Device Whitelist Controller
  
  1. Description:
  
  Implement a cgroup to track and enforce open and mknod restrictions
  on device files.  A device cgroup associates a device access
  whitelist with each cgroup.  A whitelist entry has 4 fields.
  'type' is a (all), c (char), or b (block).  'all' means it applies
  to all types and all major and minor numbers.  Major and minor are
  either an integer or * for all.  Access is a composition of r
  (read), w (write), and m (mknod).
  
  The root device cgroup starts with rwm to 'all'.  A child device
  cgroup gets a copy of the parent.  Administrators can then remove
  devices from the whitelist or add new entries.  A child cgroup can

  never receive a device access which is denied by its parent.  However

  when a device access is removed from a parent it will not also be
  removed from the child(ren).
  
  2. User Interface
  
  An entry is added using devices.allow, and removed using
  devices.deny.  For instance

  	echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow

  
  allows cgroup 1 to read and mknod the device usually known as
  /dev/null.  Doing

  	echo a > /sys/fs/cgroup/1/devices.deny

  will remove the default 'a *:* rwm' entry. Doing

  	echo a > /sys/fs/cgroup/1/devices.allow

  
  will add the 'a *:* rwm' entry to the whitelist.

  
  3. Security
  
  Any task can move itself between cgroups.  This clearly won't
  suffice, but we can decide the best way to adequately restrict
  movement as people get some experience with this.  We may just want
  to require CAP_SYS_ADMIN, which at least is a separate bit from
  CAP_MKNOD.  We may want to just refuse moving to a cgroup which

  isn't a descendant of the current one.  Or we may want to use

  CAP_MAC_ADMIN, since we really are trying to lock down root.
  
  CAP_SYS_ADMIN is needed to modify the whitelist or move another
  task to a new cgroup.  (Again we'll probably want to change that).
  
  A cgroup may not be granted more permissions than the cgroup's
  parent has.