net: Preserve CHECKSUM_COMPLETE at validation

Currently when the first checksum in a packet is validated using
CHECKSUM_COMPLETE, ip_summed is overwritten to be CHECKSUM_UNNECESSARY
so that any subsequent checksums in the packet are not correctly
validated.

This patch adds csum_valid flag in sk_buff and uses that to indicate
validated checksum instead of setting CHECKSUM_UNNECESSARY. The bit
is set accordingly in the skb_checksum_validate_* functions. The flag
is checked in skb_checksum_complete, so that validation is communicated
between checksum_init and checksum_complete sequence in TCP and UDP.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -572,7 +572,8 @@ struct sk_buff {
 	 */
 	__u8			encapsulation:1;
 	__u8			encap_hdr_csum:1;
-	/* 5/7 bit hole (depending on ndisc_nodetype presence) */
+	__u8			csum_valid:1;
+	/* 4/6 bit hole (depending on ndisc_nodetype presence) */
 	kmemcheck_bitfield_end(flags2);
 
 #if defined CONFIG_NET_DMA || defined CONFIG_NET_RX_BUSY_POLL
@@ -2735,7 +2736,7 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb);
 
 static inline int skb_csum_unnecessary(const struct sk_buff *skb)
 {
-	return skb->ip_summed & CHECKSUM_UNNECESSARY;
+	return ((skb->ip_summed & CHECKSUM_UNNECESSARY) || skb->csum_valid);
 }
 
 /**
@@ -2769,10 +2770,8 @@ static inline bool __skb_checksum_validate_needed(struct sk_buff *skb,
 						  bool zero_okay,
 						  __sum16 check)
 {
-	if (skb_csum_unnecessary(skb)) {
-		return false;
-	} else if (zero_okay && !check) {
-		skb->ip_summed = CHECKSUM_UNNECESSARY;
+	if (skb_csum_unnecessary(skb) || (zero_okay && !check)) {
+		skb->csum_valid = 1;
 		return false;
 	}
 
@@ -2799,15 +2798,20 @@ static inline __sum16 __skb_checksum_validate_complete(struct sk_buff *skb,
 {
 	if (skb->ip_summed == CHECKSUM_COMPLETE) {
 		if (!csum_fold(csum_add(psum, skb->csum))) {
-			skb->ip_summed = CHECKSUM_UNNECESSARY;
+			skb->csum_valid = 1;
 			return 0;
 		}
 	}
 
 	skb->csum = psum;
 
-	if (complete || skb->len <= CHECKSUM_BREAK)
-		return __skb_checksum_complete(skb);
+	if (complete || skb->len <= CHECKSUM_BREAK) {
+		__sum16 csum;
+
+		csum = __skb_checksum_complete(skb);
+		skb->csum_valid = !csum;
+		return csum;
+	}
 
 	return 0;
 }
@@ -2831,6 +2835,7 @@ static inline __wsum null_compute_pseudo(struct sk_buff *skb, int proto)
 				zero_okay, check, compute_pseudo)	\
 ({									\
 	__sum16 __ret = 0;						\
+	skb->csum_valid = 0;						\
 	if (__skb_checksum_validate_needed(skb, zero_okay, check))	\
 		__ret = __skb_checksum_validate_complete(skb,		\
 				complete, compute_pseudo(skb, proto));	\