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.  However
17when a device access is removed from a parent it will not also be
18removed from the child(ren).
19
202. User Interface
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22An entry is added using devices.allow, and removed using
23devices.deny.  For instance
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25	echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
26
27allows cgroup 1 to read and mknod the device usually known as
28/dev/null.  Doing
29
30	echo a > /sys/fs/cgroup/1/devices.deny
31
32will remove the default 'a *:* rwm' entry. Doing
33
34	echo a > /sys/fs/cgroup/1/devices.allow
35
36will add the 'a *:* rwm' entry to the whitelist.
37
383. Security
39
40Any task can move itself between cgroups.  This clearly won't
41suffice, but we can decide the best way to adequately restrict
42movement as people get some experience with this.  We may just want
43to require CAP_SYS_ADMIN, which at least is a separate bit from
44CAP_MKNOD.  We may want to just refuse moving to a cgroup which
45isn't a descendant of the current one.  Or we may want to use
46CAP_MAC_ADMIN, since we really are trying to lock down root.
47
48CAP_SYS_ADMIN is needed to modify the whitelist or move another
49task to a new cgroup.  (Again we'll probably want to change that).
50
51A cgroup may not be granted more permissions than the cgroup's
52parent has.
53