53 lines
1.9 KiB
Plaintext
53 lines
1.9 KiB
Plaintext
Device Whitelist Controller
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1. Description:
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Implement a cgroup to track and enforce open and mknod restrictions
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on device files. A device cgroup associates a device access
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whitelist with each cgroup. A whitelist entry has 4 fields.
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'type' is a (all), c (char), or b (block). 'all' means it applies
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to all types and all major and minor numbers. Major and minor are
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either an integer or * for all. Access is a composition of r
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(read), w (write), and m (mknod).
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The root device cgroup starts with rwm to 'all'. A child device
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cgroup gets a copy of the parent. Administrators can then remove
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devices from the whitelist or add new entries. A child cgroup can
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never receive a device access which is denied by its parent. However
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when a device access is removed from a parent it will not also be
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removed from the child(ren).
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2. User Interface
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An entry is added using devices.allow, and removed using
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devices.deny. For instance
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echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow
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allows cgroup 1 to read and mknod the device usually known as
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/dev/null. Doing
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echo a > /sys/fs/cgroup/1/devices.deny
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will remove the default 'a *:* rwm' entry. Doing
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echo a > /sys/fs/cgroup/1/devices.allow
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will add the 'a *:* rwm' entry to the whitelist.
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3. Security
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Any task can move itself between cgroups. This clearly won't
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suffice, but we can decide the best way to adequately restrict
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movement as people get some experience with this. We may just want
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to require CAP_SYS_ADMIN, which at least is a separate bit from
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CAP_MKNOD. We may want to just refuse moving to a cgroup which
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isn't a descendant of the current one. Or we may want to use
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CAP_MAC_ADMIN, since we really are trying to lock down root.
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CAP_SYS_ADMIN is needed to modify the whitelist or move another
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task to a new cgroup. (Again we'll probably want to change that).
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A cgroup may not be granted more permissions than the cgroup's
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parent has.
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