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, it enters a new cgroup names-
       pace  in  which  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  viewing /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) we create a 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
           # sh -c '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:

           # unshare -Cm bash

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

           $ cat /proc/self/cgroup | grep freezer
           7:freezer:/
           $ cat /proc/1/cgroup | grep freezer
           7:freezer:/..
           $ 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 names-
       pace is also /sub.  Thus, the new shell's  cgroup  membership  is  dis-
       played as '/'.)

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

           # 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 our initial shell process's
       cgroup namespace (whose cgroup filesystem is indeed rooted in the  par-
       ent  directory of sub).  We need to remount the freezer cgroup filesys-
       tem inside this cgroup namespace,  after  which  we  see  the  expected
       results:

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

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

CONFORMING TO
       Namespaces are a Linux-specific feature.

NOTES
       Among the purposes served by  the  virtualization  provided  by  cgroup
       namespaces are the following:

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

       * It eases tasks such as container migration.  The virtualization  pro-
         vided  by  cgroup  namespaces  allows  containers to be isolated from
         knowledge of the pathnames of ancestor cgroups.  Without such  isola-
         tion,  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:

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

           o We  have a process, X, also owned by user ID 9000, that is names-
             paced 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_NEWC-
             GROUP 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, then 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 pre-
         vents process X from escaping the limits imposed by ancestor cgroups.

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

COLOPHON
       This  page  is  part of release 4.15 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                             2017-09-15              CGROUP_NAMESPACES(7)
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