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.
17
182. User Interface
19
20An entry is added using devices.allow, and removed using
21devices.deny.  For instance
22
23	echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
24
25allows cgroup 1 to read and mknod the device usually known as
26/dev/null.  Doing
27
28	echo a > /sys/fs/cgroup/1/devices.deny
29
30will remove the default 'a *:* rwm' entry. Doing
31
32	echo a > /sys/fs/cgroup/1/devices.allow
33
34will add the 'a *:* rwm' entry to the whitelist.
35
363. Security
37
38Any task can move itself between cgroups.  This clearly won't
39suffice, but we can decide the best way to adequately restrict
40movement as people get some experience with this.  We may just want
41to require CAP_SYS_ADMIN, which at least is a separate bit from
42CAP_MKNOD.  We may want to just refuse moving to a cgroup which
43isn't a descendant of the current one.  Or we may want to use
44CAP_MAC_ADMIN, since we really are trying to lock down root.
45
46CAP_SYS_ADMIN is needed to modify the whitelist or move another
47task to a new cgroup.  (Again we'll probably want to change that).
48
49A cgroup may not be granted more permissions than the cgroup's
50parent has.
51
524. Hierarchy
53
54device cgroups maintain hierarchy by making sure a cgroup never has more
55access permissions than its parent.  Every time an entry is written to
56a cgroup's devices.deny file, all its children will have that entry removed
57from their whitelist and all the locally set whitelist entries will be
58re-evaluated.  In case one of the locally set whitelist entries would provide
59more access than the cgroup's parent, it'll be removed from the whitelist.
60
61Example:
62      A
63     / \
64        B
65
66    group        behavior	exceptions
67    A            allow		"b 8:* rwm", "c 116:1 rw"
68    B            deny		"c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm"
69
70If a device is denied in group A:
71	# echo "c 116:* r" > A/devices.deny
72it'll propagate down and after revalidating B's entries, the whitelist entry
73"c 116:2 rwm" will be removed:
74
75    group        whitelist entries                        denied devices
76    A            all                                      "b 8:* rwm", "c 116:* rw"
77    B            "c 1:3 rwm", "b 3:* rwm"                 all the rest
78
79In case parent's exceptions change and local exceptions are not allowed
80anymore, they'll be deleted.
81
82Notice that new whitelist entries will not be propagated:
83      A
84     / \
85        B
86
87    group        whitelist entries                        denied devices
88    A            "c 1:3 rwm", "c 1:5 r"                   all the rest
89    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
90
91when adding "c *:3 rwm":
92	# echo "c *:3 rwm" >A/devices.allow
93
94the result:
95    group        whitelist entries                        denied devices
96    A            "c *:3 rwm", "c 1:5 r"                   all the rest
97    B            "c 1:3 rwm", "c 1:5 r"                   all the rest
98
99but now it'll be possible to add new entries to B:
100	# echo "c 2:3 rwm" >B/devices.allow
101	# echo "c 50:3 r" >B/devices.allow
102or even
103	# echo "c *:3 rwm" >B/devices.allow
104
105Allowing or denying all by writing 'a' to devices.allow or devices.deny will
106not be possible once the device cgroups has children.
107
1084.1 Hierarchy (internal implementation)
109
110device cgroups is implemented internally using a behavior (ALLOW, DENY) and a
111list of exceptions.  The internal state is controlled using the same user
112interface to preserve compatibility with the previous whitelist-only
113implementation.  Removal or addition of exceptions that will reduce the access
114to devices will be propagated down the hierarchy.
115For every propagated exception, the effective rules will be re-evaluated based
116on current parent's access rules.
117