Ubuntu Manpage: pam_cgfs - cgroup management for unprivileged LXC containers.

NAME

       pam_cgfs - cgroup management for unprivileged LXC containers.

SYNOPSIS

       pam_cgfs.so {-c kernel_controller,name=named_controller}

DESCRIPTION

LXC has supported fully unprivileged containers since LXC 1.0. Fully unprivileged containers are the
safest containers and are run by normal (non-root) users. This is achieved by using user namespaces by
mapping between a range of UIDs and GIDs on the host to a different (unprivileged) range of UIDs and GIDs
in the container. That means the uid 0 (root) in the container is mapped to an unprivileged user id
(something like 1000000) outside of the container and only has rights on resources that it owns itself.

Cgroup management of fully unprivileged containers means restricting the resources used by these contain‐
ers like limiting the CPU usage of a container, or the number of processes it is allowed to spawn, or the
memory it is allowed to consume. It is clear that the fully unprivileged containers are run by normal
users and there is a need to limit and manage resource consumption among the containers. But unprivi‐
leged cgroup management is not easy with most init systems. So, the pam_cgfs.so came into existence.

The pam_cgfs.so module can handle pure cgroupfs v1 (/sys/fs/cgroup/$controller) and mixed mounts, where
some controllers are mounted in a standard cgroupfs v1 hierarchy (/sys/fs/cgroup/$controller) and others
in cgroupfs v2 hierarchy (/sys/fs/cgroup/unified). Writeable cgroups are either created for all con‐
trollers or, if specified, for only controllers listed as arguments on the command line. Pure cgroup v2
mount is not covered by the pam_cgfs.so module.

The cgroup created user/$user/n will be for the nth session under cgroup kernel controller hierarchy.

Systems with a systemd init system are treated specifically, both with respect to cgroupfs v1 and
cgroupfs v2. For both, cgroupfs v1 and cgroupfs v2, the module checks whether systemd already placed the
user in a cgroup it created user.slice/user-$uid/session-n.scope by checking whether $uid == login uid.
If so, the login user chown the session-n.scope, else a cgroup is created as outlined above
(user/$user/n) and chown it to login uid. If the init system has already placed the login user inside a
session specific group, the pam_cgfs.so module is smart enough to detect it and re-use the cgroup.

In essence, the pam_cgfs.so module takes care of placing unprivileged (non-root) users into writable
cgroups at login and also cleaning up these cgroup hierarchies on logout, so they are free to delegate
resources to containers as needed that have been provided to them.

OPTIONS

       -c controller-list
              Takes  a string argument which sets the list of kernel controllers and named controllers delimited
              by commas in-between “,”. Named controllers need to be  specified  in  the  form  “name=$namedcon‐
              troller”.  Can  use  “all” enable all cgroup resource controller hierarchies. Specifying “all” and
              other controllers explicitly returns PAM_SESSION_ERR.

MODULE TYPES PROVIDED

       Only session module type is provided (and needed).

RETURN VALUES

       PAM_SUCCESS
              Writeable cgroups have been created for the user.

       PAM_SESSION_ERR
              Failed to create writable cgroups for the user.

FILES

       /etc/pam.d/common-session{,-noninteractive}
              Default configuration is added at the end of these files.

EXAMPLE

       session optional    pam_cgfs.so -c freezer,memory,named=systemd
       # default configuration
       # user writable cgroups are created under freezer, memory and named cgroup systemd hierarchies.
       # /sys/fs/cgroup/$controller/user/$user/n for freezer,memory.
       # /sys/fs/cgroup/systemd/user.slice/user-$uid/session-n.scope for systemd.

       session optional    pam_cgfs.so -c all
       # user writable cgroups are created under all cgroup controllers.

       session optional    pam_cgfs.so -c all,memory,freezer
       # invalid argument and returns PAM_SESSION_ERR