Add hints about protocol-version-related SSL connection failures.

OpenSSL's native reports about problems related to protocol version
restrictions are pretty opaque and inconsistent.  When we get an
SSL error that is plausibly due to this, emit a hint message that
includes the range of SSL protocol versions we (think we) are
allowing.  This should at least get the user thinking in the right
direction to resolve the problem, even if the hint isn't totally
accurate, which it might not be for assorted reasons.

Back-patch to v13 where we increased the default minimum protocol
version, thereby increasing the risk of this class of failure.

Patch by me, reviewed by Daniel Gustafsson

Discussion: https://postgr.es/m/a9408304-4381-a5af-d259-e55d349ae4ce@2ndquadrant.com

src/backend/libpq/be-secure-openssl.c

@@ -72,6 +72,7 @@ static bool dummy_ssl_passwd_cb_called = false;
 static bool ssl_is_server_start;
 
 static int	ssl_protocol_version_to_openssl(int v);
+static const char *ssl_protocol_version_to_string(int v);
 
 /* ------------------------------------------------------------ */
 /*						 Public interface						*/
@@ -365,6 +366,7 @@ be_tls_open_server(Port *port)
 	int			err;
 	int			waitfor;
 	unsigned long ecode;
+	bool		give_proto_hint;
 
 	Assert(!port->ssl);
 	Assert(!port->peer);
@@ -451,10 +453,50 @@ aloop:
 							 errmsg("could not accept SSL connection: EOF detected")));
 				break;
 			case SSL_ERROR_SSL:
+				switch (ERR_GET_REASON(ecode))
+				{
+						/*
+						 * UNSUPPORTED_PROTOCOL, WRONG_VERSION_NUMBER, and
+						 * TLSV1_ALERT_PROTOCOL_VERSION have been observed
+						 * when trying to communicate with an old OpenSSL
+						 * library, or when the client and server specify
+						 * disjoint protocol ranges.  NO_PROTOCOLS_AVAILABLE
+						 * occurs if there's a local misconfiguration (which
+						 * can happen despite our checks, if openssl.cnf
+						 * injects a limit we didn't account for).  It's not
+						 * very clear what would make OpenSSL return the other
+						 * codes listed here, but a hint about protocol
+						 * versions seems like it's appropriate for all.
+						 */
+					case SSL_R_NO_PROTOCOLS_AVAILABLE:
+					case SSL_R_UNSUPPORTED_PROTOCOL:
+					case SSL_R_BAD_PROTOCOL_VERSION_NUMBER:
+					case SSL_R_UNKNOWN_PROTOCOL:
+					case SSL_R_UNKNOWN_SSL_VERSION:
+					case SSL_R_UNSUPPORTED_SSL_VERSION:
+					case SSL_R_VERSION_TOO_HIGH:
+					case SSL_R_VERSION_TOO_LOW:
+					case SSL_R_WRONG_SSL_VERSION:
+					case SSL_R_WRONG_VERSION_NUMBER:
+					case SSL_R_TLSV1_ALERT_PROTOCOL_VERSION:
+						give_proto_hint = true;
+						break;
+					default:
+						give_proto_hint = false;
+						break;
+				}
 				ereport(COMMERROR,
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),
 						 errmsg("could not accept SSL connection: %s",
-								SSLerrmessage(ecode))));
+								SSLerrmessage(ecode)),
+						 give_proto_hint ?
+						 errhint("This may indicate that the client does not support any SSL protocol version between %s and %s.",
+								 ssl_min_protocol_version ?
+								 ssl_protocol_version_to_string(ssl_min_protocol_version) :
+								 MIN_OPENSSL_TLS_VERSION,
+								 ssl_max_protocol_version ?
+								 ssl_protocol_version_to_string(ssl_max_protocol_version) :
+								 MAX_OPENSSL_TLS_VERSION) : 0));
 				break;
 			case SSL_ERROR_ZERO_RETURN:
 				ereport(COMMERROR,
@@ -1328,6 +1370,29 @@ ssl_protocol_version_to_openssl(int v)
 	return -1;
 }
 
