net: openvswitch: fix csum updates for MPLS actions

Skbs may have their checksum value populated by HW. If this is a checksum
calculated over the entire packet then the CHECKSUM_COMPLETE field is
marked. Changes to the data pointer on the skb throughout the network
stack still try to maintain this complete csum value if it is required
through functions such as skb_postpush_rcsum.

The MPLS actions in Open vSwitch modify a CHECKSUM_COMPLETE value when
changes are made to packet data without a push or a pull. This occurs when
the ethertype of the MAC header is changed or when MPLS lse fields are
modified.

The modification is carried out using the csum_partial function to get the
csum of a buffer and add it into the larger checksum. The buffer is an
inversion of the data to be removed followed by the new data. Because the
csum is calculated over 16 bits and these values align with 16 bits, the
effect is the removal of the old value from the CHECKSUM_COMPLETE and
addition of the new value.

However, the csum fed into the function and the outcome of the
calculation are also inverted. This would only make sense if it was the
new value rather than the old that was inverted in the input buffer.

Fix the issue by removing the bit inverts in the csum_partial calculation.

The bug was verified and the fix tested by comparing the folded value of
the updated CHECKSUM_COMPLETE value with the folded value of a full
software checksum calculation (reset skb->csum to 0 and run
skb_checksum_complete(skb)). Prior to the fix the outcomes differed but
after they produce the same result.

Fixes: 25cd9ba0 ("openvswitch: Add basic MPLS support to kernel")
Fixes: bc7cc599 ("openvswitch: update checksum in {push,pop}_mpls")
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/openvswitch/actions.c

@@ -166,8 +166,7 @@ static void update_ethertype(struct sk_buff *skb, struct ethhdr *hdr,
 	if (skb->ip_summed == CHECKSUM_COMPLETE) {
 		__be16 diff[] = { ~(hdr->h_proto), ethertype };
 
-		skb->csum = ~csum_partial((char *)diff, sizeof(diff),
-					~skb->csum);
+		skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
 	}
 
 	hdr->h_proto = ethertype;
@@ -259,8 +258,7 @@ static int set_mpls(struct sk_buff *skb, struct sw_flow_key *flow_key,
 	if (skb->ip_summed == CHECKSUM_COMPLETE) {
 		__be32 diff[] = { ~(stack->label_stack_entry), lse };
 
-		skb->csum = ~csum_partial((char *)diff, sizeof(diff),
-					  ~skb->csum);
+		skb->csum = csum_partial((char *)diff, sizeof(diff), skb->csum);
 	}
 
 	stack->label_stack_entry = lse;