Fix use-after-free bug with AfterTriggersTableData.storeslot

AfterTriggerSaveEvent() wrongly allocates the slot in execution-span
memory context, whereas the correct thing is to allocate it in
a transaction-span context, because that's where the enclosing
AfterTriggersTableData instance belongs into.

Backpatch to 12 (the test back to 11, where it works well with no code
changes, and it's good to have to confirm that the case was previously
well supported); this bug seems introduced by commit ff11e7f4b9ae.

Reported-by: Bertrand Drouvot <bdrouvot@amazon.com>
Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/39a71864-b120-5a5c-8cc5-c632b6f16761@amazon.com

1 files changed, 33 insertions, 19 deletions

diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 8908847c6c6..4e4e05844c5 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -3536,6 +3536,8 @@ static void AfterTriggerExecute(EState *estate,
 								TupleTableSlot *trig_tuple_slot2);
 static AfterTriggersTableData *GetAfterTriggersTableData(Oid relid,
 														 CmdType cmdType);
+static TupleTableSlot *GetAfterTriggersStoreSlot(AfterTriggersTableData *table,
+												 TupleDesc tupdesc);
 static void AfterTriggerFreeQuery(AfterTriggersQueryData *qs);
 static SetConstraintState SetConstraintStateCreate(int numalloc);
 static SetConstraintState SetConstraintStateCopy(SetConstraintState state);
@@ -4336,6 +4338,31 @@ GetAfterTriggersTableData(Oid relid, CmdType cmdType)
 	return table;
 }
 
+/*
+ * Returns a TupleTableSlot suitable for holding the tuples to be put
+ * into AfterTriggersTableData's transition table tuplestores.
+ */
+static TupleTableSlot *
+GetAfterTriggersStoreSlot(AfterTriggersTableData *table,
+						  TupleDesc tupdesc)
+{
+	/* Create it if not already done. */
+	if (!table->storeslot)
+	{
+		MemoryContext	oldcxt;
+
+		/*
+		 * We only need this slot only until AfterTriggerEndQuery, but making
+		 * it last till end-of-subxact is good enough.  It'll be freed by
+		 * AfterTriggerFreeQuery().
+		 */
+		oldcxt = MemoryContextSwitchTo(CurTransactionContext);
+		table->storeslot = MakeSingleTupleTableSlot(tupdesc, &TTSOpsVirtual);
+		MemoryContextSwitchTo(oldcxt);
+	}
+
+	return table->storeslot;
+}
 
 /*
  * MakeTransitionCaptureState
@@ -4625,6 +4652,8 @@ AfterTriggerFreeQuery(AfterTriggersQueryData *qs)
 		table->new_tuplestore = NULL;
 		if (ts)
 			tuplestore_end(ts);
+		if (table->storeslot)
+			ExecDropSingleTupleTableSlot(table->storeslot);
 	}
 
 	/*
@@ -5474,17 +5503,10 @@ AfterTriggerSaveEvent(EState *estate, ResultRelInfo *relinfo,
 
 			if (map != NULL)
 			{
+				AfterTriggersTableData *table = transition_capture->tcs_private;
 				TupleTableSlot *storeslot;
 
-				storeslot = transition_capture->tcs_private->storeslot;
-				if (!storeslot)
-				{
-					storeslot = ExecAllocTableSlot(&estate->es_tupleTable,
-												   map->outdesc,
-												   &TTSOpsVirtual);
-					transition_capture->tcs_private->storeslot = storeslot;
-				}
-
+				storeslot = GetAfterTriggersStoreSlot(table, map->outdesc);
 				execute_attr_map_slot(map->attrMap, oldslot, storeslot);
 				tuplestore_puttupleslot(old_tuplestore, storeslot);
 			}
@@ -5504,18 +5526,10 @@ AfterTriggerSaveEvent(EState *estate, ResultRelInfo *relinfo,
 										original_insert_tuple);
 			else if (map != NULL)
 			{
+				AfterTriggersTableData *table = transition_capture->tcs_private;
 				TupleTableSlot *storeslot;
 
-				storeslot = transition_capture->tcs_private->storeslot;
-
-				if (!storeslot)
-				{
-					storeslot = ExecAllocTableSlot(&estate->es_tupleTable,
-												   map->outdesc,
-												   &TTSOpsVirtual);
-					transition_capture->tcs_private->storeslot = storeslot;
-				}
-
+				storeslot = GetAfterTriggersStoreSlot(table, map->outdesc);
 				execute_attr_map_slot(map->attrMap, newslot, storeslot);
 				tuplestore_puttupleslot(new_tuplestore, storeslot);
 			}