[NET]: Fix CHECKSUM_HW wrt. packet trimming.

When something, on receive, between a CHECKSUM_HW using
driver and the code validating skb->csum trims the head
of the packet we have to reset skb->ip_summed back to
CHECKSUM_NONE because this kind of change invalidates
the skb->csum value calculated by the driver.

Various spots handle this properly, but not all.  So
abstract these actions into helper routines in order
to avoid code duplication.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppoe.c

@@ -338,7 +338,9 @@ static int pppoe_rcv_core(struct sock *sk, struct sk_buff *skb)
 		struct pppoe_hdr *ph = (struct pppoe_hdr *) skb->nh.raw;
 		int len = ntohs(ph->length);
 		skb_pull(skb, sizeof(struct pppoe_hdr));
-		skb_trim(skb, len);
+		skb_postpull_rcsum(skb, ph, sizeof(*ph));
+		if (pskb_trim_rcsum(skb, len))
+			goto abort_kfree;
 
 		ppp_input(&po->chan, skb);
 	} else if (sk->sk_state & PPPOX_RELAY) {

include/linux/skbuff.h

@@ -1105,6 +1105,42 @@ static inline int skb_linearize(struct sk_buff *skb, int gfp)
 	return __skb_linearize(skb, gfp);
 }
 
+/**
+ *	skb_postpull_rcsum - update checksum for received skb after pull
+ *	@skb: buffer to update
+ *	@start: start of data before pull
+ *	@len: length of data pulled
+ *
+ *	After doing a pull on a received packet, you need to call this to
+ *	update the CHECKSUM_HW checksum, or set ip_summed to CHECKSUM_NONE
+ *	so that it can be recomputed from scratch.
+ */
+
+static inline void skb_postpull_rcsum(struct sk_buff *skb,
+					 const void *start, int len)
+{
+	if (skb->ip_summed == CHECKSUM_HW)
+		skb->csum = csum_sub(skb->csum, csum_partial(start, len, 0));
+}
+
+/**
+ *	pskb_trim_rcsum - trim received skb and update checksum
+ *	@skb: buffer to trim
+ *	@len: new length
+ *
+ *	This is exactly the same as pskb_trim except that it ensures the
+ *	checksum of received packets are still valid after the operation.
+ */
+
+static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	if (len >= skb->len)
+		return 0;
+	if (skb->ip_summed == CHECKSUM_HW)
+		skb->ip_summed = CHECKSUM_NONE;
+	return __pskb_trim(skb, len);
+}
+
 static inline void *kmap_skb_frag(const skb_frag_t *frag)
 {
 #ifdef CONFIG_HIGHMEM

net/ipv4/ip_gre.c

@@ -618,10 +618,8 @@ static int ipgre_rcv(struct sk_buff *skb)
 
 		skb->mac.raw = skb->nh.raw;
 		skb->nh.raw = __pskb_pull(skb, offset);
+		skb_postpull_rcsum(skb, skb->mac.raw, offset);
 		memset(&(IPCB(skb)->opt), 0, sizeof(struct ip_options));
-		if (skb->ip_summed == CHECKSUM_HW)
-			skb->csum = csum_sub(skb->csum,
-					     csum_partial(skb->mac.raw, skb->nh.raw-skb->mac.raw, 0));
 		skb->pkt_type = PACKET_HOST;
 #ifdef CONFIG_NET_IPGRE_BROADCAST
 		if (MULTICAST(iph->daddr)) {

net/ipv4/ip_input.c

@@ -410,10 +410,9 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt)
 		 * is IP we can trim to the true length of the frame.
 		 * Note this now means skb->len holds ntohs(iph->tot_len).
 		 */
-		if (skb->len > len) {
-			__pskb_trim(skb, len);
-			if (skb->ip_summed == CHECKSUM_HW)
-				skb->ip_summed = CHECKSUM_NONE;
+		if (pskb_trim_rcsum(skb, len)) {
+			IP_INC_STATS_BH(IPSTATS_MIB_INDISCARDS);
+			goto drop;
 		}
 	}
 

net/ipv6/ip6_input.c

@@ -95,15 +95,11 @@ int ipv6_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt
 	if (pkt_len || hdr->nexthdr != NEXTHDR_HOP) {
 		if (pkt_len + sizeof(struct ipv6hdr) > skb->len)
 			goto truncated;
-		if (pkt_len + sizeof(struct ipv6hdr) < skb->len) {
-			if (__pskb_trim(skb, pkt_len + sizeof(struct ipv6hdr))){
-				IP6_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
-				goto drop;
-			}
-			hdr = skb->nh.ipv6h;
-			if (skb->ip_summed == CHECKSUM_HW)
-				skb->ip_summed = CHECKSUM_NONE;
+		if (pskb_trim_rcsum(skb, pkt_len + sizeof(struct ipv6hdr))) {
+			IP6_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
+			goto drop;
 		}
+		hdr = skb->nh.ipv6h;
 	}
 
 	if (hdr->nexthdr == NEXTHDR_HOP) {
@@ -138,7 +134,6 @@ static inline int ip6_input_finish(struct sk_buff *skb)
 	unsigned int nhoff;
 	int nexthdr;
 	u8 hash;
-	int cksum_sub = 0;
 
 	skb->h.raw = skb->nh.raw + sizeof(struct ipv6hdr);
 
@@ -173,11 +168,8 @@ static inline int ip6_input_finish(struct sk_buff *skb)
 		if (ipprot->flags & INET6_PROTO_FINAL) {
 			struct ipv6hdr *hdr;	
 
-			if (!cksum_sub && skb->ip_summed == CHECKSUM_HW) {
-				skb->csum = csum_sub(skb->csum,
-						     csum_partial(skb->nh.raw, skb->h.raw-skb->nh.raw, 0));
-				cksum_sub++;
-			}
+			skb_postpull_rcsum(skb, skb->nh.raw,
+					   skb->h.raw - skb->nh.raw);
 			hdr = skb->nh.ipv6h;
 			if (ipv6_addr_is_multicast(&hdr->daddr) &&
 			    !ipv6_chk_mcast_addr(skb->dev, &hdr->daddr,