Support channel binding 'tls-unique' in SCRAM

This is the basic feature set using OpenSSL to support the feature.  In
order to allow the frontend and the backend to fetch the sent and
expected TLS Finished messages, a PG-like API is added to be able to
make the interface pluggable for other SSL implementations.

This commit also adds a infrastructure to facilitate the addition of
future channel binding types as well as libpq parameters to control the
SASL mechanism names and channel binding names.  Those will be added by
upcoming commits.

Some tests are added to the SSL test suite to test SCRAM authentication
with channel binding.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>

Author: petere

doc/src/sgml/protocol.sgml

@@ -1461,10 +1461,11 @@ SELCT 1/0;
 
 <para>
 <firstterm>SASL</firstterm> is a framework for authentication in connection-oriented
-protocols. At the moment, <productname>PostgreSQL</productname> implements only one SASL
-authentication mechanism, SCRAM-SHA-256, but more might be added in the
-future. The below steps illustrate how SASL authentication is performed in
-general, while the next subsection gives more details on SCRAM-SHA-256.
+protocols. At the moment, <productname>PostgreSQL</productname> implements two SASL
+authentication mechanisms, SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. More
+might be added in the future. The below steps illustrate how SASL
+authentication is performed in general, while the next subsection gives
+more details on SCRAM-SHA-256 and SCRAM-SHA-256-PLUS.
 </para>
 
 <procedure>
@@ -1518,9 +1519,10 @@ ErrorMessage.
   <title>SCRAM-SHA-256 authentication</title>
 
   <para>
-    <firstterm>SCRAM-SHA-256</firstterm> (called just <firstterm>SCRAM</firstterm> from now on) is
-    the only implemented SASL mechanism, at the moment. It is described in detail
-    in RFC 7677 and RFC 5802.
+   The implemented SASL mechanisms at the moment
+   are <literal>SCRAM-SHA-256</literal> and its variant with channel
+   binding <literal>SCRAM-SHA-256-PLUS</literal>. They are described in
+   detail in RFC 7677 and RFC 5802.
   </para>
 
   <para>
@@ -1547,7 +1549,10 @@ the password is in.
   </para>
 
   <para>
-<firstterm>Channel binding</firstterm> has not been implemented yet.
+<firstterm>Channel binding</firstterm> is supported in PostgreSQL builds with
+SSL support. The SASL mechanism name for SCRAM with channel binding
+is <literal>SCRAM-SHA-256-PLUS</literal>.  The only channel binding type
+supported at the moment is <literal>tls-unique</literal>, defined in RFC 5929.
   </para>
 
 <procedure>
@@ -1556,13 +1561,19 @@ the password is in.
 <para>
   The server sends an AuthenticationSASL message. It includes a list of
   SASL authentication mechanisms that the server can accept.
+  This will be <literal>SCRAM-SHA-256-PLUS</literal>
+  and <literal>SCRAM-SHA-256</literal> if the server is built with SSL
+  support, or else just the latter.
 </para>
 </step>
 <step id="scram-client-first">
 <para>
   The client responds by sending a SASLInitialResponse message, which
-  indicates the chosen mechanism, <literal>SCRAM-SHA-256</literal>. In the Initial
-  Client response field, the message contains the SCRAM
+  indicates the chosen mechanism, <literal>SCRAM-SHA-256</literal> or
+  <literal>SCRAM-SHA-256-PLUS</literal>. (A client is free to choose either
+  mechanism, but for better security it should choose the channel-binding
+  variant if it can support it.) In the Initial Client response field,
+  the message contains the SCRAM
   <structname>client-first-message</structname>.
 </para>
 </step>

src/backend/libpq/auth-scram.c

@@ -17,8 +17,6 @@
  *	 by the SASLprep profile, we skip the SASLprep pre-processing and use
  *	 the raw bytes in calculating the hash.
  *
- * - Channel binding is not supported yet.
- *
  *
  * The password stored in pg_authid consists of the iteration count, salt,
  * StoredKey and ServerKey.
@@ -112,6 +110,11 @@ typedef struct
 
