mptcp: receive checksum for DSS

In mptcp_parse_option, adjust the expected_opsize, and always parse the
data checksum value from the receiving DSS regardless of csum presence.
Then save it in mp_opt->csum.

Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -182,10 +182,8 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 				expected_opsize += TCPOLEN_MPTCP_DSS_MAP32;
 		}
 
-		/* RFC 6824, Section 3.3:
-		 * If a checksum is present, but its use had
-		 * not been negotiated in the MP_CAPABLE handshake,
-		 * the checksum field MUST be ignored.
+		/* Always parse any csum presence combination, we will enforce
+		 * RFC 8684 Section 3.3.0 checks later in subflow_data_ready
 		 */
 		if (opsize != expected_opsize &&
 		    opsize != expected_opsize + TCPOLEN_MPTCP_DSS_CHECKSUM)
@@ -220,9 +218,15 @@ static void mptcp_parse_option(const struct sk_buff *skb,
 			mp_opt->data_len = get_unaligned_be16(ptr);
 			ptr += 2;
 
-			pr_debug("data_seq=%llu subflow_seq=%u data_len=%u",
+			if (opsize == expected_opsize + TCPOLEN_MPTCP_DSS_CHECKSUM) {
+				mp_opt->csum_reqd = 1;
+				mp_opt->csum = (__force __sum16)get_unaligned_be16(ptr);
+				ptr += 2;
+			}
+
+			pr_debug("data_seq=%llu subflow_seq=%u data_len=%u csum=%d:%u",
 				 mp_opt->data_seq, mp_opt->subflow_seq,
-				 mp_opt->data_len);
+				 mp_opt->data_len, mp_opt->csum_reqd, mp_opt->csum);
 		}
 
 		break;