[PATCH] cpuset: Enforce that a child's cpus must be a subset of the parent

[Waiman Long]

It was found that the cpuset.cpus could contain CPUs that are not listed
in their parent's cpu list as shown by the command sequence below:

  # echo "+cpuset" >cgroup.subtree_control
  # mkdir g1
  # echo 0-5 >g1/cpuset.cpus
  # mkdir g1/g11
  # echo "+cpuset" > g1/cgroup.subtree_control
  # echo 6-11 >g1/g11/cpuset.cpus
  # grep -R . g1 | grep "\.cpus"
  g1/cpuset.cpus:0-5
  g1/cpuset.cpus.effective:0-5
  g1/g11/cpuset.cpus:6-11
  g1/g11/cpuset.cpus.effective:0-5

As the intersection of g11's cpus and that of g1 is empty, the effective
cpus of g11 is just that of g1. The check in update_cpumask() is now
corrected to make sure that cpus in a child cpus must be a subset of
its parent's cpus. The error "write error: Invalid argument" will now
be reported in the above case.

Reported-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 71fb2d0..ceec438 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1185,12 +1185,17 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	if (!*buf) {
 		cpumask_clear(trialcs->cpus_allowed);
 	} else {
+		struct cpuset *parent = parent_cs(cs);
+
 		retval = cpulist_parse(buf, trialcs->cpus_allowed);
 		if (retval < 0)
 			return retval;
 
+		/*
+		 * The cpu list must be a subset of the parent.
+		 */
 		if (!cpumask_subset(trialcs->cpus_allowed,
-				    top_cpuset.cpus_allowed))
+				    parent->cpus_allowed))
 			return -EINVAL;
 	}
 
-- 
1.8.3.1