net: Fix remcsum in GRO path to not change packet

Remote checksum offload processing is currently the same for both
the GRO and non-GRO path. When the remote checksum offload option
is encountered, the checksum field referred to is modified in
the packet. So in the GRO case, the packet is modified in the
GRO path and then the operation is skipped when the packet goes
through the normal path based on skb->remcsum_offload. There is
a problem in that the packet may be modified in the GRO path, but
then forwarded off host still containing the remote checksum option.
A remote host will again perform RCO but now the checksum verification
will fail since GRO RCO already modified the checksum.

To fix this, we ensure that GRO restores a packet to it's original
state before returning. In this model, when GRO processes a remote
checksum option it still changes the checksum per the algorithm
but on return from lower layer processing the checksum is restored
to its original value.

In this patch we add define gro_remcsum structure which is passed
to skb_gro_remcsum_process to save offset and delta for the checksum
being changed. After lower layer processing, skb_gro_remcsum_cleanup
is called to restore the checksum before returning from GRO.

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

1 files changed, 23 insertions, 2 deletions

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d115256ed5a2..3aa02457fe4f 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2321,8 +2321,19 @@ do {									\
 					   compute_pseudo(skb, proto));	\
 } while (0)
 
+struct gro_remcsum {
+	int offset;
+	__wsum delta;
+};
+
+static inline void skb_gro_remcsum_init(struct gro_remcsum *grc)
+{
+	grc->delta = 0;
+}
+
 static inline void skb_gro_remcsum_process(struct sk_buff *skb, void *ptr,
-					   int start, int offset)
+					   int start, int offset,
+					   struct gro_remcsum *grc)
 {
 	__wsum delta;
 
@@ -2331,10 +2342,20 @@ static inline void skb_gro_remcsum_process(struct sk_buff *skb, void *ptr,
 	delta = remcsum_adjust(ptr, NAPI_GRO_CB(skb)->csum, start, offset);
 
 	/* Adjust skb->csum since we changed the packet */
-	skb->csum = csum_add(skb->csum, delta);
 	NAPI_GRO_CB(skb)->csum = csum_add(NAPI_GRO_CB(skb)->csum, delta);
+
+	grc->offset = (ptr + offset) - (void *)skb->head;
+	grc->delta = delta;
 }
 
+static inline void skb_gro_remcsum_cleanup(struct sk_buff *skb,
+					   struct gro_remcsum *grc)
+{
+	if (!grc->delta)
+		return;
+
+	remcsum_unadjust((__sum16 *)(skb->head + grc->offset), grc->delta);
+}
 
 static inline int dev_hard_header(struct sk_buff *skb, struct net_device *dev,
 				  unsigned short type,