fq_codel: reject silly quantum parameters

syzbot found that forcing a big quantum attribute would crash hosts fast,
essentially using this:

tc qd replace dev eth0 root fq_codel quantum 4294967295

This is because fq_codel_dequeue() would have to loop
~2^31 times in :

	if (flow->deficit <= 0) {
		flow->deficit += q->quantum;
		list_move_tail(&flow->flowchain, &q->old_flows);
		goto begin;
	}

SFQ max quantum is 2^19 (half a megabyte)
Lets adopt a max quantum of one megabyte for FQ_CODEL.

Fixes: 4b549a2e ("fq_codel: Fair Queue Codel AQM")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/pkt_sched.h

@@ -827,6 +827,8 @@ struct tc_codel_xstats {
 
 /* FQ_CODEL */
 
+#define FQ_CODEL_QUANTUM_MAX (1 << 20)
+
 enum {
 	TCA_FQ_CODEL_UNSPEC,
 	TCA_FQ_CODEL_TARGET,

net/sched/sch_fq_codel.c

@@ -369,6 +369,7 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
 {
 	struct fq_codel_sched_data *q = qdisc_priv(sch);
 	struct nlattr *tb[TCA_FQ_CODEL_MAX + 1];
+	u32 quantum = 0;
 	int err;
 
 	if (!opt)
@@ -386,6 +387,13 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
 		    q->flows_cnt > 65536)
 			return -EINVAL;
 	}
+	if (tb[TCA_FQ_CODEL_QUANTUM]) {
+		quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
+		if (quantum > FQ_CODEL_QUANTUM_MAX) {
+			NL_SET_ERR_MSG(extack, "Invalid quantum");
+			return -EINVAL;
+		}
+	}
 	sch_tree_lock(sch);
 
 	if (tb[TCA_FQ_CODEL_TARGET]) {
@@ -412,8 +420,8 @@ static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_FQ_CODEL_ECN])
 		q->cparams.ecn = !!nla_get_u32(tb[TCA_FQ_CODEL_ECN]);
 
-	if (tb[TCA_FQ_CODEL_QUANTUM])
-		q->quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
+	if (quantum)
+		q->quantum = quantum;
 
 	if (tb[TCA_FQ_CODEL_DROP_BATCH_SIZE])
 		q->drop_batch_size = max(1U, nla_get_u32(tb[TCA_FQ_CODEL_DROP_BATCH_SIZE]));