[PATCH 4.7 002/141] cgroup: fix invalid controller enable rejections with cgroup namespace

From: Greg Kroah-Hartman
Date: Thu Oct 06 2016 - 04:32:40 EST

4.7-stable review patch. If anyone has any objections, please let me know.


From: Tejun Heo <tj@xxxxxxxxxx>

commit 9157056da8f8c4a6305f15619e269f164b63a6de upstream.

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 a79a908fd2b0 ("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

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Evgeny Vereshchagin <evvers@xxxxx>
Cc: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
Cc: Aditya Kali <adityakali@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Fixes: a79a908fd2b0 ("cgroup: introduce cgroup namespaces")
Link: https://github.com/systemd/systemd/pull/3589#issuecomment-249089541
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

kernel/cgroup.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3452,9 +3452,28 @@ static ssize_t cgroup_subtree_control_wr
* 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 */
@@ -3905,7 +3924,9 @@ void cgroup_file_notify(struct cgroup_fi
* 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)