Fixups for commit 93db6cbda0.

Ensure to set always-secure search path for both local and remote
connections during slot synchronization, so that malicious users can't
redirect user code (e.g. operators).

In the passing, improve the name of define, remove spurious return
statement, and a minor change in one of the comments.

Author: Bertrand Drouvot and Shveta Malik
Reviewed-by: Amit Kapila, Peter Smith
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/ZdcejBDCr+wlVGnO@ip-10-97-1-34.eu-west-3.compute.internal
Discussion: https://postgr.es/m/CAJpy0uBNP=nrkNJkJSfF=jSocEh8vU2Owa8Rtpi=63fG=SvfVQ@mail.gmail.com

src/backend/access/transam/xlogrecovery.c

@@ -1472,13 +1472,14 @@ FinishWalRecovery(void)
 	 * Shutdown the slot sync worker to drop any temporary slots acquired by
 	 * it and to prevent it from keep trying to fetch the failover slots.
 	 *
-	 * We do not update the 'synced' column from true to false here, as any
-	 * failed update could leave 'synced' column false for some slots. This
-	 * could cause issues during slot sync after restarting the server as a
-	 * standby. While updating the 'synced' column after switching to the new
-	 * timeline is an option, it does not simplify the handling for the
-	 * 'synced' column. Therefore, we retain the 'synced' column as true after
-	 * promotion as it may provide useful information about the slot origin.
+	 * We do not update the 'synced' column in 'pg_replication_slots' system
+	 * view from true to false here, as any failed update could leave 'synced'
+	 * column false for some slots. This could cause issues during slot sync
+	 * after restarting the server as a standby. While updating the 'synced'
+	 * column after switching to the new timeline is an option, it does not
+	 * simplify the handling for the 'synced' column. Therefore, we retain the
+	 * 'synced' column as true after promotion as it may provide useful
+	 * information about the slot origin.
 	 */
 	ShutDownSlotSync();
 

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -271,7 +271,11 @@ libpqrcv_connect(const char *conninfo, bool replication, bool logical,
 				 errhint("Target server's authentication method must be changed, or set password_required=false in the subscription parameters.")));
 	}
 
-	if (logical)
+	/*
+	 * Set always-secure search path for the cases where the connection is
+	 * used to run SQL queries, so malicious users can't get control.
+	 */
+	if (!replication || logical)
 	{
 		PGresult   *res;
 

src/backend/replication/logical/slotsync.c

@@ -105,10 +105,10 @@ bool		sync_replication_slots = false;
  * (within a MIN/MAX range) according to slot activity. See
  * wait_for_slot_activity() for details.
  */
-#define MIN_WORKER_NAPTIME_MS  200
-#define MAX_WORKER_NAPTIME_MS  30000	/* 30s */
+#define MIN_SLOTSYNC_WORKER_NAPTIME_MS  200
+#define MAX_SLOTSYNC_WORKER_NAPTIME_MS  30000	/* 30s */
 
-static long sleep_ms = MIN_WORKER_NAPTIME_MS;
+static long sleep_ms = MIN_SLOTSYNC_WORKER_NAPTIME_MS;
 
 /* The restart interval for slot sync work used by postmaster */
 #define SLOTSYNC_RESTART_INTERVAL_SEC 10
@@ -924,12 +924,9 @@ ValidateSlotSyncParams(int elevel)
 	 * in this case regardless of elevel provided by caller.
 	 */
 	if (wal_level < WAL_LEVEL_LOGICAL)
-	{
 		ereport(ERROR,
 				errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				errmsg("slot synchronization requires wal_level >= \"logical\""));
-		return false;
-	}
 
 	/*
 	 * A physical replication slot(primary_slot_name) is required on the
@@ -1082,7 +1079,7 @@ wait_for_slot_activity(bool some_slot_updated)
 		 * No slots were updated, so double the sleep time, but not beyond the
 		 * maximum allowable value.
 		 */
-		sleep_ms = Min(sleep_ms * 2, MAX_WORKER_NAPTIME_MS);
+		sleep_ms = Min(sleep_ms * 2, MAX_SLOTSYNC_WORKER_NAPTIME_MS);
 	}
 	else
 	{
@@ -1090,7 +1087,7 @@ wait_for_slot_activity(bool some_slot_updated)
 		 * Some slots were updated since the last sleep, so reset the sleep
 		 * time.
 		 */
-		sleep_ms = MIN_WORKER_NAPTIME_MS;
+		sleep_ms = MIN_SLOTSYNC_WORKER_NAPTIME_MS;
 	}
 
 	rc = WaitLatch(MyLatch,
@@ -1215,6 +1212,16 @@ ReplSlotSyncWorkerMain(int argc, char *argv[])
 	 */
 	sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
 
+	/*
+	 * Set always-secure search path, so malicious users can't redirect user
+	 * code (e.g. operators).
+	 *
+	 * It's not strictly necessary since we won't be scanning or writing to
+	 * any user table locally, but it's good to retain it here for added
+	 * precaution.
+	 */
+	SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);
+
 	dbname = CheckAndGetDbnameFromConninfo();
 
 	/*

src/test/recovery/t/040_standby_failover_slots_sync.pl

@@ -361,6 +361,60 @@ ok( $stderr =~
 
 $cascading_standby->stop;
 
+##################################################
+# Test to confirm that the slot synchronization is protected from malicious
+# users.
+##################################################
+
+$primary->psql('postgres', "CREATE DATABASE slotsync_test_db");
+$primary->wait_for_replay_catchup($standby1);
+
+$standby1->stop;
+
+# On the primary server, create '=' operator in another schema mapped to
+# inequality function and redirect the queries to use new operator by setting
+# search_path. The new '=' operator is created with leftarg as 'bigint' and
+# right arg as 'int' to redirect 'count(*) = 1' in slot sync's query to use
+# new '=' operator.
+$primary->safe_psql(
+	'slotsync_test_db', q{
+
+CREATE ROLE repl_role REPLICATION LOGIN;
+CREATE SCHEMA myschema;
+
+CREATE FUNCTION myschema.myintne(bigint, int) RETURNS bool as $$
+		BEGIN
+		  RETURN $1 <> $2;
+		END;
+	  $$ LANGUAGE plpgsql immutable;
+
+CREATE OPERATOR myschema.= (
+	  leftarg    = bigint,
+	  rightarg   = int,
+	  procedure  = myschema.myintne);
+
+ALTER DATABASE slotsync_test_db SET SEARCH_PATH TO myschema,pg_catalog;
+GRANT USAGE on SCHEMA myschema TO repl_role;
+});
+
+# Start the standby with changed primary_conninfo.
+$standby1->append_conf('postgresql.conf', "primary_conninfo = '$connstr_1 dbname=slotsync_test_db user=repl_role'");
+$standby1->start;
+
+# Run the synchronization function. If the sync flow was not prepared
+# to handle such attacks, it would have failed during the validation
+# of the primary_slot_name itself resulting in
+# ERROR:  slot synchronization requires valid primary_slot_name
+$standby1->safe_psql('slotsync_test_db', "SELECT pg_sync_replication_slots();");
+
+# Reset the dbname and user in primary_conninfo to the earlier values.
+$standby1->append_conf('postgresql.conf', "primary_conninfo = '$connstr_1 dbname=postgres'");
+$standby1->reload;
+
+# Drop the newly created database.
+$primary->psql('postgres',
+	q{DROP DATABASE slotsync_test_db;});
+
 ##################################################
 # Test to confirm that the slot sync worker exits on invalid GUC(s) and
 # get started again on valid GUC(s).