Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.

tglsfdc · tglsfdc · commit 2f1997b1551a · 2020-12-08T17:50:54.000-05:00
array_get_element and array_get_slice qualify as leakproof, since they will silently return NULL for bogus subscripts. But array_set_element and array_set_slice throw errors for such cases, making them clearly not leakproof. contain_leaked_vars was evidently written with only the former case in mind, as it gave the wrong answer for assignment SubscriptingRefs (nee ArrayRefs). This would be a live security bug, were it not that assignment SubscriptingRefs can only occur in INSERT and UPDATE target lists, while we only care about leakproofness for qual expressions; so the wrong answer can't occur in practice. Still, that's a rather shaky answer for a security-related question; and maybe in future somebody will want to ask about leakproofness of a tlist. So it seems wise to fix and even back-patch this correction. (We would need some change here anyway for the upcoming generic-subscripting patch, since extensions might make different tradeoffs about whether to throw errors. Commit 558d77f attempted to lay groundwork for that by asking check_functions_in_node whether a SubscriptingRef contains leaky functions; but that idea fails now that the implementation methods of a SubscriptingRef are not SQL-visible functions that could be marked leakproof or not.) Back-patch to 9.6. While 9.5 has the same issue, the code's a bit different. It seems quite unlikely that we'd introduce any actual bug in the short time 9.5 has left to live, so the work/risk/reward balance isn't attractive for changing 9.5. Discussion: https://postgr.es/m/3143742.1607368115@sss.pgh.pa.us
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
@@ -1411,7 +1411,6 @@ contain_leaked_vars_walker(Node *node, void *context)
 		case T_ScalarArrayOpExpr:
 		case T_CoerceViaIO:
 		case T_ArrayCoerceExpr:
-		case T_SubscriptingRef:
 
 			/*
 			 * If node contains a leaky function call, and there's any Var
@@ -1423,6 +1422,23 @@ contain_leaked_vars_walker(Node *node, void *context)
 				return true;
 			break;
 
+		case T_SubscriptingRef:
+			{
+				SubscriptingRef *sbsref = (SubscriptingRef *) node;
+
+				/*
+				 * subscripting assignment is leaky, but subscripted fetches
+				 * are not
+				 */
+				if (sbsref->refassgnexpr != NULL)
+				{
+					/* Node is leaky, so reject if it contains Vars */
+					if (contain_var_clause(node))
+						return true;
+				}
+			}
+			break;
+
 		case T_RowCompareExpr:
 			{
 				/*
