dblink: SCRAM authentication pass-through

This enables SCRAM authentication for dblink (using dblink_fdw) when
connecting to a foreign server without having to store a plain-text
password on user mapping options

This uses the same approach as it was implemented for postgres_fdw in
commit 761c79508e7.  (It also contains the equivalent of the
subsequent fixes 76563f88cfb and d2028e9bbc1.)

Author: Matheus Alcantara <mths.dev@pm.me>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAFY6G8ercA1KES%3DE_0__R9QCTR805TTyYr1No8qF8ZxmMg8z2Q%40mail.gmail.com

contrib/dblink/Makefile

@@ -13,6 +13,7 @@ PGFILEDESC = "dblink - connect to other PostgreSQL databases"
 
 REGRESS = dblink
 REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
+TAP_TESTS = 1
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config

contrib/dblink/dblink.c

@@ -43,6 +43,8 @@
 #include "catalog/pg_foreign_server.h"
 #include "catalog/pg_type.h"
 #include "catalog/pg_user_mapping.h"
+#include "commands/defrem.h"
+#include "common/base64.h"
 #include "executor/spi.h"
 #include "foreign/foreign.h"
 #include "funcapi.h"
@@ -126,6 +128,11 @@ static bool is_valid_dblink_option(const PQconninfoOption *options,
 								   const char *option, Oid context);
 static int	applyRemoteGucs(PGconn *conn);
 static void restoreLocalGucs(int nestlevel);
+static bool UseScramPassthrough(ForeignServer *foreign_server, UserMapping *user);
+static void appendSCRAMKeysInfo(StringInfo buf);
+static bool is_valid_dblink_fdw_option(const PQconninfoOption *options, const char *option,
+									   Oid context);
+static bool dblink_connstr_has_required_scram_options(const char *connstr);
 
 /* Global */
 static remoteConn *pconn = NULL;
@@ -1964,7 +1971,7 @@ dblink_fdw_validator(PG_FUNCTION_ARGS)
 	{
 		DefElem    *def = (DefElem *) lfirst(cell);
 
-		if (!is_valid_dblink_option(options, def->defname, context))
+		if (!is_valid_dblink_fdw_option(options, def->defname, context))
 		{
 			/*
 			 * Unknown option, or invalid option for the context specified, so
@@ -2596,6 +2603,67 @@ deleteConnection(const char *name)
 				 errmsg("undefined connection name")));
 }
 
+ /*
+  * Ensure that require_auth and SCRAM keys are correctly set on connstr.
+  * SCRAM keys used to pass-through are coming from the initial connection
+  * from the client with the server.
+  *
+  * All required SCRAM options are set by dblink, so we just need to ensure
+  * that these options are not overwritten by the user.
+  *
+  * See appendSCRAMKeysInfo and its usage for more.
+  */
+bool
+dblink_connstr_has_required_scram_options(const char *connstr)
+{
+	PQconninfoOption *options;
+	bool		has_scram_server_key = false;
+	bool		has_scram_client_key = false;
+	bool		has_require_auth = false;
+	bool		has_scram_keys = false;
+
+	options = PQconninfoParse(connstr, NULL);
+	if (options)
+	{
+		/*
+		 * Continue iterating even if we found the keys that we need to
+		 * validate to make sure that there is no other declaration of these
+		 * keys that can overwrite the first.
+		 */
+		for (PQconninfoOption *option = options; option->keyword != NULL; option++)
+		{
+			if (strcmp(option->keyword, "require_auth") == 0)
+			{
+				if (option->val != NULL && strcmp(option->val, "scram-sha-256") == 0)
+					has_require_auth = true;
+				else
+					has_require_auth = false;
+			}
+
+			if (strcmp(option->keyword, "scram_client_key") == 0)
+			{
+				if (option->val != NULL && option->val[0] != '\0')
+					has_scram_client_key = true;
+				else
+					has_scram_client_key = false;
+			}
+
+			if (strcmp(option->keyword, "scram_server_key") == 0)
+			{
+				if (option->val != NULL && option->val[0] != '\0')
+					has_scram_server_key = true;
+				else
+					has_scram_server_key = false;
+			}
+		}
+		PQconninfoFree(options);
+	}
+
+	has_scram_keys = has_scram_client_key && has_scram_server_key && MyProcPort->has_scram_keys;
+
+	return (has_scram_keys && has_require_auth);
+}
+
 /*
  * We need to make sure that the connection made used credentials
  * which were provided by the user, so check what credentials were
@@ -2612,6 +2680,18 @@ dblink_security_check(PGconn *conn, remoteConn *rconn, const char *connstr)
 	if (PQconnectionUsedPassword(conn) && dblink_connstr_has_pw(connstr))
 		return;
 
