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.
133