[lxc-devel] cgroup management daemon

Serge E. Hallyn serge at hallyn.com
Mon Nov 25 22:43:35 UTC 2013


as i've mentioned several times, I want to write a standalone cgroup
management daemon.  Basic requirements are that it be a standalone
program; that a single instance running on the host be usable from
containers nested at any depth; that it not allow escaping ones
assigned limits; that it not allow subjegating tasks which do not
belong to you; and that, within your limits, you be able to parcel
those limits to your tasks as you like.  

Additionally, Tejun has specified that we do not want users to be
too closely tied to the cgroupfs implementation.  Therefore
commands will be just a hair more general than specifying cgroupfs
filenames and values.  I may go so far as to avoid specifying
specific controllers, as AFAIK there should be no redundancy in
features.  On the other hand, I don't want to get too general.
So I'm basing the API loosely on the lmctfy command line API.

One of the driving goals is to enable nested lxc as simply and safely as
possible.  If this project is a success, then a large chunk of code can
be removed from lxc.  I'm considering this project a part of the larger
lxc project, but given how central it is to systems management that
doesn't mean that I'll consider anyone else's needs as less important
than our own.

This document consists of two parts.  The first describes how I
intend the daemon (cgmanager) to be structured and how it will
enforce the safety requirements.  The second describes the commands 
which clients will be able to send to the manager.  The list of
controller keys which can be set is very incomplete at this point,
serving mainly to show the approach I was thinking of taking.


Each 'host' (identified by a separate instance of the linux kernel) will
have exactly one running daemon to manage control groups.  This daemon
will answer cgroup management requests over a dbus socket, located at
/sys/fs/cgroup/manager.  This socket can be bind-mounted into various
containers, so that one daemon can support the whole system.

Programs will be able to make cgroup requests using dbus calls, or
indirectly by linking against lmctfy which will be modified to use the
dbus calls if available.

  . A single manager, cgmanager, is started on the host, very early
    during boot.  It has very few dependencies, and requires only
    /proc, /run, and /sys to be mounted, with /etc ro.  It will mount
    the cgroup hierarchies in a private namespace and set defaults
    (clone_children, use_hierarchy, sane_behavior, release_agent?) It
    will open a socket at /sys/fs/cgroup/cgmanager (in a small tmpfs).
  . A client (requestor 'r') can make cgroup requests over
    /sys/fs/cgroup/manager using dbus calls.  Detailed privilege
    requirements for r are listed below.
  . The client request will pertain an existing or new cgroup A.  r's
    privilege over the cgroup must be checked.  r is said to have
    privilege over A if A is owned by r's uid, or if A's owner is mapped
    into r's user namespace, and r is root in that user namespace.
  . The client request may pertain a victim task v, which may be moved
    to a new cgroup.  In that case r's privilege over both the cgroup
    and v must be checked.  r is said to have privilege over v if v
    is mapped in r's pid namespace, v's uid is mapped into r's user ns,
    and r is root in its userns.  Or if r and v have the same uid
    and v is mapped in r's pid namespace.
  . r's credentials will be taken from socket's peercred, ensuring that
    pid and uid are translated.
  . r passes PID(v) as a SCM_CREDENTIAL, so that cgmanager receives the
    translated global pid.  It will then read UID(v) from /proc/PID(v)/status,
    which is the global uid, and check /proc/PID(r)/uid_map to see whether
    UID is mapped there.
  . dbus-send can be enhanced to send a pid as SCM_CREDENTIAL to have
    the kernel translate it for the reader.  Only 'move task v to cgroup
    A' will require a SCM_CREDENTIAL to be sent.

