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Fix GH-21776: use-after-free in zend_std_read_property magic __isset #21786

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Fix GH-21776: use-after-free in zend_std_read_property magic __isset #21786

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16 changes: 16 additions & 0 deletions Zend/tests/gh21776.phpt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
--TEST--
GH-21776 (Heap use-after-free in zend_object_is_lazy via magic __isset)
--FILE--
<?php
class C {
function __isset($x) {
$GLOBALS['o'] = 0;
return true;
}
}
$o = new C;
$o->a ?? 0;
echo "OK\n";
?>
--EXPECT--
OK
14 changes: 10 additions & 4 deletions Zend/zend_object_handlers.c
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Original file line number Diff line number Diff line change
Expand Up @@ -743,6 +743,7 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
uintptr_t property_offset;
const zend_property_info *prop_info = NULL;
uint32_t *guard = NULL;
bool release_zobj = false;

#if DEBUG_OBJECT_HANDLERS
fprintf(stderr, "Read object #%d property: %s\n", zobj->handle, ZSTR_VAL(name));
Expand Down Expand Up @@ -937,7 +938,7 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
if (zobj->ce->__get && !((*guard) & IN_GET)) {
goto call_getter;
}
OBJ_RELEASE(zobj);
release_zobj = true;
} else if (zobj->ce->__get && !((*guard) & IN_GET)) {
goto call_getter_addref;
}
Expand Down Expand Up @@ -986,7 +987,7 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
zend_object *instance = zend_lazy_object_init(zobj);
if (!instance) {
retval = &EG(uninitialized_zval);
goto exit;
goto release_zobj_exit;
}

if (UNEXPECTED(guard && (instance->ce->ce_flags & ZEND_ACC_USE_GUARDS))) {
Expand All @@ -999,11 +1000,12 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
(*guard) |= guard_type;
retval = zend_std_read_property(instance, name, type, cache_slot, rv);
(*guard) &= ~guard_type;
return retval;
goto release_zobj_exit;
}
}

return zend_std_read_property(instance, name, type, cache_slot, rv);
retval = zend_std_read_property(instance, name, type, cache_slot, rv);
goto release_zobj_exit;
}
}
if (type != BP_VAR_IS) {
Expand All @@ -1015,6 +1017,10 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
}
retval = &EG(uninitialized_zval);

release_zobj_exit:
if (release_zobj) {
OBJ_RELEASE(zobj);
}
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@iluuu1994 iluuu1994 Apr 17, 2026

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You could move this above exit with a new label and adjust the gotos in if (UNEXPECTED(zend_lazy_object_must_init(zobj))) { to go there instead. This check is not necessary the majority of the time.

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@iliaal iliaal Apr 17, 2026

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Fair enough, let me adjust, I just looked at bug report and it mention 8.5 so started from there

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@iliaal iliaal Apr 17, 2026

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Moved the release into a release_zobj_exit: label above exit: and routed the three lazy-init-block jumps there. The isset-false, getter, and wrong-offset paths still goto exit and skip the flag check. Retargeted to master.

exit:
return retval;
}
Expand Down
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