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Documentation / cgroups / devices.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

1	Device Whitelist Controller
2	
3	1. Description:
4	
5	Implement a cgroup to track and enforce open and mknod restrictions
6	on device files.  A device cgroup associates a device access
7	whitelist with each cgroup.  A whitelist entry has 4 fields.
8	'type' is a (all), c (char), or b (block).  'all' means it applies
9	to all types and all major and minor numbers.  Major and minor are
10	either an integer or * for all.  Access is a composition of r
11	(read), w (write), and m (mknod).
12	
13	The root device cgroup starts with rwm to 'all'.  A child device
14	cgroup gets a copy of the parent.  Administrators can then remove
15	devices from the whitelist or add new entries.  A child cgroup can
16	never receive a device access which is denied by its parent.
17	
18	2. User Interface
19	
20	An entry is added using devices.allow, and removed using
21	devices.deny.  For instance
22	
23		echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
24	
25	allows 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	
30	will remove the default 'a *:* rwm' entry. Doing
31	
32		echo a > /sys/fs/cgroup/1/devices.allow
33	
34	will add the 'a *:* rwm' entry to the whitelist.
35	
36	3. Security
37	
38	Any task can move itself between cgroups.  This clearly won't
39	suffice, but we can decide the best way to adequately restrict
40	movement as people get some experience with this.  We may just want
41	to require CAP_SYS_ADMIN, which at least is a separate bit from
42	CAP_MKNOD.  We may want to just refuse moving to a cgroup which
43	isn't a descendant of the current one.  Or we may want to use
44	CAP_MAC_ADMIN, since we really are trying to lock down root.
45	
46	CAP_SYS_ADMIN is needed to modify the whitelist or move another
47	task to a new cgroup.  (Again we'll probably want to change that).
48	
49	A cgroup may not be granted more permissions than the cgroup's
50	parent has.
51	
52	4. Hierarchy
53	
54	device cgroups maintain hierarchy by making sure a cgroup never has more
55	access permissions than its parent.  Every time an entry is written to
56	a cgroup's devices.deny file, all its children will have that entry removed
57	from their whitelist and all the locally set whitelist entries will be
58	re-evaluated.  In case one of the locally set whitelist entries would provide
59	more access than the cgroup's parent, it'll be removed from the whitelist.
60	
61	Example:
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	
70	If a device is denied in group A:
71		# echo "c 116:* r" > A/devices.deny
72	it'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	
79	In case parent's exceptions change and local exceptions are not allowed
80	anymore, they'll be deleted.
81	
82	Notice 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	
91	when adding "c *:3 rwm":
92		# echo "c *:3 rwm" >A/devices.allow
93	
94	the 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	
99	but 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
102	or even
103		# echo "c *:3 rwm" >B/devices.allow
104	
105	Allowing or denying all by writing 'a' to devices.allow or devices.deny will
106	not be possible once the device cgroups has children.
107	
108	4.1 Hierarchy (internal implementation)
109	
110	device cgroups is implemented internally using a behavior (ALLOW, DENY) and a
111	list of exceptions.  The internal state is controlled using the same user
112	interface to preserve compatibility with the previous whitelist-only
113	implementation.  Removal or addition of exceptions that will reduce the access
114	to devices will be propagated down the hierarchy.
115	For every propagated exception, the effective rules will be re-evaluated based
116	on current parent's access rules.
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