net/sched: act_skbmod: Skip non-Ethernet packets

Currently tcf_skbmod_act() assumes that packets use Ethernet as their L2
protocol, which is not always the case.  As an example, for CAN devices:

	$ ip link add dev vcan0 type vcan
	$ ip link set up vcan0
	$ tc qdisc add dev vcan0 root handle 1: htb
	$ tc filter add dev vcan0 parent 1: protocol ip prio 10 \
		matchall action skbmod swap mac

Doing the above silently corrupts all the packets.  Do not perform skbmod
actions for non-Ethernet packets.

Fixes: 86da71b5 ("net_sched: Introduce skbmod action")
Reviewed-by: NCong Wang <cong.wang@bytedance.com>
Signed-off-by: NPeilin Ye <peilin.ye@bytedance.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/act_skbmod.c

@@ -6,6 +6,7 @@
 */
 
 #include <linux/module.h>
+#include <linux/if_arp.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/skbuff.h>
@@ -33,6 +34,13 @@ static int tcf_skbmod_act(struct sk_buff *skb, const struct tc_action *a,
 	tcf_lastuse_update(&d->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(d->common.cpu_bstats), skb);
 
+	action = READ_ONCE(d->tcf_action);
+	if (unlikely(action == TC_ACT_SHOT))
+		goto drop;
+
+	if (!skb->dev || skb->dev->type != ARPHRD_ETHER)
+		return action;
+
 	/* XXX: if you are going to edit more fields beyond ethernet header
 	 * (example when you add IP header replacement or vlan swap)
 	 * then MAX_EDIT_LEN needs to change appropriately
@@ -41,10 +49,6 @@ static int tcf_skbmod_act(struct sk_buff *skb, const struct tc_action *a,
 	if (unlikely(err)) /* best policy is to drop on the floor */
 		goto drop;
 
-	action = READ_ONCE(d->tcf_action);
-	if (unlikely(action == TC_ACT_SHOT))
-		goto drop;
-
 	p = rcu_dereference_bh(d->skbmod_p);
 	flags = p->flags;
 	if (flags & SKBMOD_F_DMAC)