net-sched: sch_cbq: avoid infinite loop

Its possible to setup a bad cbq configuration leading to
an infinite loop in cbq_classify()

DEV_OUT=eth0
ICMP="match ip protocol 1 0xff"
U32="protocol ip u32"
DST="match ip dst"
tc qdisc add dev $DEV_OUT root handle 1: cbq avpkt 1000 \
bandwidth 100mbit
tc class add dev $DEV_OUT parent 1: classid 1:1 cbq \
rate 512kbit allot 1500 prio 5 bounded isolated
tc filter add dev $DEV_OUT parent 1: prio 3 $U32 \
$ICMP $DST 192.168.3.234 flowid 1:

Reported-by: Denys Fedoryschenko <[e-mail]>
Tested-by: Denys Fedoryschenko <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sched/sch_cbq.c

···
 			else if ((cl = defmap[res.classid & TC_PRIO_MAX]) == NULL)
 				cl = defmap[TC_PRIO_BESTEFFORT];
 
-			if (cl == NULL || cl->level >= head->level)
+			if (cl == NULL)
 				goto fallback;
 		}
-
+		if (cl->level >= head->level)
+			goto fallback;
 #ifdef CONFIG_NET_CLS_ACT
 		switch (result) {
 		case TC_ACT_QUEUED: