Commit d1897c95 authored Feb 21, 2018 by Tejun Heo

cgroup: fix rule checking for threaded mode switching



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
cgroup_enable_threaded().

Reported-by: Michael Kerrisk (man-pages) <mtk.manpages@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 8cfd8147 ("cgroup: implement cgroup v2 thread support")
Cc: stable@vger.kernel.org # v4.14+

parent c53593e5

kernel/cgroup/cgroup.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -3183,6 +3183,16 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
		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) \|\|
		!cgroup_can_be_thread_root(dom_cgrp))

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