Allow SCRAM authentication, when pg_hba.conf says 'md5'.

If a user has a SCRAM verifier in pg_authid.rolpassword, there's no reason
we cannot attempt to perform SCRAM authentication instead of MD5. The worst
that can happen is that the client doesn't support SCRAM, and the
authentication will fail. But previously, it would fail for sure, because
we would not even try. SCRAM is strictly more secure than MD5, so there's
no harm in trying it. This allows for a more graceful transition from MD5
passwords to SCRAM, as user passwords can be changed to SCRAM verifiers
incrementally, without changing pg_hba.conf.

Refactor the code in auth.c to support that better. Notably, we now have to
look up the user's pg_authid entry before sending the password challenge,
also when performing MD5 authentication. Also simplify the concept of a
"doomed" authentication. Previously, if a user had a password, but it had
expired, we still performed SCRAM authentication (but always returned error
at the end) using the salt and iteration count from the expired password.
Now we construct a fake salt, like we do when the user doesn't have a
password or doesn't exist at all. That simplifies get_role_password(), and
we can don't need to distinguish the  "user has expired password", and
"user does not exist" cases in auth.c.

On second thoughts, also rename uaSASL to uaSCRAM. It refers to the
mechanism specified in pg_hba.conf, and while we use SASL for SCRAM
authentication at the protocol level, the mechanism should be called SCRAM,
not SASL. As a comparison, we have uaLDAP, even though it looks like the
plain 'password' authentication at the protocol level.

Discussion: https://www.postgresql.org/message-id/6425.1489506016@sss.pgh.pa.us
Reviewed-by: Michael Paquier

src/backend/libpq/auth-scram.c

@@ -130,79 +130,91 @@ static char *scram_MockSalt(const char *username);
  * after the beginning of the exchange with verifier data.
  *
  * 'username' is the provided by the client.  'shadow_pass' is the role's
- * password verifier, from pg_authid.rolpassword.  If 'doomed' is true, the
- * authentication must fail, as if an incorrect password was given.
- * 'shadow_pass' may be NULL, when 'doomed' is set.
+ * password verifier, from pg_authid.rolpassword.  If 'shadow_pass' is NULL, we
+ * still perform an authentication exchange, but it will fail, as if an
+ * incorrect password was given.
  */
 void *
-pg_be_scram_init(const char *username, const char *shadow_pass, bool doomed)
+pg_be_scram_init(const char *username, const char *shadow_pass)
 {
 	scram_state *state;
-	int			password_type;
+	bool		got_verifier;
 
 	state = (scram_state *) palloc0(sizeof(scram_state));
 	state->state = SCRAM_AUTH_INIT;
 	state->username = username;
 
