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Documentation / controllers / devices.txt

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Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.

1	Device Whitelist Controller
3	1. Description:
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).
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.  However
17	when a device access is removed from a parent it will not also be
18	removed from the child(ren).
20	2. User Interface
22	An entry is added using devices.allow, and removed using
23	devices.deny.  For instance
25		echo 'c 1:3 mr' > /cgroups/1/devices.allow
27	allows cgroup 1 to read and mknod the device usually known as
28	/dev/null.  Doing
30		echo a > /cgroups/1/devices.deny
32	will remove the default 'a *:* rwm' entry. Doing
34		echo a > /cgroups/1/devices.allow
36	will add the 'a *:* rwm' entry to the whitelist.
38	3. Security
40	Any task can move itself between cgroups.  This clearly won't
41	suffice, but we can decide the best way to adequately restrict
42	movement as people get some experience with this.  We may just want
43	to require CAP_SYS_ADMIN, which at least is a separate bit from
44	CAP_MKNOD.  We may want to just refuse moving to a cgroup which
45	isn't a descendent of the current one.  Or we may want to use
46	CAP_MAC_ADMIN, since we really are trying to lock down root.
48	CAP_SYS_ADMIN is needed to modify the whitelist or move another
49	task to a new cgroup.  (Again we'll probably want to change that).
51	A cgroup may not be granted more permissions than the cgroup's
52	parent has.
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