In security-restricted operations, block enqueue of at-commit user code.

Specifically, this blocks DECLARE ... WITH HOLD and firing of deferred
triggers within index expressions and materialized view queries.  An
attacker having permission to create non-temp objects in at least one
schema could execute arbitrary SQL functions under the identity of the
bootstrap superuser.  One can work around the vulnerability by disabling
autovacuum and not manually running ANALYZE, CLUSTER, REINDEX, CREATE
INDEX, VACUUM FULL, or REFRESH MATERIALIZED VIEW.  (Don't restore from
pg_dump, since it runs some of those commands.)  Plain VACUUM (without
FULL) is safe, and all commands are fine when a trusted user owns the
target object.  Performance may degrade quickly under this workaround,
however.  Back-patch to 9.5 (all supported versions).

Reviewed by Robert Haas.  Reported by Etienne Stalmans.

Security: CVE-2020-25695

2 files changed, 17 insertions, 0 deletions

diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index 46369cf3dbe..3d01a782da0 100644
--- a/src/backend/commands/portalcmds.c
+++ b/src/backend/commands/portalcmds.c
@@ -27,6 +27,7 @@
 #include "commands/portalcmds.h"
 #include "executor/executor.h"
 #include "executor/tstoreReceiver.h"
+#include "miscadmin.h"
 #include "rewrite/rewriteHandler.h"
 #include "tcop/pquery.h"
 #include "tcop/tcopprot.h"
@@ -64,6 +65,10 @@ PerformCursorOpen(DeclareCursorStmt *cstmt, ParamListInfo params,
 	 */
 	if (!(cstmt->options & CURSOR_OPT_HOLD))
 		RequireTransactionChain(isTopLevel, "DECLARE CURSOR");
+	else if (InSecurityRestrictedOperation())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("cannot create a cursor WITH HOLD within security-restricted operation")));
 
 	/*
 	 * Parse analysis was done already, but we still have to run the rule
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index f8384062534..9c04eee4842 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -4144,6 +4144,7 @@ afterTriggerMarkEvents(AfterTriggerEventList *events,
 					   bool immediate_only)
 {
 	bool		found = false;
+	bool		deferred_found = false;
 	AfterTriggerEvent event;
 	AfterTriggerEventChunk *chunk;
 
@@ -4179,6 +4180,7 @@ afterTriggerMarkEvents(AfterTriggerEventList *events,
 		 */
 		if (defer_it && move_list != NULL)
 		{
+			deferred_found = true;
 			/* add it to move_list */
 			afterTriggerAddEvent(move_list, event, evtshared);
 			/* mark original copy "done" so we don't do it again */
@@ -4186,6 +4188,16 @@ afterTriggerMarkEvents(AfterTriggerEventList *events,
 		}
 	}
 
+	/*
+	 * We could allow deferred triggers if, before the end of the
+	 * security-restricted operation, we were to verify that a SET CONSTRAINTS
+	 * ... IMMEDIATE has fired all such triggers.  For now, don't bother.
+	 */
+	if (deferred_found && InSecurityRestrictedOperation())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("cannot fire deferred trigger within security-restricted operation")));
+
 	return found;
 }