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Subject[PATCH 4.15 044/105] cgroup: fix rule checking for threaded mode switching
4.15-stable review patch.  If anyone has any objections, please let me know.


From: Tejun Heo <>

commit d1897c9538edafd4ae6bbd03cc075962ddde2c21 upstream.

A domain cgroup isn't allowed to be turned threaded if its subtree is
populated or domain controllers are enabled. cgroup_enable_threaded()
depended on cgroup_can_be_thread_root() test to enforce this rule. A
parent which has populated domain descendants or have domain
controllers enabled can't become a thread root, so the above rules are
enforced automatically.

However, for the root cgroup which can host mixed domain and threaded
children, cgroup_can_be_thread_root() doesn't check any of those
conditions and thus first level cgroups ends up escaping those rules.

This patch fixes the bug by adding explicit checks for those rules in

Reported-by: Michael Kerrisk (man-pages) <>
Signed-off-by: Tejun Heo <>
Fixes: 8cfd8147df67 ("cgroup: implement cgroup v2 thread support")
Cc: # v4.14+
Signed-off-by: Greg Kroah-Hartman <>

kernel/cgroup/cgroup.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3183,6 +3183,16 @@ static int cgroup_enable_threaded(struct
if (cgroup_is_threaded(cgrp))
return 0;

+ /*
+ * If @cgroup is populated or has domain controllers enabled, it
+ * can't be switched. While the below cgroup_can_be_thread_root()
+ * test can catch the same conditions, that's only when @parent is
+ * not mixable, so let's check it explicitly.
+ */
+ if (cgroup_is_populated(cgrp) ||
+ cgrp->subtree_control & ~cgrp_dfl_threaded_ss_mask)
+ return -EOPNOTSUPP;
/* we're joining the parent's domain, ensure its validity */
if (!cgroup_is_valid_domain(dom_cgrp) ||

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