 	/*
-	 * Perform sanity checks on the provided password after catalog lookup.
-	 * The authentication is bound to fail if the lookup itself failed or if
-	 * the password stored is MD5-encrypted.  Authentication is possible for
-	 * users with a valid plain password though.
+	 * Parse the stored password verifier.
 	 */
-
-	if (shadow_pass == NULL || doomed)
-		password_type = -1;
-	else
-		password_type = get_password_type(shadow_pass);
-
-	if (password_type == PASSWORD_TYPE_SCRAM)
+	if (shadow_pass)
 	{
-		if (!parse_scram_verifier(shadow_pass, &state->salt, &state->iterations,
-								  state->StoredKey, state->ServerKey))
+		int			password_type = get_password_type(shadow_pass);
+
+		if (password_type == PASSWORD_TYPE_SCRAM)
+		{
+			if (parse_scram_verifier(shadow_pass, &state->salt, &state->iterations,
+									 state->StoredKey, state->ServerKey))
+				got_verifier = true;
+			else
+			{
+				/*
+				 * The password looked like a SCRAM verifier, but could not be
+				 * parsed.
+				 */
+				elog(LOG, "invalid SCRAM verifier for user \"%s\"", username);
+				got_verifier = false;
+			}
+		}
+		else if (password_type == PASSWORD_TYPE_PLAINTEXT)
 		{
 			/*
-			 * The password looked like a SCRAM verifier, but could not be
-			 * parsed.
+			 * The stored password is in plain format.  Generate a fresh SCRAM
+			 * verifier from it, and proceed with that.
 			 */
-			elog(LOG, "invalid SCRAM verifier for user \"%s\"", username);
-			doomed = true;
+			char	   *verifier;
+
+			verifier = scram_build_verifier(username, shadow_pass, 0);
+
+			(void) parse_scram_verifier(verifier, &state->salt, &state->iterations,
+										state->StoredKey, state->ServerKey);
+			pfree(verifier);
+
+			got_verifier = true;
+		}
+		else
+		{
+			/*
+			 * The user doesn't have SCRAM verifier, nor could we generate
+			 * one. (You cannot do SCRAM authentication with an MD5 hash.)
+			 */
+			state->logdetail = psprintf(_("User \"%s\" does not have a valid SCRAM verifier."),
+										state->username);
+			got_verifier = false;
 		}
 	}
-	else if (password_type == PASSWORD_TYPE_PLAINTEXT)
-	{
-		char	   *verifier;
-
-		/*
-		 * The password provided is in plain format, in which case a fresh
-		 * SCRAM verifier can be generated and used for the rest of the
-		 * processing.
-		 */
-		verifier = scram_build_verifier(username, shadow_pass, 0);
-
-		(void) parse_scram_verifier(verifier, &state->salt, &state->iterations,
-									state->StoredKey, state->ServerKey);
-		pfree(verifier);
-	}
 	else
-		doomed = true;
-
-	if (doomed)
 	{
 		/*
-		 * We don't have a valid SCRAM verifier, nor could we generate one, or
-		 * the caller requested us to perform a dummy authentication.
-		 *
-		 * The authentication is bound to fail, but to avoid revealing
-		 * information to the attacker, go through the motions with a fake
-		 * SCRAM verifier, and fail as if the password was incorrect.
+		 * The caller requested us to perform a dummy authentication.  This is
+		 * considered normal, since the caller requested it, so don't set log
+		 * detail.
 		 */
-		state->logdetail = psprintf(_("User \"%s\" does not have a valid SCRAM verifier."),
-									state->username);
+		got_verifier = false;
+	}
+
+	/*
+	 * If the user did not have a valid SCRAM verifier, we still go through
+	 * the motions with a mock one, and fail as if the client supplied an
+	 * incorrect password.  This is to avoid revealing information to an
+	 * attacker.
+	 */
+	if (!got_verifier)
+	{
 		mock_scram_verifier(username, &state->salt, &state->iterations,
 							state->StoredKey, state->ServerKey);
+		state->doomed = true;
 	}
-	state->doomed = doomed;
 
 	return state;
 }

src/backend/libpq/auth.c

@@ -24,6 +24,7 @@
 #include <sys/select.h>
 #endif
 
+#include "commands/user.h"
 #include "common/ip.h"
 #include "common/md5.h"
 #include "libpq/auth.h"
@@ -49,17 +50,15 @@ static char *recv_password_packet(Port *port);
 
 
 /*----------------------------------------------------------------
- * MD5 authentication
+ * Password-based authentication methods (password, md5, and scram)
  *----------------------------------------------------------------
  */
-static int	CheckMD5Auth(Port *port, char **logdetail);
-
-/*----------------------------------------------------------------
- * Plaintext password authentication
- *----------------------------------------------------------------
- */
-
 static int	CheckPasswordAuth(Port *port, char **logdetail);
+static int	CheckPWChallengeAuth(Port *port, char **logdetail);
+
+static int	CheckMD5Auth(Port *port, char *shadow_pass, char **logdetail);
+static int	CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail);
+
 
 /*----------------------------------------------------------------
  * Ident authentication
@@ -200,12 +199,6 @@ static int	CheckRADIUSAuth(Port *port);
 static int	PerformRadiusTransaction(char *server, char *secret, char *portstr, char *identifier, char *user_name, char *passwd);
 
