[PPP] generic: Fix receive path data clobbering & non-linear handling

This patch adds missing pskb_may_pull calls to deal with non-linear
packets that may arrive from pppoe or pppol2tp.

It also copies cloned packets before writing over them.

Signed-off-by: Herbert Xu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ppp_generic.c

···
 static void
 ppp_receive_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
 {
-	if (skb->len >= 2) {
+	if (pskb_may_pull(skb, 2)) {
 #ifdef CONFIG_PPP_MULTILINK
 		/* XXX do channel-level decompression here */
 		if (PPP_PROTO(skb) == PPP_MP)
···
 		if (ppp->vj == 0 || (ppp->flags & SC_REJ_COMP_TCP))
 			goto err;
 
-		if (skb_tailroom(skb) < 124) {
+		if (skb_tailroom(skb) < 124 || skb_cloned(skb)) {
 			/* copy to a new sk_buff with more tailroom */
 			ns = dev_alloc_skb(skb->len + 128);
 			if (ns == 0) {
···
 		/* check if the packet passes the pass and active filters */
 		/* the filter instructions are constructed assuming
 		   a four-byte PPP header on each packet */
-		*skb_push(skb, 2) = 0;
-		if (ppp->pass_filter
-		    && sk_run_filter(skb, ppp->pass_filter,
-				     ppp->pass_len) == 0) {
-			if (ppp->debug & 1)
-				printk(KERN_DEBUG "PPP: inbound frame not passed\n");
-			kfree_skb(skb);
-			return;
-		}
-		if (!(ppp->active_filter
-		      && sk_run_filter(skb, ppp->active_filter,
-				       ppp->active_len) == 0))
-			ppp->last_recv = jiffies;
-		skb_pull(skb, 2);
-#else
-		ppp->last_recv = jiffies;
+		if (ppp->pass_filter || ppp->active_filter) {
+			if (skb_cloned(skb) &&
+			    pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
+				goto err;
+
+			*skb_push(skb, 2) = 0;
+			if (ppp->pass_filter
+			    && sk_run_filter(skb, ppp->pass_filter,
+					     ppp->pass_len) == 0) {
+				if (ppp->debug & 1)
+					printk(KERN_DEBUG "PPP: inbound frame "
+					       "not passed\n");
+				kfree_skb(skb);
+				return;
+			}
+			if (!(ppp->active_filter
+			      && sk_run_filter(skb, ppp->active_filter,
+					       ppp->active_len) == 0))
+				ppp->last_recv = jiffies;
+			__skb_pull(skb, 2);
+		} else
 #endif /* CONFIG_PPP_FILTER */
+			ppp->last_recv = jiffies;
 
 		if ((ppp->dev->flags & IFF_UP) == 0
 		    || ppp->npmode[npi] != NPMODE_PASS) {
···
 	struct channel *ch;
 	int mphdrlen = (ppp->flags & SC_MP_SHORTSEQ)? MPHDRLEN_SSN: MPHDRLEN;
 
-	if (!pskb_may_pull(skb, mphdrlen) || ppp->mrru == 0)
+	if (!pskb_may_pull(skb, mphdrlen + 1) || ppp->mrru == 0)
 		goto err;		/* no good, throw it away */
 
 	/* Decode sequence number and begin/end bits */