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

3 files changed, 24 insertions, 6 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 3c823f56821..7181718d6e2 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -1993,9 +1993,10 @@ CommitTransaction(void)
 
 	/*
 	 * Do pre-commit processing that involves calling user-defined code, such
-	 * as triggers.  Since closing cursors could queue trigger actions,
-	 * triggers could open cursors, etc, we have to keep looping until there's
-	 * nothing left to do.
+	 * as triggers.  SECURITY_RESTRICTED_OPERATION contexts must not queue an
+	 * action that would run here, because that would bypass the sandbox.
+	 * Since closing cursors could queue trigger actions, triggers could open
+	 * cursors, etc, we have to keep looping until there's nothing left to do.
 	 */
 	for (;;)
 	{
@@ -2013,9 +2014,6 @@ CommitTransaction(void)
 			break;
 	}
 
-	CallXactCallbacks(is_parallel_worker ? XACT_EVENT_PARALLEL_PRE_COMMIT
-					  : XACT_EVENT_PRE_COMMIT);
-
 	/*
 	 * The remaining actions cannot call any user-defined code, so it's safe
 	 * to start shutting down within-transaction services.  But note that most
@@ -2023,6 +2021,9 @@ CommitTransaction(void)
 	 * the transaction-abort path.
 	 */
 
+	CallXactCallbacks(is_parallel_worker ? XACT_EVENT_PARALLEL_PRE_COMMIT
+					  : XACT_EVENT_PRE_COMMIT);
+
 	/* If we might have parallel workers, clean them up now. */
 	if (IsInParallelMode())
 		AtEOXact_Parallel(true);
diff --git a/src/backend/commands/portalcmds.c b/src/backend/commands/portalcmds.c
index 568499761ff..e00ca83f0d8 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))
 		RequireTransactionBlock(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 2a1bf8bf462..acfb9b26140 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -4383,6 +4383,7 @@ afterTriggerMarkEvents(AfterTriggerEventList *events,
 					   bool immediate_only)
 {
 	bool		found = false;
+	bool		deferred_found = false;
 	AfterTriggerEvent event;
 	AfterTriggerEventChunk *chunk;
 
@@ -4418,6 +4419,7 @@ 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 */
@@ -4425,6 +4427,16 @@ 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;
 }