 
-/*----------------------------------------------------------------
- * SASL authentication
- *----------------------------------------------------------------
- */
-static int	CheckSASLAuth(Port *port, char **logdetail);
-
 /*
  * Maximum accepted size of GSS and SSPI authentication tokens.
  *
@@ -291,7 +284,7 @@ auth_failed(Port *port, int status, char *logdetail)
 			break;
 		case uaPassword:
 		case uaMD5:
-		case uaSASL:
+		case uaSCRAM:
 			errstr = gettext_noop("password authentication failed for user \"%s\"");
 			/* We use it to indicate if a .pgpass password failed. */
 			errcode_return = ERRCODE_INVALID_PASSWORD;
@@ -552,17 +545,14 @@ ClientAuthentication(Port *port)
 			break;
 
 		case uaMD5:
-			status = CheckMD5Auth(port, &logdetail);
+		case uaSCRAM:
+			status = CheckPWChallengeAuth(port, &logdetail);
 			break;
 
 		case uaPassword:
 			status = CheckPasswordAuth(port, &logdetail);
 			break;
 
-		case uaSASL:
-			status = CheckSASLAuth(port, &logdetail);
-			break;
-
 		case uaPAM:
 #ifdef USE_PAM
 			status = CheckPAMAuth(port, port->user_name, "");
@@ -710,16 +700,116 @@ recv_password_packet(Port *port)
 
 
 /*----------------------------------------------------------------
- * MD5 authentication
+ * Password-based authentication mechanisms
  *----------------------------------------------------------------
  */
 
+/*
+ * Plaintext password authentication.
+ */
 static int
-CheckMD5Auth(Port *port, char **logdetail)
+CheckPasswordAuth(Port *port, char **logdetail)
+{
+	char	   *passwd;
+	int			result;
+	char	   *shadow_pass;
+
+	sendAuthRequest(port, AUTH_REQ_PASSWORD, NULL, 0);
+
+	passwd = recv_password_packet(port);
+	if (passwd == NULL)
+		return STATUS_EOF;		/* client wouldn't send password */
+
+	shadow_pass = get_role_password(port->user_name, logdetail);
+	if (shadow_pass)
+	{
+		result = plain_crypt_verify(port->user_name, shadow_pass, passwd,
+									logdetail);
+	}
+	else
+		result = STATUS_ERROR;
+
+	if (shadow_pass)
+		pfree(shadow_pass);
+	pfree(passwd);
+
+	return result;
+}
+
+/*
+ * MD5 and SCRAM authentication.
+ */
+static int
+CheckPWChallengeAuth(Port *port, char **logdetail)
+{
+	int			auth_result;
+	char	   *shadow_pass;
+	PasswordType pwtype;
+
+	Assert(port->hba->auth_method == uaSCRAM ||
+		   port->hba->auth_method == uaMD5);
+
+	/* First look up the user's password. */
+	shadow_pass = get_role_password(port->user_name, logdetail);
+
+	/*
+	 * If the user does not exist, or has no password, we still go through the
+	 * motions of authentication, to avoid revealing to the client that the
+	 * user didn't exist.  If 'md5' is allowed, we choose whether to use 'md5'
+	 * or 'scram' authentication based on current password_encryption setting.
+	 * The idea is that most genuine users probably have a password of that
+	 * type, if we pretend that this user had a password of that type, too, it
+	 * "blends in" best.
+	 *
+	 * If the user had a password, but it was expired, we'll use the details
+	 * of the expired password for the authentication, but report it as
+	 * failure to the client even if correct password was given.
+	 */
+	if (!shadow_pass)
+		pwtype = Password_encryption;
+	else
+		pwtype = get_password_type(shadow_pass);
+
+	/*
+	 * If 'md5' authentication is allowed, decide whether to perform 'md5' or
+	 * 'scram' authentication based on the type of password the user has.  If
+	 * it's an MD5 hash, we must do MD5 authentication, and if it's a SCRAM
+	 * verifier, we must do SCRAM authentication.  If it's stored in
+	 * plaintext, we could do either one, so we opt for the more secure
+	 * mechanism, SCRAM.
+	 *
+	 * If MD5 authentication is not allowed, always use SCRAM.  If the user
+	 * had an MD5 password, CheckSCRAMAuth() will fail.
+	 */
+	if (port->hba->auth_method == uaMD5 && pwtype == PASSWORD_TYPE_MD5)
+	{
+		auth_result = CheckMD5Auth(port, shadow_pass, logdetail);
+	}
+	else
+	{
+		auth_result = CheckSCRAMAuth(port, shadow_pass, logdetail);
+	}
+
+	if (shadow_pass)
+		pfree(shadow_pass);
+
+	/*
+	 * If get_role_password() returned error, return error, even if the
+	 * authentication succeeded.
+	 */
+	if (!shadow_pass)
+	{
+		Assert(auth_result != STATUS_OK);
+		return STATUS_ERROR;
+	}
+	return auth_result;
+}
+
+static int
+CheckMD5Auth(Port *port, char *shadow_pass, char **logdetail)
 {
 	char		md5Salt[4];		/* Password salt */
 	char	   *passwd;
-	char	   *shadow_pass;
 	int			result;
 
