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Diffstat (limited to 'Documentation/cgroup-v1/devices.txt')
| -rw-r--r-- | Documentation/cgroup-v1/devices.txt | 116 |
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diff --git a/Documentation/cgroup-v1/devices.txt b/Documentation/cgroup-v1/devices.txt deleted file mode 100644 index 3c1095ca02ea..000000000000 --- a/Documentation/cgroup-v1/devices.txt +++ /dev/null @@ -1,116 +0,0 @@ -Device Whitelist Controller - -1. Description: - -Implement a cgroup to track and enforce open and mknod restrictions -on device files. A device cgroup associates a device access -whitelist with each cgroup. A whitelist entry has 4 fields. -'type' is a (all), c (char), or b (block). 'all' means it applies -to all types and all major and minor numbers. Major and minor are -either an integer or * for all. Access is a composition of r -(read), w (write), and m (mknod). - -The root device cgroup starts with rwm to 'all'. A child device -cgroup gets a copy of the parent. Administrators can then remove -devices from the whitelist or add new entries. A child cgroup can -never receive a device access which is denied by its parent. - -2. User Interface - -An entry is added using devices.allow, and removed using -devices.deny. For instance - - echo 'c 1:3 mr' > /sys/fs/cgroup/1/devices.allow - -allows cgroup 1 to read and mknod the device usually known as -/dev/null. Doing - - echo a > /sys/fs/cgroup/1/devices.deny - -will remove the default 'a *:* rwm' entry. Doing - - echo a > /sys/fs/cgroup/1/devices.allow - -will add the 'a *:* rwm' entry to the whitelist. - -3. Security - -Any task can move itself between cgroups. This clearly won't -suffice, but we can decide the best way to adequately restrict -movement as people get some experience with this. We may just want -to require CAP_SYS_ADMIN, which at least is a separate bit from -CAP_MKNOD. We may want to just refuse moving to a cgroup which -isn't a descendant of the current one. Or we may want to use -CAP_MAC_ADMIN, since we really are trying to lock down root. - -CAP_SYS_ADMIN is needed to modify the whitelist or move another -task to a new cgroup. (Again we'll probably want to change that). - -A cgroup may not be granted more permissions than the cgroup's -parent has. - -4. Hierarchy - -device cgroups maintain hierarchy by making sure a cgroup never has more -access permissions than its parent. Every time an entry is written to -a cgroup's devices.deny file, all its children will have that entry removed -from their whitelist and all the locally set whitelist entries will be -re-evaluated. In case one of the locally set whitelist entries would provide -more access than the cgroup's parent, it'll be removed from the whitelist. - -Example: - A - / \ - B - - group behavior exceptions - A allow "b 8:* rwm", "c 116:1 rw" - B deny "c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm" - -If a device is denied in group A: - # echo "c 116:* r" > A/devices.deny -it'll propagate down and after revalidating B's entries, the whitelist entry -"c 116:2 rwm" will be removed: - - group whitelist entries denied devices - A all "b 8:* rwm", "c 116:* rw" - B "c 1:3 rwm", "b 3:* rwm" all the rest - -In case parent's exceptions change and local exceptions are not allowed -anymore, they'll be deleted. - -Notice that new whitelist entries will not be propagated: - A - / \ - B - - group whitelist entries denied devices - A "c 1:3 rwm", "c 1:5 r" all the rest - B "c 1:3 rwm", "c 1:5 r" all the rest - -when adding "c *:3 rwm": - # echo "c *:3 rwm" >A/devices.allow - -the result: - group whitelist entries denied devices - A "c *:3 rwm", "c 1:5 r" all the rest - B "c 1:3 rwm", "c 1:5 r" all the rest - -but now it'll be possible to add new entries to B: - # echo "c 2:3 rwm" >B/devices.allow - # echo "c 50:3 r" >B/devices.allow -or even - # echo "c *:3 rwm" >B/devices.allow - -Allowing or denying all by writing 'a' to devices.allow or devices.deny will -not be possible once the device cgroups has children. - -4.1 Hierarchy (internal implementation) - -device cgroups is implemented internally using a behavior (ALLOW, DENY) and a -list of exceptions. The internal state is controlled using the same user -interface to preserve compatibility with the previous whitelist-only -implementation. Removal or addition of exceptions that will reduce the access -to devices will be propagated down the hierarchy. -For every propagated exception, the effective rules will be re-evaluated based -on current parent's access rules. |