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
       containers 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
       unprivileged  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
       controllers  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=$namedcontroller”.  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

SEE ALSO

       lxc-cgroup(1), cgroups(7), user_namespaces(7), namespaces(7), pam(8)

AUTHOR

       Venkata Harshavardhan Reddy Allu <venkataharshavardhan_ven@srmuniv.edu.in>

                                                   2024-10-30                                        pam_cgfs(8)