+	/*
+	 * Password was not used to connect, check if SCRAM pass-through is in
+	 * use.
+	 *
+	 * If dblink_connstr_has_required_scram_options is true we assume that
+	 * UseScramPassthrough is also true because the required SCRAM keys are
+	 * only added if UseScramPassthrough is set, and the user is not allowed
+	 * to add the SCRAM keys on fdw and user mapping options.
+	 */
+	if (MyProcPort->has_scram_keys && dblink_connstr_has_required_scram_options(connstr))
+		return;
+
 #ifdef ENABLE_GSS
 	/* If GSSAPI creds used to connect, make sure it was one delegated */
 	if (PQconnectionUsedGSSAPI(conn) && be_gssapi_get_delegation(MyProcPort))
@@ -2664,12 +2744,14 @@ dblink_connstr_has_pw(const char *connstr)
 }
 
 /*
- * For non-superusers, insist that the connstr specify a password, except
+ * For non-superusers, insist that the connstr specify a password, except if
- * if GSSAPI credentials have been delegated (and we check that they are used
+ * GSSAPI credentials have been delegated (and we check that they are used for
- * for the connection in dblink_security_check later).  This prevents a
+ * the connection in dblink_security_check later) or if SCRAM pass-through is
- * password or GSSAPI credentials from being picked up from .pgpass, a
+ * being used.  This prevents a password or GSSAPI credentials from being
- * service file, the environment, etc.  We don't want the postgres user's
+ * picked up from .pgpass, a service file, the environment, etc.  We don't want
- * passwords or Kerberos credentials to be accessible to non-superusers.
+ * the postgres user's passwords or Kerberos credentials to be accessible to
+ * non-superusers. In case of SCRAM pass-through insist that the connstr
+ * has the required SCRAM pass-through options.
  */
 static void
 dblink_connstr_check(const char *connstr)
@@ -2680,6 +2762,9 @@ dblink_connstr_check(const char *connstr)
 	if (dblink_connstr_has_pw(connstr))
 		return;
 
+	if (MyProcPort->has_scram_keys && dblink_connstr_has_required_scram_options(connstr))
+		return;
+
 #ifdef ENABLE_GSS
 	if (be_gssapi_get_delegation(MyProcPort))
 		return;
@@ -2832,6 +2917,14 @@ get_connect_string(const char *servername)
 		if (aclresult != ACLCHECK_OK)
 			aclcheck_error(aclresult, OBJECT_FOREIGN_SERVER, foreign_server->servername);
 
+		/*
+		 * First append hardcoded options needed for SCRAM pass-through, so if
+		 * the user overwrites these options we can ereport on
+		 * dblink_connstr_check and dblink_security_check.
+		 */
+		if (MyProcPort->has_scram_keys && UseScramPassthrough(foreign_server, user_mapping))
+			appendSCRAMKeysInfo(&buf);
+
 		foreach(cell, fdw->options)
 		{
 			DefElem    *def = lfirst(cell);
@@ -3016,6 +3109,20 @@ is_valid_dblink_option(const PQconninfoOption *options, const char *option,
 	return true;
 }
 
