Staging: batman-adv: ensure that eth_type_trans gets linear memory

eth_type_trans tries to pull data with the length of the ethernet header
from the skb. We only ensured that enough data for the first ethernet
header and the batman header is available in non-paged memory of the skb
and not for the ethernet after the batman header.

eth_type_trans would fail sometimes with drivers which don't ensure that
all there data is perfectly linearised.

The failure was noticed through a kernel bug Oops generated by the
skb_pull inside eth_type_trans.

Reported-by: Rafal Lesniak <[e-mail]>
Signed-off-by: Marek Lindner <[e-mail]>
Signed-off-by: Sven Eckelmann <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/batman-adv/soft-interface.c

···
 	struct bat_priv *priv = netdev_priv(soft_iface);
 
 	/* check if enough space is available for pulling, and pull */
-	if (!pskb_may_pull(skb, hdr_size)) {
-		kfree_skb(skb);
-		return;
-	}
+	if (!pskb_may_pull(skb, hdr_size))
+		goto dropped;
+
 	skb_pull_rcsum(skb, hdr_size);
 /*	skb_set_mac_header(skb, -sizeof(struct ethhdr));*/
 
 	/* skb->dev & skb->pkt_type are set here */
+	if (unlikely(!pskb_may_pull(skb, ETH_HLEN)))
+		goto dropped;
 	skb->protocol = eth_type_trans(skb, soft_iface);
 
 	/* should not be neccesary anymore as we use skb_pull_rcsum()
···
 	soft_iface->last_rx = jiffies;
 
 	netif_rx(skb);
+	return;
+
+dropped:
+	kfree_skb(skb);
+	return;
 }
 
 #ifdef HAVE_NET_DEVICE_OPS