devices.rst

  1===========================
  2Device Whitelist Controller
  3===========================
  4
  51. Description
  6==============
  7
  8Implement a cgroup to track and enforce open and mknod restrictions
  9on device files.  A device cgroup associates a device access
 10whitelist with each cgroup.  A whitelist entry has 4 fields.
 11'type' is a (all), c (char), or b (block).  'all' means it applies
 12to all types and all major and minor numbers.  Major and minor are
 13either an integer or * for all.  Access is a composition of r
 14(read), w (write), and m (mknod).
 15
 16The root device cgroup starts with rwm to 'all'.  A child device
 17cgroup gets a copy of the parent.  Administrators can then remove
 18devices from the whitelist or add new entries.  A child cgroup can
 19never receive a device access which is denied by its parent.
 20
 212. User Interface
 22=================
 23
 24An entry is added using devices.allow, and removed using
 25devices.deny.  For instance::
 26
 27	echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
 28
 29allows cgroup 1 to read and mknod the device usually known as
 30/dev/null.  Doing::
 31
 32	echo a > /sys/fs/cgroup/1/devices.deny
 33
 34will remove the default 'a *:* rwm' entry. Doing::
 35
 36	echo a > /sys/fs/cgroup/1/devices.allow
 37
 38will add the 'a *:* rwm' entry to the whitelist.
 39
 403. Security
 41===========
 42
 43Any task can move itself between cgroups.  This clearly won't
 44suffice, but we can decide the best way to adequately restrict
 45movement as people get some experience with this.  We may just want
 46to require CAP_SYS_ADMIN, which at least is a separate bit from
 47CAP_MKNOD.  We may want to just refuse moving to a cgroup which
 48isn't a descendant of the current one.  Or we may want to use
 49CAP_MAC_ADMIN, since we really are trying to lock down root.
 50
 51CAP_SYS_ADMIN is needed to modify the whitelist or move another
 52task to a new cgroup.  (Again we'll probably want to change that).
 53
 54A cgroup may not be granted more permissions than the cgroup's
 55parent has.
 56
 574. Hierarchy
 58============
 59
 60device cgroups maintain hierarchy by making sure a cgroup never has more
 61access permissions than its parent.  Every time an entry is written to
 62a cgroup's devices.deny file, all its children will have that entry removed
 63from their whitelist and all the locally set whitelist entries will be
 64re-evaluated.  In case one of the locally set whitelist entries would provide
 65more access than the cgroup's parent, it'll be removed from the whitelist.
 66
 67Example::
 68
 69      A
 70     / \
 71        B
 72
 73    group        behavior	exceptions
 74    A            allow		"b 8:* rwm", "c 116:1 rw"
 75    B            deny		"c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm"
 76
 77If a device is denied in group A::
 78
 79	# echo "c 116:* r" > A/devices.deny
 80
 81it'll propagate down and after revalidating B's entries, the whitelist entry
 82"c 116:2 rwm" will be removed::
 83
 84    group        whitelist entries                        denied devices
 85    A            all                                      "b 8:* rwm", "c 116:* rw"
 86    B            "c 1:3 rwm", "b 3:* rwm"                 all the rest
 87
 88In case parent's exceptions change and local exceptions are not allowed
 89anymore, they'll be deleted.
 90
 91Notice that new whitelist entries will not be propagated::
 92
 93      A
 94     / \
 95        B
 96
 97    group        whitelist entries                        denied devices
 98    A            "c 1:3 rwm", "c 1:5 r"                   all the rest
 99    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
100
101when adding ``c *:3 rwm``::
102
103	# echo "c *:3 rwm" >A/devices.allow
104
105the result::
106
107    group        whitelist entries                        denied devices
108    A            "c *:3 rwm", "c 1:5 r"                   all the rest
109    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
110
111but now it'll be possible to add new entries to B::
112
113	# echo "c 2:3 rwm" >B/devices.allow
114	# echo "c 50:3 r" >B/devices.allow
115
116or even::
117
118	# echo "c *:3 rwm" >B/devices.allow
119
120Allowing or denying all by writing 'a' to devices.allow or devices.deny will
121not be possible once the device cgroups has children.
122
1234.1 Hierarchy (internal implementation)
124---------------------------------------
125
126device cgroups is implemented internally using a behavior (ALLOW, DENY) and a
127list of exceptions.  The internal state is controlled using the same user
128interface to preserve compatibility with the previous whitelist-only
129implementation.  Removal or addition of exceptions that will reduce the access
130to devices will be propagated down the hierarchy.
131For every propagated exception, the effective rules will be re-evaluated based
132on current parent's access rules.