cgroup_namespaces

CGROUP_NAMESPACES(7)       Linux Programmer's Manual      CGROUP_NAMESPACES(7)



NAME
       cgroup_namespaces - overview of Linux cgroup namespaces

DESCRIPTION
       For an overview of namespaces, see namespaces(7).

       Cgroup namespaces virtualize the view of a process's cgroups (see
       cgroups(7)) as seen via /proc/[pid]/cgroup and /proc/[pid]/mountinfo.

       Each cgroup namespace has its own set of cgroup root directories.
       These root directories are the base points for the relative locations
       displayed in the corresponding records in the /proc/[pid]/cgroup file.
       When a process creates a new cgroup namespace using clone(2) or
       unshare(2) with the CLONE_NEWCGROUP flag, its current cgroups
       directories become the cgroup root directories of the new namespace.
       (This applies both for the cgroups version 1 hierarchies and the
       cgroups version 2 unified hierarchy.)

       When reading the cgroup memberships of a "target" process from
       /proc/[pid]/cgroup, the pathname shown in the third field of each
       record will be relative to the reading process's root directory for the
       corresponding cgroup hierarchy.  If the cgroup directory of the target
       process lies outside the root directory of the reading process's cgroup
       namespace, then the pathname will show ../ entries for each ancestor
       level in the cgroup hierarchy.

       The following shell session demonstrates the effect of creating a new
       cgroup namespace.

       First, (as superuser) in a shell in the initial cgroup namespace, we
       create a child cgroup in the freezer hierarchy, and place a process in
       that cgroup that we will use as part of the demonstration below:

           # mkdir -p /sys/fs/cgroup/freezer/sub2
           # sleep 10000 &     # Create a process that lives for a while
           [1] 20124
           # echo 20124 > /sys/fs/cgroup/freezer/sub2/cgroup.procs

       We then create another child cgroup in the freezer hierarchy and put
       the shell into that cgroup:

           # mkdir -p /sys/fs/cgroup/freezer/sub
           # echo $$                      # Show PID of this shell
           30655
           # echo 30655 > /sys/fs/cgroup/freezer/sub/cgroup.procs
           # cat /proc/self/cgroup | grep freezer
           7:freezer:/sub

       Next, we use unshare(1) to create a process running a new shell in new
       cgroup and mount namespaces:

           # PS1="sh2# " unshare -Cm bash

       From the new shell started by unshare(1), we then inspect the
       /proc/[pid]/cgroup files of, respectively, the new shell, a process
       that is in the initial cgroup namespace (init, with PID 1), and the
       process in the sibling cgroup (sub2):

           sh2# cat /proc/self/cgroup | grep freezer
           7:freezer:/
           sh2# cat /proc/1/cgroup | grep freezer
           7:freezer:/..
           sh2# cat /proc/20124/cgroup | grep freezer
           7:freezer:/../sub2

       From the output of the first command, we see that the freezer cgroup
       membership of the new shell (which is in the same cgroup as the initial
       shell) is shown defined relative to the freezer cgroup root directory
       that was established when the new cgroup namespace was created.  (In
       absolute terms, the new shell is in the /sub freezer cgroup, and the
       root directory of the freezer cgroup hierarchy in the new cgroup
       namespace is also /sub.  Thus, the new shell's cgroup membership is
       displayed as '/'.)

       However, when we look in /proc/self/mountinfo we see the following
       anomaly:

           sh2# cat /proc/self/mountinfo | grep freezer
           155 145 0:32 /.. /sys/fs/cgroup/freezer ...

       The fourth field of this line (/..)  should show the directory in the
       cgroup filesystem which forms the root of this mount.  Since by the
       definition of cgroup namespaces, the process's current freezer cgroup
       directory became its root freezer cgroup directory, we should see '/'
       in this field.  The problem here is that we are seeing a mount entry
       for the cgroup filesystem corresponding to the initial cgroup namespace
       (whose cgroup filesystem is indeed rooted at the parent directory of
       sub).  To fix this problem, we must remount the freezer cgroup
       filesystem from the new shell (i.e., perform the mount from a process
       that is in the new cgroup namespace), after which we see the expected
       results:

           sh2# mount --make-rslave /     # Don't propagate mount events
                                          # to other namespaces
           sh2# umount /sys/fs/cgroup/freezer
           sh2# mount -t cgroup -o freezer freezer /sys/fs/cgroup/freezer
           sh2# cat /proc/self/mountinfo | grep freezer
           155 145 0:32 / /sys/fs/cgroup/freezer rw,relatime ...

CONFORMING TO
       Namespaces are a Linux-specific feature.

NOTES
       Use of cgroup namespaces requires a kernel that is configured with the
       CONFIG_CGROUPS option.

       The virtualization provided by cgroup namespaces serves a number of
       purposes:

       * It prevents information leaks whereby cgroup directory paths outside
         of a container would otherwise be visible to processes in the
         container.  Such leakages could, for example, reveal information
         about the container framework to containerized applications.

       * It eases tasks such as container migration.  The virtualization
         provided by cgroup namespaces allows containers to be isolated from
         knowledge of the pathnames of ancestor cgroups.  Without such
         isolation, the full cgroup pathnames (displayed in
         /proc/self/cgroups) would need to be replicated on the target system
         when migrating a container; those pathnames would also need to be
         unique, so that they don't conflict with other pathnames on the
         target system.

       * It allows better confinement of containerized processes, because it
         is possible to mount the container's cgroup filesystems such that the
         container processes can't gain access to ancestor cgroup directories.
         Consider, for example, the following scenario:

           • We have a cgroup directory, /cg/1, that is owned by user ID 9000.

           • We have a process, X, also owned by user ID 9000, that is
             namespaced under the cgroup /cg/1/2 (i.e., X was placed in a new
             cgroup namespace via clone(2) or unshare(2) with the
             CLONE_NEWCGROUP flag).

         In the absence of cgroup namespacing, because the cgroup directory
         /cg/1 is owned (and writable) by UID 9000 and process X is also owned
         by user ID 9000, process X would be able to modify the contents of
         cgroups files (i.e., change cgroup settings) not only in /cg/1/2 but
         also in the ancestor cgroup directory /cg/1.  Namespacing process X
         under the cgroup directory /cg/1/2, in combination with suitable
         mount operations for the cgroup filesystem (as shown above), prevents
         it modifying files in /cg/1, since it cannot even see the contents of
         that directory (or of further removed cgroup ancestor directories).
         Combined with correct enforcement of hierarchical limits, this
         prevents process X from escaping the limits imposed by ancestor
         cgroups.

SEE ALSO
       unshare(1), clone(2), setns(2), unshare(2), proc(5), cgroups(7),
       credentials(7), namespaces(7), user_namespaces(7)

COLOPHON
       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-08-02              CGROUP_NAMESPACES(7)