UBUNTU: SAUCE: quota: Require that qids passed to dqget() be valid and map into s_user_ns

BugLink: http://bugs.launchpad.net/bugs/1566505

This ensures that quotas that aren't valid on disk never make it
into the hash table. inodes with ids which don't map into
s_user_ns will now have no quota enforcement.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

fs/quota/dquot.c

@@ -834,6 +834,12 @@ struct dquot *dqget(struct super_block *sb, struct kqid qid)
 	unsigned int hashent = hashfn(sb, qid);
 	struct dquot *dquot, *empty = NULL;
 
+	/* qid must be valid and must map into sb's userns */
+	if (!qid_valid(qid))
+		return ERR_PTR(-EINVAL);
+	if (from_kqid(sb->s_user_ns, qid) == (qid_t)-1)
+		return ERR_PTR(-EOVERFLOW);
+
         if (!sb_has_quota_active(sb, qid.type))
 		return ERR_PTR(-ESRCH);
 we_slept: