Fix RLS policy usage in MERGE.

If MERGE executes an UPDATE action on a table with row-level security,
the code incorrectly applied the WITH CHECK clauses from the target
table's INSERT policies to new rows, instead of the clauses from the
table's UPDATE policies. In addition, it failed to check new rows
against the target table's SELECT policies, if SELECT permissions were
required (likely to always be the case).

In addition, if MERGE executes a DO NOTHING action for matched rows,
the code incorrectly applied the USING clauses from the target table's
DELETE policies to existing target tuples. These policies were applied
as checks that would throw an error, if they did not pass.

Fix this, so that a MERGE UPDATE action applies the same RLS policies
as a plain UPDATE query with a WHERE clause, and a DO NOTHING action
does not apply any RLS checks (other than adding clauses from SELECT
policies to the join).

Back-patch to v15, where MERGE was introduced.

Dean Rasheed, reviewed by Stephen Frost.

Security: CVE-2023-39418

1 files changed, 4 insertions, 3 deletions

diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index ca62cb707a4..daf48dc0f0e 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -2842,13 +2842,14 @@ lmerge_matched:;
 		 * UPDATE/DELETE RLS policies. If those checks fail, we throw an
 		 * error.
 		 *
-		 * The WITH CHECK quals are applied in ExecUpdate() and hence we need
-		 * not do anything special to handle them.
+		 * The WITH CHECK quals for UPDATE RLS policies are applied in
+		 * ExecUpdateAct() and hence we need not do anything special to handle
+		 * them.
 		 *
 		 * NOTE: We must do this after WHEN quals are evaluated, so that we
 		 * check policies only when they matter.
 		 */
-		if (resultRelInfo->ri_WithCheckOptions)
+		if (resultRelInfo->ri_WithCheckOptions && commandType != CMD_NOTHING)
 		{
 			ExecWithCheckOptions(commandType == CMD_UPDATE ?
 								 WCO_RLS_MERGE_UPDATE_CHECK : WCO_RLS_MERGE_DELETE_CHECK,