroot will be used.

Warning

Singularity requires that a range of at least 65536 IDs is used for each mapping. Larger ranges may be defined without error.

It is also important to ensure that the subuid and subgid ranges defined in these files don’t overlap with eachother, or any real UIDs and GIDs on the host system.

So if you want to add another user bar, /etc/subuid and /etc/subgid will look like:

foo:100000:65536
bar:165536:65536

Resulting in the following allocation:

User

Host UID

Sub UID/GID range

foo

1000

100000 to 165535

bar

1001

165536 to 231071

Inside a user namespace / container, foo and bar can now act as any UID/GID between 0 and 65536, but these UIDs are confined to the container. For foo UID 0 in the container will map to the host foo UID 1000 and 1 to 65536 will map to 100000-165535 outside of the container etc. This impacts the ownership of files, which will have different IDs inside and outside of the container.

Note

If you are managing large numbers of fakeroot mappings you may wish to specify users by UID rather than username in the /etc/subuid and /etc/subgid files. The man page for subuid advises:

When large number of entries (10000-100000 or more) are defined in /etc/subuid, parsing performance penalty will become noticeable. In this case it is recommended to use UIDs instead of login names. Benchmarks have shown speed-ups up to 20x.

Filesystem considerations

Based on the above range, here we can see what happens when the user foo create files with --fakeroot feature:

Create file with container UID

Created host file owned by UID

0 (default)

1000

1 (daemon)

100000

2 (bin)

100001

Outside of the fakeroot container the user may not be able to remove directories and files created with a subuid, as they do not match with the user’s UID on the host. The user can remove these files by using a container shell running with fakeroot.

Network configuration

With fakeroot, users can request a container network named fakeroot, other networks are restricted and can only be used by the real host root user. By default the fakeroot network is configured to use a network veth pair.

Warning

Do not change the fakeroot network type in etc/singularity/network/40_fakeroot.conflist without considering the security implications.

Note

Unprivileged installations of Singularity cannot use fakeroot network as it requires privilege during container creation to setup the network.

Configuration with config fakeroot

Singularity 3.5 and above provides a config fakeroot command that can be used by a root user to administer local system /etc/subuid and /etc/subgid files in a simple manner. This allows users to be granted the ability to use Singularity’s fakeroot functionality without editing the files manually. The config fakeroot command will automatically ensure that generated subuid/subgid ranges are an approriate size, and do not overlap.

config fakeroot must be run as the root user, or via sudo singularity config fakeroot as the /etc/subuid and /etc/subgid files form part of the system configuration, and are security sensitive. You may --add or --remove user subuid/subgid mappings. You can also --enable or --disable existing mappings.

Note

If you deploy Singularity to a cluster you will need to make arrangements to synchronize /etc/subid and /etc/subgid mapping files to all nodes.

At this time, the glibc name service switch functionality does not support subuid or subgid mappings, so they cannot be definied in a central directory such as LDAP.

Adding a fakeroot mapping

Use the -a/--add <user> option to config fakeroot to create new mapping entries so that <user> can use the fakeroot feature of Singularity:

$ sudo singularity config fakeroot --add dave

# Show generated `/etc/subuid`
$ cat /etc/subuid
1000:4294836224:65536

# Show generated `/etc/subgid`
$ cat /etc/subgid
1000:4294836224:65536

The first subuid range will be set to the top of the 32-bit UID space. Subsequent subuid ranges for additional users will be created working down from this value. This minimizes the change of overlap with real UIDs on most systems.

Note

The config fakeroot command generates mappings specified using the user’s uid, rather than their username. This is the preferred format for faster lookups when configuring a large number of mappings, and the command can be used to manipulate these by username.

Deleting, disabling, enabling mappings

Use the -r/--remove <user> option to config fakeroot to completely remove mapping entries. The <user> will no longer be able to use the fakeroot feature of Singularity:

$ sudo singularity config fakeroot --remove dave

Warning

If a fakeroot mapping is removed, the subuid/subgid range may be assigned to another user via --add. Any remaining files from the prior user that were created with this mapping will be accessible to the new user via fakeroot.

The -d/--disable and -e/--enable options will comment and uncomment entries in the mapping files, to temporarily disable and subsequently re-enable fakeroot functionality for a user. This can be useful to disable fakeroot for a user, but ensure the subuid/subgid range assigned to them is reserved, and not re-assigned to a different user.

# Disable dave
$ sudo singularity config fakeroot --disable dave

# Entry is commented
$ cat /etc/subuid
!1000:4294836224:65536

# Enable dave
$ sudo singularity config fakeroot --enable dave

# Entry is active
$ cat /etc/subuid
1000:4294836224:65536