net_sched: refetch skb protocol for each filter

Martin reported a set of filters don't work after changing
from reclassify to continue. Looking into the code, it
looks like skb protocol is not always fetched for each
iteration of the filters. But, as demonstrated by Martin,
TC actions could modify skb->protocol, for example act_vlan,
this means we have to refetch skb protocol in each iteration,
rather than using the one we fetch in the beginning of the loop.

This bug is _not_ introduced by commit 3b3ae88026
("net: sched: consolidate tc_classify{,_compat}"), technically,
if act_vlan is the only action that modifies skb protocol, then
it is commit c7e2b9689e ("sched: introduce vlan action") which
introduced this bug.

Reported-by: Martin Olsson <martin.olsson+netdev@sentorsecurity.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -1277,7 +1277,6 @@ EXPORT_SYMBOL(tcf_block_cb_unregister);
 int tcf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 		 struct tcf_result *res, bool compat_mode)
 {
-	__be16 protocol = tc_skb_protocol(skb);
 #ifdef CONFIG_NET_CLS_ACT
 	const int max_reclassify_loop = 4;
 	const struct tcf_proto *orig_tp = tp;
@@ -1287,6 +1286,7 @@ int tcf_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 reclassify:
 #endif
 	for (; tp; tp = rcu_dereference_bh(tp->next)) {
+		__be16 protocol = tc_skb_protocol(skb);
 		int err;
 
 		if (tp->protocol != protocol &&
@@ -1319,7 +1319,6 @@ reset:
 	}
 
 	tp = first_tp;
-	protocol = tc_skb_protocol(skb);
 	goto reclassify;
 #endif
 }