net/sched: sch_qfq: reintroduce lmax bound check for MTU

2536989 deletes a check for the case where no 'lmax' is
specified which 3037933 previously fixed as 'lmax'
could be set to the device's MTU without any bound checking
for QFQ_LMAX_MIN and QFQ_LMAX_MAX. Therefore, reintroduce the check.

Fixes: 2536989 ("net/sched: sch_qfq: refactor parsing of netlink parameters")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: tammela

net/sched/sch_qfq.c

@@ -423,10 +423,17 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 	else
 		weight = 1;
 
-	if (tb[TCA_QFQ_LMAX])
+	if (tb[TCA_QFQ_LMAX]) {
 		lmax = nla_get_u32(tb[TCA_QFQ_LMAX]);
-	else
+	} else {
+		/* MTU size is user controlled */
 		lmax = psched_mtu(qdisc_dev(sch));
+		if (lmax < QFQ_MIN_LMAX || lmax > QFQ_MAX_LMAX) {
+			NL_SET_ERR_MSG_MOD(extack,
+					   "MTU size out of bounds for qfq");
+			return -EINVAL;
+		}
+	}
 
 	inv_w = ONE_FP / weight;
 	weight = ONE_FP / inv_w;