pidns: Deny strange cases when creating pid namespaces.

task_active_pid_ns(current) != current->ns_proxy->pid_ns will
soon be allowed to support unshare and setns.

The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of current->ns_proxy->pid_ns.  However
that leads to strange cases like trying to have a single process be
init in multiple pid namespaces, which is racy and hard to think
about.

The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of task_active_pid_ns(current).  While
that seems less racy it does not provide any utility.

Therefore define the semantics of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns to be that the
pid namespace creation fails.  That is easy to implement and easy
to think about.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/pid_namespace.c

@@ -146,6 +146,8 @@ struct pid_namespace *copy_pid_ns(unsigned long flags,
 		return get_pid_ns(old_ns);
 	if (flags & (CLONE_THREAD|CLONE_PARENT))
 		return ERR_PTR(-EINVAL);
+	if (task_active_pid_ns(current) != old_ns)
+		return ERR_PTR(-EINVAL);
 	return create_pid_namespace(user_ns, old_ns);
 }