+/*
+ * Likewise provide a mapping to strings.
+ */
+static const char *
+ssl_protocol_version_to_string(int v)
+{
+	switch (v)
+	{
+		case PG_TLS_ANY:
+			return "any";
+		case PG_TLS1_VERSION:
+			return "TLSv1";
+		case PG_TLS1_1_VERSION:
+			return "TLSv1.1";
+		case PG_TLS1_2_VERSION:
+			return "TLSv1.2";
+		case PG_TLS1_3_VERSION:
+			return "TLSv1.3";
+	}
+
+	return "(unrecognized)";
+}
+
 
 static void
 default_openssl_tls_init(SSL_CTX *context, bool isServerStart)

src/include/common/openssl.h

@@ -17,12 +17,33 @@
 #ifdef USE_OPENSSL
 #include <openssl/ssl.h>
 
+/*
+ * OpenSSL doesn't provide any very nice way to identify the min/max
+ * protocol versions the library supports, so we fake it as best we can.
+ * Note in particular that this doesn't account for restrictions that
+ * might be specified in the installation's openssl.cnf.
+ *
+ * We disable SSLv3 and older in library setup, so TLSv1 is the oldest
+ * protocol version of interest.
+ */
+#define MIN_OPENSSL_TLS_VERSION  "TLSv1"
+
+#if defined(TLS1_3_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.3"
+#elif defined(TLS1_2_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.2"
+#elif defined(TLS1_1_VERSION)
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1.1"
+#else
+#define MAX_OPENSSL_TLS_VERSION  "TLSv1"
+#endif
+
 /* src/common/protocol_openssl.c */
 #ifndef SSL_CTX_set_min_proto_version
 extern int	SSL_CTX_set_min_proto_version(SSL_CTX *ctx, int version);
 extern int	SSL_CTX_set_max_proto_version(SSL_CTX *ctx, int version);
 #endif
 
-#endif
+#endif							/* USE_OPENSSL */
 
 #endif							/* COMMON_OPENSSL_H */

src/interfaces/libpq/fe-secure-openssl.c

@@ -1304,6 +1304,45 @@ open_client_SSL(PGconn *conn)
 									  libpq_gettext("SSL error: %s\n"),
 									  err);
 					SSLerrfree(err);
+					switch (ERR_GET_REASON(ecode))
+					{
+							/*
+							 * UNSUPPORTED_PROTOCOL, WRONG_VERSION_NUMBER, and
+							 * TLSV1_ALERT_PROTOCOL_VERSION have been observed
+							 * when trying to communicate with an old OpenSSL
+							 * library, or when the client and server specify
+							 * disjoint protocol ranges.
+							 * NO_PROTOCOLS_AVAILABLE occurs if there's a
+							 * local misconfiguration (which can happen
+							 * despite our checks, if openssl.cnf injects a
+							 * limit we didn't account for).  It's not very
+							 * clear what would make OpenSSL return the other
+							 * codes listed here, but a hint about protocol
+							 * versions seems like it's appropriate for all.
+							 */
+						case SSL_R_NO_PROTOCOLS_AVAILABLE:
+						case SSL_R_UNSUPPORTED_PROTOCOL:
+						case SSL_R_BAD_PROTOCOL_VERSION_NUMBER:
+						case SSL_R_UNKNOWN_PROTOCOL:
+						case SSL_R_UNKNOWN_SSL_VERSION:
+						case SSL_R_UNSUPPORTED_SSL_VERSION:
+						case SSL_R_VERSION_TOO_HIGH:
+						case SSL_R_VERSION_TOO_LOW:
+						case SSL_R_WRONG_SSL_VERSION:
+						case SSL_R_WRONG_VERSION_NUMBER:
+						case SSL_R_TLSV1_ALERT_PROTOCOL_VERSION:
+							appendPQExpBuffer(&conn->errorMessage,
+											  libpq_gettext("This may indicate that the server does not support any SSL protocol version between %s and %s.\n"),
+											  conn->ssl_min_protocol_version ?
+											  conn->ssl_min_protocol_version :
+											  MIN_OPENSSL_TLS_VERSION,
+											  conn->ssl_max_protocol_version ?
+											  conn->ssl_max_protocol_version :
+											  MAX_OPENSSL_TLS_VERSION);
+							break;
+						default:
+							break;
+					}
 					pgtls_close(conn);
 					return PGRES_POLLING_FAILED;
 				}