Commit 9157056d authored Sep 23, 2016 by Tejun Heo

cgroup: fix invalid controller enable rejections with cgroup namespace



On the v2 hierarchy, "cgroup.subtree_control" rejects controller
enables if the cgroup has processes in it.  The enforcement of this
logic assumes that the cgroup wouldn't have any css_sets associated
with it if there are no tasks in the cgroup, which is no longer true
since a79a908f ("cgroup: introduce cgroup namespaces").

When a cgroup namespace is created, it pins the css_set of the
creating task to use it as the root css_set of the namespace.  This
extra reference stays as long as the namespace is around and makes
"cgroup.subtree_control" think that the namespace root cgroup is not
empty even when it is and thus reject controller enables.

Fix it by making cgroup_subtree_control() walk and test emptiness of
each css_set instead of testing whether the list_head is empty.

While at it, update the comment of cgroup_task_count() to indicate
that the returned value may be higher than the number of tasks, which
has always been true due to temporary references and doesn't break
anything.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Evgeny Vereshchagin <evvers@ya.ru>
Cc: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: Aditya Kali <adityakali@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: stable@vger.kernel.org # v4.6+
Fixes: a79a908f ("cgroup: introduce cgroup namespaces")
Link: https://github.com/systemd/systemd/pull/3589#issuecomment-249089541

parent 8a15b817

kernel/cgroup.c

+25 −4

Original line number	Diff line number	Diff line
		@@ -3446,8 +3446,27 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
		* Except for the root, subtree_control must be zero for a cgroup
		* with tasks so that child cgroups don't compete against tasks.
		*/
		if (enable && cgroup_parent(cgrp) && !list_empty(&cgrp->cset_links)) {
		if (enable && cgroup_parent(cgrp)) {
		struct cgrp_cset_link *link;

		/*
		* Because namespaces pin csets too, @cgrp->cset_links
		* might not be empty even when @cgrp is empty. Walk and
		* verify each cset.
		*/
		spin_lock_irq(&css_set_lock);

		ret = 0;
		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
		if (css_set_populated(link->cset)) {
		ret = -EBUSY;
		break;
		}
		}

		spin_unlock_irq(&css_set_lock);

		if (ret)
		goto out_unlock;
		}

		@@ -3899,7 +3918,9 @@ void cgroup_file_notify(struct cgroup_file *cfile)
		* cgroup_task_count - count the number of tasks in a cgroup.
		* @cgrp: the cgroup in question
		*
		* Return the number of tasks in the cgroup.
		* Return the number of tasks in the cgroup. The returned number can be
		* higher than the actual number of tasks due to css_set references from
		* namespace roots and temporary usages.
		*/
		static int cgroup_task_count(const struct cgroup *cgrp)
		{

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