net: Ensure partial checksum offset is inside the skb head

On Thu, Jun 04, 2009 at 09:06:00PM +1000, Herbert Xu wrote:
>
> tun: Optimise handling of bogus gso->hdr_len
>
> As all current versions of virtio_net generate a value for the
> header length that's too small, we should optimise this so that
> we don't copy it twice.  This can be done by ensuring that it is
> at least as large as the place where we'll write the checksum.
>
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

With this applied we can strengthen the partial checksum check:

In skb_partial_csum_set we check to see if the checksum offset
is within the packet.  However, we really should check that it
is within the skb head as that's the only bit we can modify
without copying.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -3026,12 +3026,12 @@ EXPORT_SYMBOL_GPL(skb_tstamp_tx);
  */
 bool skb_partial_csum_set(struct sk_buff *skb, u16 start, u16 off)
 {
-	if (unlikely(start > skb->len - 2) ||
-	    unlikely((int)start + off > skb->len - 2)) {
+	if (unlikely(start > skb_headlen(skb)) ||
+	    unlikely((int)start + off > skb_headlen(skb) - 2)) {
 		if (net_ratelimit())
 			printk(KERN_WARNING
 			       "bad partial csum: csum=%u/%u len=%u\n",
-			       start, off, skb->len);
+			       start, off, skb_headlen(skb));
 		return false;
 	}
 	skb->ip_summed = CHECKSUM_PARTIAL;