udp: Avoid post-GRO UDP checksum recalculation

Currently, when resegmenting an unexpected UDP GRO packet, the full UDP
checksum will be calculated for every new SKB created by skb_segment()
because the netdev features passed in by udp_rcv_segment() lack any
information about checksum offload capabilities.

Usually, we have no need to perform this calculation again, as
  1) The GRO implementation guarantees that any packets making it to the
     udp_rcv_segment() function had correct checksums, and, more
     importantly,
  2) Upon the successful return of udp_rcv_segment(), we immediately pull
     the UDP header off and either queue the segment to the socket or
     hand it off to a new protocol handler.

Unless userspace has set the IP_CHECKSUM sockopt to indicate that they
want the final checksum values, we can pass the needed netdev feature
flags to __skb_gso_segment() to avoid checksumming each segment in
skb_segment().

Fixes: cf329aa42b ("udp: cope with UDP GRO packet misdirection")
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udp.h

@@ -471,12 +471,19 @@ void udpv6_encap_enable(void);
 static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
 					      struct sk_buff *skb, bool ipv4)
 {
+	netdev_features_t features = NETIF_F_SG;
 	struct sk_buff *segs;
 
+	/* Avoid csum recalculation by skb_segment unless userspace explicitly
+	 * asks for the final checksum values
+	 */
+	if (!inet_get_convert_csum(sk))
+		features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
+
 	/* the GSO CB lays after the UDP one, no need to save and restore any
 	 * CB fragment
 	 */
-	segs = __skb_gso_segment(skb, NETIF_F_SG, false);
+	segs = __skb_gso_segment(skb, features, false);
 	if (unlikely(IS_ERR_OR_NULL(segs))) {
 		int segs_nr = skb_shinfo(skb)->gso_segs;