117 lines
4.3 KiB
Plaintext
117 lines
4.3 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.
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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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4. Hierarchy
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device cgroups maintain hierarchy by making sure a cgroup never has more
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access permissions than its parent. Every time an entry is written to
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a cgroup's devices.deny file, all its children will have that entry removed
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from their whitelist and all the locally set whitelist entries will be
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re-evaluated. In case one of the locally set whitelist entries would provide
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more access than the cgroup's parent, it'll be removed from the whitelist.
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Example:
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A
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/ \
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B
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group behavior exceptions
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A allow "b 8:* rwm", "c 116:1 rw"
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B deny "c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm"
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If a device is denied in group A:
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# echo "c 116:* r" > A/devices.deny
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it'll propagate down and after revalidating B's entries, the whitelist entry
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"c 116:2 rwm" will be removed:
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group whitelist entries denied devices
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A all "b 8:* rwm", "c 116:* rw"
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B "c 1:3 rwm", "b 3:* rwm" all the rest
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In case parent's exceptions change and local exceptions are not allowed
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anymore, they'll be deleted.
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Notice that new whitelist entries will not be propagated:
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A
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/ \
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B
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group whitelist entries denied devices
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A "c 1:3 rwm", "c 1:5 r" all the rest
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B "c 1:3 rwm", "c 1:5 r" all the rest
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when adding "c *:3 rwm":
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# echo "c *:3 rwm" >A/devices.allow
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the result:
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group whitelist entries denied devices
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A "c *:3 rwm", "c 1:5 r" all the rest
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B "c 1:3 rwm", "c 1:5 r" all the rest
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but now it'll be possible to add new entries to B:
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# echo "c 2:3 rwm" >B/devices.allow
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# echo "c 50:3 r" >B/devices.allow
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or even
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# echo "c *:3 rwm" >B/devices.allow
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Allowing or denying all by writing 'a' to devices.allow or devices.deny will
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not be possible once the device cgroups has children.
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4.1 Hierarchy (internal implementation)
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device cgroups is implemented internally using a behavior (ALLOW, DENY) and a
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list of exceptions. The internal state is controlled using the same user
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interface to preserve compatibility with the previous whitelist-only
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implementation. Removal or addition of exceptions that will reduce the access
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to devices will be propagated down the hierarchy.
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For every propagated exception, the effective rules will be re-evaluated based
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on current parent's access rules.
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