+/*
+ * Same as is_valid_dblink_option but also check for only dblink_fdw specific
+ * options.
+ */
+static bool
+is_valid_dblink_fdw_option(const PQconninfoOption *options, const char *option,
+						   Oid context)
+{
+	if (strcmp(option, "use_scram_passthrough") == 0)
+		return true;
+
+	return is_valid_dblink_option(options, option, context);
+}
+
 /*
  * Copy the remote session's values of GUCs that affect datatype I/O
  * and apply them locally in a new GUC nesting level.  Returns the new
@@ -3085,3 +3192,66 @@ restoreLocalGucs(int nestlevel)
 	if (nestlevel > 0)
 		AtEOXact_GUC(true, nestlevel);
 }
+
+/*
+ * Append SCRAM client key and server key information from the global
+ * MyProcPort into the given StringInfo buffer.
+ */
+static void
+appendSCRAMKeysInfo(StringInfo buf)
+{
+	int			len;
+	int			encoded_len;
+	char	   *client_key;
+	char	   *server_key;
+
+	len = pg_b64_enc_len(sizeof(MyProcPort->scram_ClientKey));
+	/* don't forget the zero-terminator */
+	client_key = palloc0(len + 1);
+	encoded_len = pg_b64_encode((const char *) MyProcPort->scram_ClientKey,
+								sizeof(MyProcPort->scram_ClientKey),
+								client_key, len);
+	if (encoded_len < 0)
+		elog(ERROR, "could not encode SCRAM client key");
+
+	len = pg_b64_enc_len(sizeof(MyProcPort->scram_ServerKey));
+	/* don't forget the zero-terminator */
+	server_key = palloc0(len + 1);
+	encoded_len = pg_b64_encode((const char *) MyProcPort->scram_ServerKey,
+								sizeof(MyProcPort->scram_ServerKey),
+								server_key, len);
+	if (encoded_len < 0)
+		elog(ERROR, "could not encode SCRAM server key");
+
+	appendStringInfo(buf, "scram_client_key='%s' ", client_key);
+	appendStringInfo(buf, "scram_server_key='%s' ", server_key);
+	appendStringInfo(buf, "require_auth='scram-sha-256' ");
+
+	pfree(client_key);
+	pfree(server_key);
+}
+
+
+static bool
+UseScramPassthrough(ForeignServer *foreign_server, UserMapping *user)
+{
+	ListCell   *cell;
+
+	foreach(cell, foreign_server->options)
+	{
+		DefElem    *def = lfirst(cell);
+
+		if (strcmp(def->defname, "use_scram_passthrough") == 0)
+			return defGetBoolean(def);
+	}
+
+	foreach(cell, user->options)
+	{
+		DefElem    *def = (DefElem *) lfirst(cell);
+
+		if (strcmp(def->defname, "use_scram_passthrough") == 0)
+			return defGetBoolean(def);
+	}
+
+	return false;
+}

contrib/dblink/meson.build

@@ -36,4 +36,9 @@ tests += {
     ],
     'regress_args': ['--dlpath', meson.build_root() / 'src/test/regress'],
   },
+  'tap': {
+    'tests': [
+      't/001_auth_scram.pl',
+    ],
+  },
 }

