mptcp: setsockopt support for TCP_MD5SIG

Supporting TCP_MD5 socket option is required when MPTCP is used by
default when creating a socket, to keep the same behaviour as with
TCP. TCP_MD5 is not compatible with MPTCP, and it will cause a
fallback to TCP at the connection request, if MPTCP was requested.
This then fixes a "regression" compared to TCP.

This patch adds setsockopt support for TCP_MD5SIG and TCP_MD5SIG_EXT
options. The implementation:
 - Allow setting these options (getsockopt remains unsupported)
 - Apply them only to the first subflow
 - Trigger fallback to TCP to maintain MD5 compatibility

Note that getsockopt for these options remains unsupported, consistent
with TCP.

Closes: multipath-tcp/mptcp_net-next#575
Fixes: d9e4c12 ("mptcp: only admit explicitly supported sockopt")
Signed-off-by: Geliang Tang <[email protected]>

Author: Geliang Tang

net/mptcp/sockopt.c

@@ -13,6 +13,7 @@
 #include <net/tcp.h>
 #include <net/mptcp.h>
 #include "protocol.h"
+#include "mib.h"
 
 #define MIN_INFO_OPTLEN_SIZE		16
 #define MIN_FULL_INFO_OPTLEN_SIZE	40
@@ -567,11 +568,12 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_FASTOPEN_CONNECT:
 		case TCP_FASTOPEN_KEY:
 		case TCP_FASTOPEN_NO_COOKIE:
+		/* MD5 will force a fallback to TCP: OK to set while not connected */
+		case TCP_MD5SIG:
+		case TCP_MD5SIG_EXT:
 			return true;
 		}
 
-		/* TCP_MD5SIG, TCP_MD5SIG_EXT are not supported, MD5 is not compatible with MPTCP */
-
 		/* TCP_REPAIR, TCP_REPAIR_QUEUE, TCP_QUEUE_SEQ, TCP_REPAIR_OPTIONS,
 		 * TCP_REPAIR_WINDOW are not supported, better avoid this mess
 		 */
@@ -836,6 +838,19 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 	case TCP_FASTOPEN_NO_COOKIE:
 		return mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname,
 						      optval, optlen);
+#ifdef CONFIG_TCP_MD5SIG
+	case TCP_MD5SIG:
+	case TCP_MD5SIG_EXT:
+		if (!((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
+			return -EINVAL;
+		ret = mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname,
+						     optval, optlen);
+		if (ret)
+			return ret;
+		if (!__mptcp_try_fallback(msk, MPTCP_MIB_MD5SIGFALLBACK))
+			return -EINVAL;
+		return 0;
+#endif
 	}
 
 	ret = mptcp_get_int_option(msk, optval, optlen, &val);