linux/Documentation/cgroups/devices.txt
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   1Device Whitelist Controller
   2
   31. Description:
   4
   5Implement a cgroup to track and enforce open and mknod restrictions
   6on device files.  A device cgroup associates a device access
   7whitelist with each cgroup.  A whitelist entry has 4 fields.
   8'type' is a (all), c (char), or b (block).  'all' means it applies
   9to all types and all major and minor numbers.  Major and minor are
  10either an integer or * for all.  Access is a composition of r
  11(read), w (write), and m (mknod).
  12
  13The root device cgroup starts with rwm to 'all'.  A child device
  14cgroup gets a copy of the parent.  Administrators can then remove
  15devices from the whitelist or add new entries.  A child cgroup can
  16never receive a device access which is denied by its parent.  However
  17when a device access is removed from a parent it will not also be
  18removed from the child(ren).
  19
  202. User Interface
  21
  22An entry is added using devices.allow, and removed using
  23devices.deny.  For instance
  24
  25        echo 'c 1:3 mr' > /cgroups/1/devices.allow
  26
  27allows cgroup 1 to read and mknod the device usually known as
  28/dev/null.  Doing
  29
  30        echo a > /cgroups/1/devices.deny
  31
  32will remove the default 'a *:* rwm' entry. Doing
  33
  34        echo a > /cgroups/1/devices.allow
  35
  36will add the 'a *:* rwm' entry to the whitelist.
  37
  383. Security
  39
  40Any task can move itself between cgroups.  This clearly won't
  41suffice, but we can decide the best way to adequately restrict
  42movement as people get some experience with this.  We may just want
  43to require CAP_SYS_ADMIN, which at least is a separate bit from
  44CAP_MKNOD.  We may want to just refuse moving to a cgroup which
  45isn't a descendent of the current one.  Or we may want to use
  46CAP_MAC_ADMIN, since we really are trying to lock down root.
  47
  48CAP_SYS_ADMIN is needed to modify the whitelist or move another
  49task to a new cgroup.  (Again we'll probably want to change that).
  50
  51A cgroup may not be granted more permissions than the cgroup's
  52parent has.
  53
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