contrib/dblink/t/001_auth_scram.pl

@@ -0,0 +1,254 @@
+# Copyright (c) 2024-2025, PostgreSQL Global Development Group
+
+# Test SCRAM authentication when opening a new connection with a foreign
+# server.
+#
+# The test is executed by testing the SCRAM authentifcation on a loopback
+# connection on the same server and with different servers.
+
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Utils;
+use PostgreSQL::Test::Cluster;
+use Test::More;
+
+if (!$use_unix_sockets)
+{
+	plan skip_all => "test requires Unix-domain sockets";
+}
+
+my $user = "user01";
+
+my $db0 = "db0";                               # For node1
+my $db1 = "db1";                               # For node1
+my $db2 = "db2";                               # For node2
+my $fdw_server = "db1_fdw";
+my $fdw_server2 = "db2_fdw";
+my $fdw_invalid_server = "db2_fdw_invalid";    # For invalid fdw options
+my $fdw_invalid_server2 =
+  "db2_fdw_invalid2";    # For invalid scram keys fdw options
+
+my $node1 = PostgreSQL::Test::Cluster->new('node1');
+my $node2 = PostgreSQL::Test::Cluster->new('node2');
+
+$node1->init;
+$node2->init;
+
+$node1->start;
+$node2->start;
+
+# Test setup
+
+$node1->safe_psql('postgres', qq'CREATE USER $user WITH password \'pass\'');
+$node2->safe_psql('postgres', qq'CREATE USER $user WITH password \'pass\'');
+$ENV{PGPASSWORD} = "pass";
+
+$node1->safe_psql('postgres', qq'CREATE DATABASE $db0');
+$node1->safe_psql('postgres', qq'CREATE DATABASE $db1');
+$node2->safe_psql('postgres', qq'CREATE DATABASE $db2');
+
+setup_table($node1, $db1, "t");
+setup_table($node2, $db2, "t2");
+
+$node1->safe_psql($db0, 'CREATE EXTENSION IF NOT EXISTS dblink');
+setup_fdw_server($node1, $db0, $fdw_server, $node1, $db1);
+setup_fdw_server($node1, $db0, $fdw_server2, $node2, $db2);
+setup_invalid_fdw_server($node1, $db0, $fdw_invalid_server, $node2, $db2);
+setup_fdw_server($node1, $db0, $fdw_invalid_server2, $node2, $db2);
+
+setup_user_mapping($node1, $db0, $fdw_server);
+setup_user_mapping($node1, $db0, $fdw_server2);
+setup_user_mapping($node1, $db0, $fdw_invalid_server);
+
+# Make the user have the same SCRAM key on both servers. Forcing to have the
+# same iteration and salt.
+my $rolpassword = $node1->safe_psql('postgres',
+	qq"SELECT rolpassword FROM pg_authid WHERE rolname = '$user';");
+$node2->safe_psql('postgres', qq"ALTER ROLE $user PASSWORD '$rolpassword'");
+
+unlink($node1->data_dir . '/pg_hba.conf');
+unlink($node2->data_dir . '/pg_hba.conf');
+
+$node1->append_conf(
+	'pg_hba.conf', qq{
+local   db0             all                                     scram-sha-256
+local   db1             all                                     scram-sha-256
+}
+);
+$node2->append_conf(
+	'pg_hba.conf', qq{
+local   db2             all                                     scram-sha-256
+}
+);
+
+$node1->restart;
+$node2->restart;
+
+# End of test setup
+
+test_scram_keys_is_not_overwritten($node1, $db0, $fdw_invalid_server2);
+
+test_fdw_auth($node1, $db0, "t", $fdw_server,
+	"SCRAM auth on the same database cluster must succeed");
+
+test_fdw_auth($node1, $db0, "t2", $fdw_server2,
+	"SCRAM auth on a different database cluster must succeed");
+
+test_fdw_auth_with_invalid_overwritten_require_auth($fdw_invalid_server);
+
+# Ensure that trust connections fail without superuser opt-in.
+unlink($node1->data_dir . '/pg_hba.conf');
+unlink($node2->data_dir . '/pg_hba.conf');
+
+$node1->append_conf(
+	'pg_hba.conf', qq{
+local   db0             all                                     scram-sha-256
+local   db1             all                                     trust
+}
+);
+$node2->append_conf(
+	'pg_hba.conf', qq{
+local   all             all                                     password
+}
+);
+
+$node1->restart;
+$node2->restart;
+
+my ($ret, $stdout, $stderr) = $node1->psql(
+	$db0,
+	"SELECT * FROM dblink('$fdw_server', 'SELECT * FROM t') AS t(a int, b int)",
+	connstr => $node1->connstr($db0) . " user=$user");
+
+is($ret, 3, 'loopback trust fails on the same cluster');
