cgroup: Fix potential overflow issue when checking max_depth

cgroup.max.depth is the maximum allowed descent depth below the current
cgroup. If the actual descent depth is equal or larger, an attempt to
create a new child cgroup will fail. However due to the cgroup->max_depth
is of int type and having the default value INT_MAX, the condition
'level > cgroup->max_depth' will never be satisfied, and it will cause
an overflow of the level after it reaches to INT_MAX.

Fix it by starting the level from 0 and using '>=' instead.

It's worth mentioning that this issue is unlikely to occur in reality,
as it's impossible to have a depth of INT_MAX hierarchy, but should be
be avoided logically.

Fixes: 1a926e0 ("cgroup: implement hierarchy limits")
Signed-off-by: Xiu Jianfeng <[email protected]>
Reviewed-by: Michal Koutný <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Xiu Jianfeng

kernel/cgroup/cgroup.c

@@ -5789,15 +5789,15 @@ static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 {
 	struct cgroup *cgroup;
 	int ret = false;
-	int level = 1;
+	int level = 0;
 
 	lockdep_assert_held(&cgroup_mutex);
 
 	for (cgroup = parent; cgroup; cgroup = cgroup_parent(cgroup)) {
 		if (cgroup->nr_descendants >= cgroup->max_descendants)
 			goto fail;
 
-		if (level > cgroup->max_depth)
+		if (level >= cgroup->max_depth)
 			goto fail;
 
 		level++;