Improve the SASL authentication protocol.

This contains some protocol changes to SASL authentiation (which is new
in v10):

* For future-proofing, in the AuthenticationSASL message that begins SASL
  authentication, provide a list of SASL mechanisms that the server
  supports, for the client to choose from. Currently, it's always just
  SCRAM-SHA-256.

* Add a separate authentication message type for the final server->client
  SASL message, which the client doesn't need to respond to. This makes
  it unambiguous whether the client is supposed to send a response or not.
  The SASL mechanism should know that anyway, but better to be explicit.

Also, in the server, support clients that don't send an Initial Client
response in the first SASLInitialResponse message. The server is supposed
to first send an empty request in that case, to which the client will
respond with the data that usually comes in the Initial Client Response.
libpq uses the Initial Client Response field and doesn't need this, and I
would assume any other sensible implementation to use Initial Client
Response, too, but let's follow the SASL spec.

Improve the documentation on SASL authentication in protocol. Add a
section describing the SASL message flow, and some details on our
SCRAM-SHA-256 implementation.

Document the different kinds of PasswordMessages that the frontend sends
in different phases of SASL authentication, as well as GSS/SSPI
authentication as separate message formats. Even though they're all 'p'
messages, and the exact format depends on the context, describing them as
separate message formats makes the documentation more clear.

Reviewed by Michael Paquier and Álvaro Hernández Tortosa.

Discussion: https://www.postgresql.org/message-id/CAB7nPqS-aFg0iM3AQOJwKDv_0WkAedRjs1W2X8EixSz+sKBXCQ@mail.gmail.com

2 files changed, 85 insertions, 10 deletions

diff --git a/src/backend/libpq/auth-scram.c b/src/backend/libpq/auth-scram.c
index a47c48d9805..338afede9de 100644
--- a/src/backend/libpq/auth-scram.c
+++ b/src/backend/libpq/auth-scram.c
@@ -254,8 +254,16 @@ pg_be_scram_init(const char *username, const char *shadow_pass)
 /*
  * Continue a SCRAM authentication exchange.
  *
- * The next message to send to client is saved in "output", for a length
- * of "outputlen".  In the case of an error, optionally store a palloc'd
+ * 'input' is the SCRAM payload sent by the client.  On the first call,
+ * 'input' contains the "Initial Client Response" that the client sent as
+ * part of the SASLInitialResponse message, or NULL if no Initial Client
+ * Response was given.  (The SASL specification distinguishes between an
+ * empty response and non-existing one.)  On subsequent calls, 'input'
+ * cannot be NULL.  For convenience in this function, the caller must
+ * ensure that there is a null terminator at input[inputlen].
+ *
+ * The next message to send to client is saved in 'output', for a length
+ * of 'outputlen'.  In the case of an error, optionally store a palloc'd
  * string at *logdetail that will be sent to the postmaster log (but not
  * the client).
  */
@@ -269,6 +277,21 @@ pg_be_scram_exchange(void *opaq, char *input, int inputlen,
 	*output = NULL;
 
 	/*
+	 * If the client didn't include an "Initial Client Response" in the
+	 * SASLInitialResponse message, send an empty challenge, to which the
+	 * client will respond with the same data that usually comes in the
+	 * Initial Client Response.
+	 */
+	if (input == NULL)
+	{
+		Assert(state->state == SCRAM_AUTH_INIT);
+
+		*output = pstrdup("");
+		*outputlen = 0;
+		return SASL_EXCHANGE_CONTINUE;
+	}
+
+	/*
 	 * Check that the input length agrees with the string length of the input.
 	 * We can ignore inputlen after this.
 	 */
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index b4c98c45c9f..848561e188e 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -620,10 +620,11 @@ sendAuthRequest(Port *port, AuthRequest areq, char *extradata, int extralen)
 	pq_endmessage(&buf);
 
 	/*
-	 * Flush message so client will see it, except for AUTH_REQ_OK, which need
-	 * not be sent until we are ready for queries.
+	 * Flush message so client will see it, except for AUTH_REQ_OK and
+	 * AUTH_REQ_SASL_FIN, which need not be sent until we are ready for
+	 * queries.
 	 */
-	if (areq != AUTH_REQ_OK)
+	if (areq != AUTH_REQ_OK && areq != AUTH_REQ_SASL_FIN)
 		pq_flush();
 
 	CHECK_FOR_INTERRUPTS();
@@ -850,7 +851,10 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 	void	   *scram_opaq;
 	char	   *output = NULL;
 	int			outputlen = 0;
+	char	   *input;
+	int			inputlen;
 	int			result;
+	bool		initial;
 
 	/*
 	 * SASL auth is not supported for protocol versions before 3, because it
@@ -866,10 +870,13 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 				 errmsg("SASL authentication is not supported in protocol version 2")));
 
 	/*
-	 * Send first the authentication request to user.
+	 * 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.
 	 */
-	sendAuthRequest(port, AUTH_REQ_SASL, SCRAM_SHA256_NAME,
-					strlen(SCRAM_SHA256_NAME) + 1);
+	sendAuthRequest(port, AUTH_REQ_SASL, SCRAM_SHA256_NAME "\0",
+					strlen(SCRAM_SHA256_NAME) + 2);
 
 	/*
 	 * Initialize the status tracker for message exchanges.
@@ -890,6 +897,7 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 	 * from the client.  All messages from client to server are password
 	 * packets (type 'p').
 	 */
+	initial = true;
 	do
 	{
 		pq_startmsgread();
@@ -921,10 +929,51 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 		elog(DEBUG4, "Processing received SASL response of length %d", buf.len);
 
 		/*
+		 * The first SASLInitialResponse message is different from the others.
+		 * It indicates which SASL mechanism the client selected, and contains
+		 * an optional Initial Client Response payload.  The subsequent
+		 * SASLResponse messages contain just the SASL payload.
+		 */
+		if (initial)
+		{
+			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)
+				ereport(COMMERROR,
+						(errcode(ERRCODE_PROTOCOL_VIOLATION),
+						 errmsg("client selected an invalid SASL authentication mechanism")));
+
+			inputlen = pq_getmsgint(&buf, 4);
+			if (inputlen == -1)
+				input = NULL;
+			else
+				input = (char *) pq_getmsgbytes(&buf, inputlen);
+
+			initial = false;
+		}
+		else
+		{
+			inputlen = buf.len;
+			input = (char *) pq_getmsgbytes(&buf, buf.len);
+		}
+		pq_getmsgend(&buf);
+
+		/*
+		 * The StringInfo guarantees that there's a \0 byte after the
+		 * response.
+		 */
+		Assert(input == NULL || input[inputlen] == '\0');
+
+		/*
 		 * we pass 'logdetail' as NULL when doing a mock authentication,
 		 * because we should already have a better error message in that case
 		 */
-		result = pg_be_scram_exchange(scram_opaq, buf.data, buf.len,
+		result = pg_be_scram_exchange(scram_opaq, input, inputlen,
 									  &output, &outputlen,
 									  logdetail);
 
@@ -938,7 +987,10 @@ CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 			 */
 			elog(DEBUG4, "sending SASL challenge of length %u", outputlen);
 
-			sendAuthRequest(port, AUTH_REQ_SASL_CONT, output, outputlen);
+			if (result == SASL_EXCHANGE_SUCCESS)
+				sendAuthRequest(port, AUTH_REQ_SASL_FIN, output, outputlen);
+			else
+				sendAuthRequest(port, AUTH_REQ_SASL_CONT, output, outputlen);
 
 			pfree(output);
 		}