capabilities: allow nice if we are privileged

We allow task A to change B's nice level if it has a supserset of
B's privileges, or of it has CAP_SYS_NICE. Also allow it if A has
CAP_SYS_NICE with respect to B - meaning it is root in the same
namespace, or it created B's namespace.

Signed-off-by: Serge Hallyn <[e-mail]>
Reviewed-by: "Eric W. Biederman" <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

security/commoncap.c

···
  */
 static int cap_safe_nice(struct task_struct *p)
 {
-	int is_subset;
+	int is_subset, ret = 0;
 
 	rcu_read_lock();
 	is_subset = cap_issubset(__task_cred(p)->cap_permitted,
 				 current_cred()->cap_permitted);
+	if (!is_subset && !ns_capable(__task_cred(p)->user_ns, CAP_SYS_NICE))
+		ret = -EPERM;
 	rcu_read_unlock();
 
-	if (!is_subset && !capable(CAP_SYS_NICE))
-		return -EPERM;
-	return 0;
+	return ret;
 }
 
 /**