 	if (Db_user_namespace)
@@ -741,54 +831,19 @@ CheckMD5Auth(Port *port, char **logdetail)
 	if (passwd == NULL)
 		return STATUS_EOF;		/* client wouldn't send password */
 
-	result = get_role_password(port->user_name, &shadow_pass, logdetail);
-	if (result == STATUS_OK)
+	if (shadow_pass)
 		result = md5_crypt_verify(port->user_name, shadow_pass, passwd,
 								  md5Salt, 4, logdetail);
+	else
+		result = STATUS_ERROR;
 
-	if (shadow_pass)
-		pfree(shadow_pass);
 	pfree(passwd);
 
 	return result;
 }
 
-/*----------------------------------------------------------------
- * Plaintext password authentication
- *----------------------------------------------------------------
- */
-
 static int
-CheckPasswordAuth(Port *port, char **logdetail)
-{
-	char	   *passwd;
-	int			result;
-	char	   *shadow_pass;
-
-	sendAuthRequest(port, AUTH_REQ_PASSWORD, NULL, 0);
-
-	passwd = recv_password_packet(port);
-	if (passwd == NULL)
-		return STATUS_EOF;		/* client wouldn't send password */
-
-	result = get_role_password(port->user_name, &shadow_pass, logdetail);
-	if (result == STATUS_OK)
-		result = plain_crypt_verify(port->user_name, shadow_pass, passwd,
-									logdetail);
-
-	if (shadow_pass)
-		pfree(shadow_pass);
-	pfree(passwd);
-
-	return result;
-}
-
-/*----------------------------------------------------------------
- * SASL authentication system
- *----------------------------------------------------------------
- */
-static int
-CheckSASLAuth(Port *port, char **logdetail)
+CheckSCRAMAuth(Port *port, char *shadow_pass, char **logdetail)
 {
 	int			mtype;
 	StringInfoData buf;
@@ -796,8 +851,6 @@ CheckSASLAuth(Port *port, char **logdetail)
 	char	   *output = NULL;
 	int			outputlen = 0;
 	int			result;
-	char	   *shadow_pass;
-	bool		doomed = false;
 
 	/*
 	 * SASL auth is not supported for protocol versions before 3, because it
@@ -827,11 +880,9 @@ CheckSASLAuth(Port *port, 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.
 	 */
-	if (get_role_password(port->user_name, &shadow_pass, logdetail) != STATUS_OK)
-		doomed = true;
 
 	/* Initialize the status tracker for message exchanges */
-	scram_opaq = pg_be_scram_init(port->user_name, shadow_pass, doomed);
+	scram_opaq = pg_be_scram_init(port->user_name, shadow_pass);
 
 	/*
 	 * Loop through SASL message exchange.  This exchange can consist of
@@ -875,7 +926,7 @@ CheckSASLAuth(Port *port, char **logdetail)
 		 */
 		result = pg_be_scram_exchange(scram_opaq, buf.data, buf.len,
 									  &output, &outputlen,
-									  doomed ? NULL : logdetail);
+									  logdetail);
 