 	const char *username;		/* username from startup packet */
 
+	bool		ssl_in_use;
+	const char *tls_finished_message;
+	size_t		tls_finished_len;
+	char	   *channel_binding_type;
+
 	int			iterations;
 	char	   *salt;			/* base64-encoded */
 	uint8		StoredKey[SCRAM_KEY_LEN];
@@ -168,14 +171,22 @@ static char *scram_mock_salt(const char *username);
  * it will fail, as if an incorrect password was given.
  */
 void *
-pg_be_scram_init(const char *username, const char *shadow_pass)
+pg_be_scram_init(const char *username,
+				 const char *shadow_pass,
+				 bool ssl_in_use,
+				 const char *tls_finished_message,
+				 size_t tls_finished_len)
 {
 	scram_state *state;
 	bool		got_verifier;
 
 	state = (scram_state *) palloc0(sizeof(scram_state));
 	state->state = SCRAM_AUTH_INIT;
 	state->username = username;
+	state->ssl_in_use = ssl_in_use;
+	state->tls_finished_message = tls_finished_message;
+	state->tls_finished_len = tls_finished_len;
+	state->channel_binding_type = NULL;
 
 	/*
 	 * Parse the stored password verifier.
@@ -773,45 +784,96 @@ read_client_first_message(scram_state *state, char *input)
 	 *------
 	 */
 
-	/* read gs2-cbind-flag */
+	/*
+	 * Read gs2-cbind-flag.  (For details see also RFC 5802 Section 6 "Channel
+	 * Binding".)
+	 */
 	switch (*input)
 	{
 		case 'n':
-			/* Client does not support channel binding */
+			/*
+			 * The client does not support channel binding or has simply
+			 * decided to not use it.  In that case just let it go.
+			 */
+			input++;
+			if (*input != ',')
+				ereport(ERROR,
+						(errcode(ERRCODE_PROTOCOL_VIOLATION),
+						 errmsg("malformed SCRAM message"),
+						 errdetail("Comma expected, but found character \"%s\".",
+								   sanitize_char(*input))));
 			input++;
 			break;
 		case 'y':
-			/* Client supports channel binding, but we're not doing it today */
+			/*
+			 * The client supports channel binding and thinks that the server
+			 * does not.  In this case, the server must fail authentication if
+			 * it supports channel binding, which in this implementation is
+			 * the case if a connection is using SSL.
+			 */
+			if (state->ssl_in_use)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
+						 errmsg("SCRAM channel binding negotiation error"),
+						 errdetail("The client supports SCRAM channel binding but thinks the server does not.  "
+								   "However, this server does support channel binding.")));
+			input++;
+			if (*input != ',')
+				ereport(ERROR,
+						(errcode(ERRCODE_PROTOCOL_VIOLATION),
+						 errmsg("malformed SCRAM message"),
+						 errdetail("Comma expected, but found character \"%s\".",
+								   sanitize_char(*input))));
 			input++;
 			break;
 		case 'p':
-
 			/*
-			 * Client requires channel binding.  We don't support it.
-			 *
-			 * RFC 5802 specifies a particular error code,
-			 * e=server-does-support-channel-binding, for this.  But it can
-			 * only be sent in the server-final message, and we don't want to
-			 * go through the motions of the authentication, knowing it will
-			 * fail, just to send that error message.
+			 * The client requires channel binding.  Channel binding type
+			 * follows, e.g., "p=tls-unique".
 			 */
-			ereport(ERROR,
-					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("client requires SCRAM channel binding, but it is not supported")));
+			{
+				char *channel_binding_type;
+
+				if (!state->ssl_in_use)
+				{
+					/*
+					 * Without SSL, we don't support channel binding.
+					 *
+					 * RFC 5802 specifies a particular error code,
+					 * e=server-does-support-channel-binding, for this.  But
+					 * it can only be sent in the server-final message, and we
+					 * don't want to go through the motions of the
+					 * authentication, knowing it will fail, just to send that
+					 * error message.
+					 */
+					ereport(ERROR,
+							(errcode(ERRCODE_PROTOCOL_VIOLATION),
+							 errmsg("client requires SCRAM channel binding, but it is not supported")));
+				}
+
+				/*
+				 * Read value provided by client; only tls-unique is supported
+				 * for now.  (It is not safe to print the name of an
+				 * unsupported binding type in the error message.  Pranksters
+				 * could print arbitrary strings into the log that way.)
+				 */
+				channel_binding_type = read_attr_value(&input, 'p');
+				if (strcmp(channel_binding_type, SCRAM_CHANNEL_BINDING_TLS_UNIQUE) != 0)
+					ereport(ERROR,
+							(errcode(ERRCODE_PROTOCOL_VIOLATION),
+							 (errmsg("unsupported SCRAM channel-binding type"))));
+
+				/* Save the name for handling of subsequent messages */
+				state->channel_binding_type = pstrdup(channel_binding_type);
+			}
+			break;
 		default:
 			ereport(ERROR,
 					(errcode(ERRCODE_PROTOCOL_VIOLATION),
 					 errmsg("malformed SCRAM message"),
 					 errdetail("Unexpected channel-binding flag \"%s\".",
 							   sanitize_char(*input))));
 	}
-	if (*input != ',')
-		ereport(ERROR,
-				(errcode(ERRCODE_PROTOCOL_VIOLATION),
-				 errmsg("malformed SCRAM message"),
-				 errdetail("Comma expected, but found character \"%s\".",
-						   sanitize_char(*input))));
-	input++;
 
