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: 8cfd8147df67 ("cgroup: implement cgroup v2 thread support")
Cc: stable@vger.kernel.org # v4.14+

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8cda3bc3ae22841f9c3a9478ae65cadba6b65f66..4bfb2908ec157204692424bc1b1a32a7ef185b29 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -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))