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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 a79a908fd2 ("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: a79a908fd2 ("cgroup: introduce cgroup namespaces")
Link: https://github.com/systemd/systemd/pull/3589#issuecomment-249089541
This commit is contained in:
Tejun Heo 2016-09-23 16:55:49 -04:00
parent 8a15b81741
commit 9157056da8
1 changed files with 25 additions and 4 deletions
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29
kernel/cgroup.c
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@ -3446,9 +3446,28 @@ 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)) {
ret = -EBUSY;
goto out_unlock;
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;
}
/* save and update control masks and prepare csses */
@ -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)
{