+like(
+	$stderr,
+	qr/failed: authentication method requirement "scram-sha-256" failed: server did not complete authentication/,
+	'expected error from loopback trust (same cluster)');
+
+($ret, $stdout, $stderr) = $node1->psql(
+	$db0,
+	"SELECT * FROM dblink('$fdw_server2', 'SELECT * FROM t2') AS t2(a int, b int)",
+	connstr => $node1->connstr($db0) . " user=$user");
+
+is($ret, 3, 'loopback password fails on a different cluster');
+like(
+	$stderr,
+	qr/authentication method requirement "scram-sha-256" failed: server requested a cleartext password/,
+	'expected error from loopback password (different cluster)');
+
+# Helper functions
+
+sub test_fdw_auth
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($node, $db, $tbl, $fdw, $testname) = @_;
+	my $connstr = $node->connstr($db) . qq' user=$user';
+
+	my $ret = $node->safe_psql(
+		$db,
+		qq"SELECT count(1) FROM dblink('$fdw', 'SELECT * FROM $tbl') AS $tbl(a int, b int)",
+		connstr => $connstr);
+
+	is($ret, '10', $testname);
+}
+
+sub test_fdw_auth_with_invalid_overwritten_require_auth
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($fdw) = @_;
+
+	my ($ret, $stdout, $stderr) = $node1->psql(
+		$db0,
+		"select * from dblink('$fdw', 'select * from t') as t(a int, b int)",
+		connstr => $node1->connstr($db0) . " user=$user");
+
+	is($ret, 3, 'loopback trust fails when overwriting require_auth');
+	like(
+		$stderr,
+		qr/password or GSSAPI delegated credentials required/,
+		'expected error when connecting to a fdw overwriting the require_auth'
+	);
+}
+
+sub test_scram_keys_is_not_overwritten
+{
+	local $Test::Builder::Level = $Test::Builder::Level + 1;
+
+	my ($node, $db, $fdw) = @_;
+
+	my ($ret, $stdout, $stderr) = $node->psql(
+		$db,
+		qq'CREATE USER MAPPING FOR $user SERVER $fdw OPTIONS (user \'$user\', scram_client_key \'key\');',
+		connstr => $node->connstr($db) . " user=$user");
+
+	is($ret, 3, 'user mapping creation fails when using scram_client_key');
+	like(
+		$stderr,
+		qr/ERROR:  invalid option "scram_client_key"/,
+		'user mapping creation fails when using scram_client_key');
+
+	($ret, $stdout, $stderr) = $node->psql(
+		$db,
+		qq'CREATE USER MAPPING FOR $user SERVER $fdw OPTIONS (user \'$user\', scram_server_key \'key\');',
+		connstr => $node->connstr($db) . " user=$user");
+
+	is($ret, 3, 'user mapping creation fails when using scram_server_key');
+	like(
+		$stderr,
+		qr/ERROR:  invalid option "scram_server_key"/,
+		'user mapping creation fails when using scram_server_key');
+}
+
+sub setup_user_mapping
+{
+	my ($node, $db, $fdw) = @_;
+
+	$node->safe_psql($db,
+		qq'CREATE USER MAPPING FOR $user SERVER $fdw OPTIONS (user \'$user\');'
+	);
+}
+
+sub setup_fdw_server
+{
+	my ($node, $db, $fdw, $fdw_node, $dbname) = @_;
+	my $host = $fdw_node->host;
+	my $port = $fdw_node->port;
+
+	$node->safe_psql(
+		$db, qq'CREATE SERVER $fdw FOREIGN DATA WRAPPER dblink_fdw options (
+		host \'$host\', port \'$port\', dbname \'$dbname\', use_scram_passthrough \'true\') '
+	);
+	$node->safe_psql($db, qq'GRANT USAGE ON FOREIGN SERVER $fdw TO $user;');
+	$node->safe_psql($db, qq'GRANT ALL ON SCHEMA public TO $user');
+}
+
+sub setup_invalid_fdw_server
+{
+	my ($node, $db, $fdw, $fdw_node, $dbname) = @_;
+	my $host = $fdw_node->host;
+	my $port = $fdw_node->port;
+
+	$node->safe_psql(
+		$db, qq'CREATE SERVER $fdw FOREIGN DATA WRAPPER dblink_fdw options (
+		host \'$host\', port \'$port\', dbname \'$dbname\', use_scram_passthrough \'true\', require_auth \'none\') '
+	);
+	$node->safe_psql($db, qq'GRANT USAGE ON FOREIGN SERVER $fdw TO $user;');
+	$node->safe_psql($db, qq'GRANT ALL ON SCHEMA public TO $user');
+}
+
+sub setup_table
+{
+	my ($node, $db, $tbl) = @_;
+
+	$node->safe_psql($db,
+		qq'CREATE TABLE $tbl AS SELECT g as a, g + 1 as b FROM generate_series(1,10) g(g)'
+	);
+	$node->safe_psql($db, qq'GRANT USAGE ON SCHEMA public TO $user');
+	$node->safe_psql($db, qq'GRANT SELECT ON $tbl TO $user');
+}
+
+done_testing();
+

