Fixed bug #79784

The fix here is essentially the same as for bug #78598, just for
the undefined variable notice, rather than the undefined index one.

NEWS

@@ -21,6 +21,8 @@ PHP                                                                        NEWS
     (Nikita)
   . Fixed bug #78598 (Changing array during undef index RW error segfaults).
     (Nikita)
+  . Fixed bug #79784 (Use after free if changing array during undef var during
+    array write fetch). (Nikita)
 
 - Fileinfo:
   . Fixed bug #79756 (finfo_file crash (FILEINFO_MIME)). (cmb)

Zend/tests/bug79784.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Bug #79784: Use after free if changing array during undef var during array write fetch
+--FILE--
+<?php
+set_error_handler(function () {
+    $GLOBALS['a'] = null;
+});
+
+$a[$c] = 'x' ;
+var_dump($a);
+$a[$c] .= 'x' ;
+var_dump($a);
+$a[$c][$c] = 'x' ;
+var_dump($a);
+
+?>
+--EXPECT--
+NULL
+NULL
+NULL

Zend/zend_execute.c

@@ -2067,12 +2067,25 @@ static ZEND_COLD void zend_binary_assign_op_dim_slow(zval *container, zval *dim
 	FREE_OP(free_op_data1);
 }
 
-static zend_never_inline zend_uchar slow_index_convert(const zval *dim, zend_value *value EXECUTE_DATA_DC)
+static zend_never_inline zend_uchar slow_index_convert(HashTable *ht, const zval *dim, zend_value *value EXECUTE_DATA_DC)
 {
 	switch (Z_TYPE_P(dim)) {
-		case IS_UNDEF:
+		case IS_UNDEF: {
+			/* The array may be destroyed while throwing the notice.
+			 * Temporarily increase the refcount to detect this situation. */
+			if (!(GC_FLAGS(ht) & IS_ARRAY_IMMUTABLE)) {
+				GC_ADDREF(ht);
+			}
 			ZVAL_UNDEFINED_OP2();
+			if (!(GC_FLAGS(ht) & IS_ARRAY_IMMUTABLE) && !GC_DELREF(ht)) {
+				zend_array_destroy(ht);
+				return IS_NULL;
+			}
+			if (EG(exception)) {
+				return IS_NULL;
+			}
 			/* break missing intentionally */
+		}
 		case IS_NULL:
 			value->str = ZSTR_EMPTY_ALLOC();
 			return IS_STRING;
@@ -2182,7 +2195,7 @@ static zend_always_inline zval *zend_fetch_dimension_address_inner(HashTable *ht
 		goto try_again;
 	} else {
 		zend_value val;
-		zend_uchar t = slow_index_convert(dim, &val EXECUTE_DATA_CC);
+		zend_uchar t = slow_index_convert(ht, dim, &val EXECUTE_DATA_CC);
 
 		if (t == IS_STRING) {
 			offset_key = val.str;