net: eth_type_trans() should use skb_header_pointer()

eth_type_trans() can read uninitialized memory as drivers
do not necessarily pull more than 14 bytes in skb->head before
calling it.

As David suggested, we can use skb_header_pointer() to
fix this without breaking some drivers that might not expect
eth_type_trans() pulling 2 additional bytes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index 8f032bae60ad8fdc8e284c5cd5ae7501660fcf50..5dc638cad2e1ff396347f53c74a0a038d3a4b77e 100644 (file)
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -156,7 +156,9 @@ EXPORT_SYMBOL(eth_rebuild_header);
  */
 __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
 {
-       struct ethhdr *eth;
+       unsigned short _service_access_point;
+       const unsigned short *sap;
+       const struct ethhdr *eth;
 
        skb->dev = dev;
        skb_reset_mac_header(skb);
@@ -194,7 +196,8 @@ __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev)
         *      layer. We look for FFFF which isn't a used 802.2 SSAP/DSAP. This
         *      won't work for fault tolerant netware but does for the rest.
         */
-       if (unlikely(skb->len >= 2 && *(unsigned short *)(skb->data) == 0xFFFF))
+       sap = skb_header_pointer(skb, 0, sizeof(*sap), &_service_access_point);
+       if (sap && *sap == 0xFFFF)
                return htons(ETH_P_802_3);
 
        /*