doc/src/sgml/dblink.sgml

@@ -150,9 +150,23 @@ dblink_connect(text connname, text connstr) returns text
     executing arbitrary SQL commands.
    </para>
 
+   <para>
+    The foreign-data wrapper <filename>dblink_fdw</filename> has an additional
+    Boolean option <literal>use_scram_passthrough</literal> that controls
+    whether <filename>dblink</filename> will use the SCRAM pass-through
+    authentication to connect to the remote database.  With SCRAM pass-through
+    authentication, <filename>dblink</filename> uses SCRAM-hashed secrets
+    instead of plain-text user passwords to connect to the remote server. This
+    avoids storing plain-text user passwords in PostgreSQL system catalogs.
+    See the documentation of the equivalent <link
+    linkend="postgres-fdw-option-use-scram-passthrough"><literal>use_scram_passthrough</literal></link>
+    option of postgres_fdw for further details and restrictions.
+   </para>
+
    <para>
     Only superusers may use <function>dblink_connect</function> to create
-    non-password-authenticated and non-GSSAPI-authenticated connections.
+    connections that use neither password authentication, SCRAM pass-through,
+    nor GSSAPI-authentication.
     If non-superusers need this capability, use
     <function>dblink_connect_u</function> instead.
    </para>
@@ -181,8 +195,9 @@ SELECT dblink_connect('myconn', 'dbname=postgres options=-csearch_path=');
 (1 row)
 
 -- FOREIGN DATA WRAPPER functionality
--- Note: local connection must require password authentication for this to work properly
+-- Note: local connections that don't use SCRAM pass-through require password
---       Otherwise, you will receive the following error from dblink_connect():
+--       authentication for this to work properly. Otherwise, you will receive
+--       the following error from dblink_connect():
 --       ERROR:  password is required
 --       DETAIL:  Non-superuser cannot connect if the server does not request a password.
 --       HINT:  Target server's authentication method must be changed.

doc/src/sgml/postgres-fdw.sgml

@@ -756,7 +756,7 @@ OPTIONS (ADD password_required 'false');
 
     <variablelist>
 
-     <varlistentry>
+     <varlistentry id="postgres-fdw-option-keep-connections">
       <term><literal>keep_connections</literal> (<type>boolean</type>)</term>
       <listitem>
        <para>
@@ -770,7 +770,7 @@ OPTIONS (ADD password_required 'false');
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="postgres-fdw-option-use-scram-passthrough">
       <term><literal>use_scram_passthrough</literal> (<type>boolean</type>)</term>
       <listitem>
        <para>