Privilege requirements by action:
    * Requestor of an action (r) over a socket may only make
      changes to cgroups over which it has privilege.
    * Requestors may be limited to a certain #/depth of cgroups
      (to limit memory usage) - DEFER?
    * Cgroup hierarchy is responsible for resource limits
    * A requestor must either be uid 0 in its userns with victim mapped
      ito its userns, or the same uid and in same/ancestor pidns as the
    * If r requests creation of cgroup '/x', /x will be interpreted
      as relative to r's cgroup.  r cannot make changes to cgroups not
      under its own current cgroup.
    * If r is not in the initial user_ns, then it may not change settings
      in its own cgroup, only descendants.  (Not strictly necessary -
      we could require the use of extra cgroups when wanted, as lxc does
    * If r requests creation of cgroup '/x', it must have write access
      to its own cgroup  (not strictly necessary)
    * If r requests chown of cgroup /x to uid Y, Y is passed in a
      ucred over the unix socket, and therefore translated to init
    * if r requests setting a limit under /x, then
      . either r must be root in its own userns, and UID(/x) be mapped
        into its userns, or else UID(r) == UID(/x)
      . /x must not be / (not strictly necessary, all users know to
        ensure an extra cgroup layer above '/')
      . setns(UIDNS(r)) would not work, due to in-kernel capable() checks
        which won't be satisfied.  Therefore we'll need to do privilege
        checks ourselves, then perform the write as the host root user.
        (see devices.allow/deny).  Further we need to support older kernels
        which don't support setns for pid.
    * If r requests action on victim V, it passes V's pid in a ucred,
      so that gets translated.
      Daemon will verify that V's uid is mapped into r's userns.  Since
      r is either root or the same uid as V, it is allowed to classify.

The above addresses
    * creating cgroups
    * chowning cgroups
    * setting cgroup limits
    * moving tasks into cgroups
  . but does not address a 'cgexec <group> -- command' type of behavior.
    * To handle that (specifically for upstart), recommend that r do:
      if (!pid) {
        request_reclassify(cgroup, getpid());
  . alternatively, the daemon could, if kernel is new enough, setns to
    the requestor's namespaces to execute a command in a new cgroup.
    The new command would be daemonized to that pid namespaces' pid 1.

Types of requests:
  * r requests creating cgroup A'/A
    . lmctfy/cli/commands/create.cc
    . Verify that UID(r) mapped to 0 in r's userns
    . R=cgroup_of(r)
    . Verify that UID(R) is mapped into r's userns
    . Create R/A'/A
    . chown R/A'/A to UID(r)
  * r requests to move task x to cgroup A.
    . lmctfy/cli/commands/enter.cc
    . r must send PID(x) as ancillary message
    . Verify that UID(r) mapped to 0 in r's userns, and UID(x) is mapped into
      that userns
      (is it safe to allow if UID(x) == UID(r))?
    . R=cgroup_of(r)
    . Verify that R/A is owned by UID(r) or UID(x)?  (not sure that's needed)
    . echo PID(x) >> /R/A/tasks
  * r requests chown of cgroup A to uid X
    . X is passed in ancillary message
      * ensures it is valid in r's userns
      * maps the userid to host for us
    . Verify that UID(r) mapped to 0 in r's userns
    . R=cgroup_of(r)
    . Chown R/A to X
  * r requests cgroup A's 'property=value'
    . Verify that either
      * A != ''
      * UID(r) == 0 on host
      In other words, r in a userns may not set root cgroup settings.
    . Verify that UID(r) mapped to 0 in r's userns
    . R=cgroup_of(r)
    . Set property=value for R/A
      * Expect kernel to guarantee hierarchical constraints
  * r requests deletion of cgroup A
    . lmctfy/cli/commands/destroy.cc (without -f)
    . same requirements as setting 'property=value'
  * r requests purge of cgroup A
    . lmctfy/cli/commands/destroy.cc (with -f)
    . same requirements as setting 'property=value'

Long-term we will want the cgroup manager to become more intelligent -
to place its own limits on clients, to address cpu and device hotplug,
etc.  Since we will not be doing that in the first prototype, the daemon
will not keep any state about the clients.

Client DBus Message API

<name>: a-zA-Z0-9
<name>: "a-zA-Z0-9 "
<controllerlist>: <controller1>[:controllerlist]
<valueentry>: key:value
<valueentry>: frozen
<valueentry>: thawed
<values>: valueentry[:values]
	cpus_allowed  # set of allowed cpus
	cpus_fraction # % of allowed cpus
	cpus_number   # number of allowed cpus
	cpu_share_percent   # percent of cpushare
	Create <name> <controllerlist> <values>
	Setvalue <name> <values>
	Getvalue <name> <readkeys>
	ListChildren <name>
	ListTasks <name>
	ListControllers <name>
	Chown <name> <uid>
	Chown <name> <uid>:<gid>
	Move <pid> <name>  [[ pid is sent as a SCM_CREDENTIAL ]]
	Delete <name>
	Delete-force <name>
	Kill <name>

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