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 set up a container without privileged operations, so that a normal user can act as root inside a container to perform administrative tasks without being root on the host outside.

User namespaces are required whenever the setuid-root component of Apptainer is not installed. The default installation of Apptainer does not include a setuid-root component. Pros and cons of a setuid-root installation are discussed in the Security section of the user guide.

User Namespace Requirements

To allow unprivileged creation of user namespaces a kernel >=3.8 is required, with >=4.18 being recommended due to support for unprivileged mounting of FUSE filesystems (needed for example for mounting SIF files). The equivalent recommendation on RHEL7 is >=3.10.0-1127 from release 7.8, where unprivileged mounting of FUSE filesystems was backported. To use unprivileged overlayFS for persistent overlays, kernel >=5.11 is recommended, but if that’s not available then Apptainer will use fuse-overlayfs instead. That feature has not been backported to RHEL7.

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/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_user_namespaces.conf'
sudo sysctl -p /etc/sysctl.d/90-max_user_namespaces.conf

Disabling network namespaces

There have been many Linux kernel exploits that have made use of unprivileged user namespaces as a point of entry, but almost all of them in the last few years have been in combination with network namespaces. Therefore even though the Apptainer project recommends enabling unprivileged user namespaces, it recommends disabling network namespaces when possible in order to substantially reduce the risk profile and need for urgent updates when vulnerabilities are announced.

Network namespaces can be disabled on most Linux-based systems like this:

echo "user.max_net_namespaces = 0" \
     >/etc/sysctl.d/90-max_net_namespaces.conf
sysctl -p /etc/sysctl.d/90-max_net_namespaces.conf

Apptainer does not by default make use of network namespaces, but it does have some little-used privileged options beginning with --net that do. Those options will not work when network namespaces are disabled. Unfortunately it is not possible to disable only unprivileged network namespaces, so this will affect programs that use them even if run as root.

Some other container runtimes such as Docker and Podman do make use of network namespaces by default. Those two runtimes can still work when network namespaces are disabled by adding the --net=host option.

Disabling network namespaces also blocks the systemd PrivateNetwork feature. To find services that use it, look for PrivateNetwork=true or PrivateNetwork=yes in /lib/systemd/system/*.service. This can be turned off for each service through a /etc/systemd/system/<service>.d/*.conf file, for example for systemd-hostnamed:

cd /etc/systemd/system
mkdir -p systemd-hostnamed.service.d
(echo "[Service]"; echo "PrivateNetwork=no") \
     >systemd-hostnamed.service.d/no-private-network.conf

If the service is enabled (that is, actively used) then restart it and check its status:

systemctl status systemd-hostnamed
systemctl daemon-reload
systemctl restart systemd-hostnamed
systemctl status systemd-hostnamed

“Rootless” Fakeroot feature

The --fakeroot option available on many Apptainer functions allows an unprivileged user to run a container with the appearance of running as root. Apptainer does this in multiple different ways depending on what is available on the system. See details in the Fakeroot feature section of the user guide.

The most complete method of emulating running as root, the method used for example by Podman and also commonly referred to as “rootless mode”, requires administrator setup and also requires some assistance from an enhanced-privilege program. The rest of the documentation on this page describes how to do that setup. If this setup is done, Apptainer will take advantage of it and use it, otherwise it will try to use one of its other methods when the --fakeroot option is used. The other methods do not require any special administrator setup.

This mode not only maps the root user to the original unprivileged user, but it also maps many additional UIDs and GIDs to otherwise unused UIDs and GIDs on the host, via user namespace UID/GID mapping.

User namespace UID/GID mapping allows a user to act as different UIDs/GIDs 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 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, for rootless fakeroot mode Apptainer must manipulate subuid and subgid maps for the user namespace it creates. When Apptainer is installed as setuid-root, it handles doing the manipulations itself. With non-suid installations of Apptainer or where allow setuid = no is set in apptainer.conf, Apptainer attempts to use external setuid binaries newuidmap and newgidmap, so you need to install those binaries on your system.

Basics

Rootless 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 user namespace. A user must have an entry in these system configuration files to use the fakeroot feature. Apptainer 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 rootless fakeroot will be used.

Warning

Apptainer 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 each other, 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 the rootless --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 rootless 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/apptainer/network/40_fakeroot.conflist without considering the security implications.

Note

Unprivileged installations of Apptainer cannot use fakeroot network as it requires privilege during container creation to set up the network.

Configuration with config fakeroot

Apptainer 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 Apptainer’s fakeroot functionality without editing the files manually. The config fakeroot command will automatically ensure that generated subuid/subgid ranges are an appropriate size, and do not overlap.

config fakeroot must be run as the root user, or via sudo apptainer 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 Apptainer to a cluster you will need to make arrangements to synchronize /etc/subuid 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 defined 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 Apptainer:

$ sudo apptainer 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 Apptainer:

$ sudo apptainer 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 apptainer config fakeroot --disable dave

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

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

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