ppp: fix BUG on non-linear SKB (multilink receive)

PPP does not correctly call pskb_may_pull() on all necessary receive paths
before reading the PPP protocol, thus causing PPP to report seemingly
random 'unsupported protocols' and eventually trigger BUG_ON(skb->len <
skb->data_len) in skb_pull_rcsum() when receiving multilink protocol in
non-linear skbs.

ppp_receive_nonmp_frame() does not call pskb_may_pull() before reading the
protocol number.  For the non-mp receive path this is not a problem, as
this check is done in ppp_receive_frame().  For the mp receive path,
ppp_mp_reconstruct() usually copies the data into a new linear skb.
However, in the case where the frame is made up of a single mp fragment,
the mp header is pulled and the existing skb used.  This skb was then
passed to ppp_receive_nonmp_frame() without checking if the encapsulated
protocol header could safely be read.
Signed-off-by: Ben McKeegan <ben@netservers.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp_generic.c

@@ -1944,8 +1944,15 @@ ppp_receive_mp_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
 	}
 
 	/* Pull completed packets off the queue and receive them. */
-	while ((skb = ppp_mp_reconstruct(ppp)))
+	while ((skb = ppp_mp_reconstruct(ppp))) {
+		if (pskb_may_pull(skb, 2))
 			ppp_receive_nonmp_frame(ppp, skb);
+		else {
+			++ppp->dev->stats.rx_length_errors;
+			kfree_skb(skb);
+			ppp_receive_error(ppp);
+		}
+	}
 
 	return;