 	/*
 	 * Forbid optional authzid (authorization identity).  We don't support it.
@@ -1032,14 +1094,73 @@ read_client_final_message(scram_state *state, char *input)
 	 */
 
 	/*
-	 * Read channel-binding.  We don't support channel binding, so it's
-	 * expected to always be "biws", which is "n,,", base64-encoded.
+	 * Read channel binding.  This repeats the channel-binding flags and is
+	 * then followed by the actual binding data depending on the type.
 	 */
 	channel_binding = read_attr_value(&p, 'c');
-	if (strcmp(channel_binding, "biws") != 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_PROTOCOL_VIOLATION),
-				 (errmsg("unexpected SCRAM channel-binding attribute in client-final-message"))));
+	if (state->channel_binding_type)
+	{
+		const char *cbind_data = NULL;
+		size_t		cbind_data_len = 0;
+		size_t		cbind_header_len;
+		char	   *cbind_input;
+		size_t		cbind_input_len;
+		char	   *b64_message;
+		int			b64_message_len;
+
+		/*
+		 * Fetch data appropriate for channel binding type
+		 */
+		if (strcmp(state->channel_binding_type, SCRAM_CHANNEL_BINDING_TLS_UNIQUE) == 0)
+		{
+			cbind_data = state->tls_finished_message;
+			cbind_data_len = state->tls_finished_len;
+		}
+		else
+		{
+			/* should not happen */
+			elog(ERROR, "invalid channel binding type");
+		}
+
+		/* should not happen */
+		if (cbind_data == NULL || cbind_data_len == 0)
+			elog(ERROR, "empty channel binding data for channel binding type \"%s\"",
+				 state->channel_binding_type);
+
+		cbind_header_len = 4 + strlen(state->channel_binding_type); /* p=type,, */
+		cbind_input_len = cbind_header_len + cbind_data_len;
+		cbind_input = palloc(cbind_input_len);
+		snprintf(cbind_input, cbind_input_len, "p=%s,,", state->channel_binding_type);
+		memcpy(cbind_input + cbind_header_len, cbind_data, cbind_data_len);
+
+		b64_message = palloc(pg_b64_enc_len(cbind_input_len) + 1);
+		b64_message_len = pg_b64_encode(cbind_input, cbind_input_len,
+										b64_message);
+		b64_message[b64_message_len] = '\0';
+
+		/*
+		 * Compare the value sent by the client with the value expected by
+		 * the server.
+		 */
+		if (strcmp(channel_binding, b64_message) != 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
+					 (errmsg("SCRAM channel binding check failed"))));
+	}
+	else
+	{
+		/*
+		 * If we are not using channel binding, the binding data is expected
+		 * to always be "biws", which is "n,," base64-encoded, or "eSws",
+		 * which is "y,,".
+		 */
+		if (strcmp(channel_binding, "biws") != 0 &&
+			strcmp(channel_binding, "eSws") != 0)
+			ereport(ERROR,
+					(errcode(ERRCODE_PROTOCOL_VIOLATION),
+					 (errmsg("unexpected SCRAM channel-binding attribute in client-final-message"))));
+	}
+
 	state->client_final_nonce = read_attr_value(&p, 'r');
 