 		/* input buffer no longer used */
 		pfree(buf.data);

src/backend/libpq/crypt.c

@@ -31,25 +31,18 @@
 /*
  * Fetch stored password for a user, for authentication.
  *
- * Returns STATUS_OK on success.  On error, returns STATUS_ERROR, and stores
- * a palloc'd string describing the reason, for the postmaster log, in
- * *logdetail.  The error reason should *not* be sent to the client, to avoid
- * giving away user information!
- *
- * If the password is expired, it is still returned in *shadow_pass, but the
- * return code is STATUS_ERROR.  On other errors, *shadow_pass is set to
- * NULL.
+ * On error, returns NULL, and stores a palloc'd string describing the reason,
+ * for the postmaster log, in *logdetail.  The error reason should *not* be
+ * sent to the client, to avoid giving away user information!
  */
-int
-get_role_password(const char *role, char **shadow_pass, char **logdetail)
+char *
+get_role_password(const char *role, char **logdetail)
 {
-	int			retval = STATUS_ERROR;
 	TimestampTz vuntil = 0;
 	HeapTuple	roleTup;
 	Datum		datum;
 	bool		isnull;
-
-	*shadow_pass = NULL;
+	char	   *shadow_pass;
 
 	/* Get role info from pg_authid */
 	roleTup = SearchSysCache1(AUTHNAME, PointerGetDatum(role));
@@ -57,7 +50,7 @@ get_role_password(const char *role, char **shadow_pass, char **logdetail)
 	{
 		*logdetail = psprintf(_("Role \"%s\" does not exist."),
 							  role);
-		return STATUS_ERROR;	/* no such user */
+		return NULL;	/* no such user */
 	}
 
 	datum = SysCacheGetAttr(AUTHNAME, roleTup,
@@ -67,9 +60,9 @@ get_role_password(const char *role, char **shadow_pass, char **logdetail)
 		ReleaseSysCache(roleTup);
 		*logdetail = psprintf(_("User \"%s\" has no password assigned."),
 							  role);
-		return STATUS_ERROR;	/* user has no password */
+		return NULL;	/* user has no password */
 	}
-	*shadow_pass = TextDatumGetCString(datum);
+	shadow_pass = TextDatumGetCString(datum);
 
 	datum = SysCacheGetAttr(AUTHNAME, roleTup,
 							Anum_pg_authid_rolvaliduntil, &isnull);
@@ -78,30 +71,25 @@ get_role_password(const char *role, char **shadow_pass, char **logdetail)
 
 	ReleaseSysCache(roleTup);
 
-	if (**shadow_pass == '\0')
+	if (*shadow_pass == '\0')
 	{
 		*logdetail = psprintf(_("User \"%s\" has an empty password."),
 							  role);
-		pfree(*shadow_pass);
-		*shadow_pass = NULL;
-		return STATUS_ERROR;	/* empty password */
+		pfree(shadow_pass);
+		return NULL;	/* empty password */
 	}
 
 	/*
-	 * Password OK, now check to be sure we are not past rolvaliduntil
+	 * Password OK, but check to be sure we are not past rolvaliduntil
 	 */
-	if (isnull)
-		retval = STATUS_OK;
-	else if (vuntil < GetCurrentTimestamp())
+	if (!isnull && vuntil < GetCurrentTimestamp())
 	{
 		*logdetail = psprintf(_("User \"%s\" has an expired password."),
 							  role);
-		retval = STATUS_ERROR;
+		return NULL;
 	}
-	else
-		retval = STATUS_OK;
 
-	return retval;
+	return shadow_pass;
 }
 
 /*

src/backend/libpq/hba.c

@@ -1328,7 +1328,7 @@ parse_hba_line(TokenizedLine *tok_line, int elevel)
 		parsedline->auth_method = uaMD5;
 	}
 	else if (strcmp(token->string, "scram") == 0)
-		parsedline->auth_method = uaSASL;
+		parsedline->auth_method = uaSCRAM;
 	else if (strcmp(token->string, "pam") == 0)
 #ifdef USE_PAM
 		parsedline->auth_method = uaPAM;