 	/* ignore optional extensions */

src/backend/libpq/auth.c

@@ -860,6 +860,8 @@ CheckMD5Auth(Port *port, char *shadow_pass, char **logdetail)
 static int
 CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 {
+	char	   *sasl_mechs;
+	char	   *p;
 	int			mtype;
 	StringInfoData buf;
 	void	   *scram_opaq;
@@ -869,6 +871,8 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 	int			inputlen;
 	int			result;
 	bool		initial;
+	char	   *tls_finished = NULL;
+	size_t		tls_finished_len = 0;
 
 	/*
 	 * SASL auth is not supported for protocol versions before 3, because it
@@ -885,12 +889,39 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 
 	/*
 	 * Send the SASL authentication request to user.  It includes the list of
-	 * authentication mechanisms (which is trivial, because we only support
-	 * SCRAM-SHA-256 at the moment).  The extra "\0" is for an empty string to
-	 * terminate the list.
+	 * authentication mechanisms that are supported.  The order of mechanisms
+	 * is advertised in decreasing order of importance.  So the
+	 * channel-binding variants go first, if they are supported.  Channel
+	 * binding is only supported in SSL builds.
 	 */
-	sendAuthRequest(port, AUTH_REQ_SASL, SCRAM_SHA256_NAME "\0",
-					strlen(SCRAM_SHA256_NAME) + 2);
+	sasl_mechs = palloc(strlen(SCRAM_SHA256_PLUS_NAME) +
+						strlen(SCRAM_SHA256_NAME) + 3);
+	p = sasl_mechs;
+
+	if (port->ssl_in_use)
+	{
+		strcpy(p, SCRAM_SHA256_PLUS_NAME);
+		p += strlen(SCRAM_SHA256_PLUS_NAME) + 1;
+	}
+
+	strcpy(p, SCRAM_SHA256_NAME);
+	p += strlen(SCRAM_SHA256_NAME) + 1;
+
+	/* Put another '\0' to mark that list is finished. */
+	p[0] = '\0';
+
+	sendAuthRequest(port, AUTH_REQ_SASL, sasl_mechs, p - sasl_mechs + 1);
+	pfree(sasl_mechs);
+
+#ifdef USE_SSL
+	/*
+	 * Get data for channel binding.
+	 */
+	if (port->ssl_in_use)
+	{
+		tls_finished = be_tls_get_peer_finished(port, &tls_finished_len);
+	}
+#endif
 
 	/*
 	 * Initialize the status tracker for message exchanges.
@@ -903,7 +934,11 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 	 * This is because we don't want to reveal to an attacker what usernames
 	 * are valid, nor which users have a valid password.
 	 */
-	scram_opaq = pg_be_scram_init(port->user_name, shadow_pass);
+	scram_opaq = pg_be_scram_init(port->user_name,
+								  shadow_pass,
+								  port->ssl_in_use,
+								  tls_finished,
+								  tls_finished_len);
 
 	/*
 	 * Loop through SASL message exchange.  This exchange can consist of
@@ -951,12 +986,9 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 		{
 			const char *selected_mech;
 
-			/*
-			 * We only support SCRAM-SHA-256 at the moment, so anything else
-			 * is an error.
-			 */
 			selected_mech = pq_getmsgrawstring(&buf);
-			if (strcmp(selected_mech, SCRAM_SHA256_NAME) != 0)
+			if (strcmp(selected_mech, SCRAM_SHA256_NAME) != 0 &&
+				strcmp(selected_mech, SCRAM_SHA256_PLUS_NAME) != 0)
 			{
 				ereport(ERROR,
 						(errcode(ERRCODE_PROTOCOL_VIOLATION),