qapi: fix double free in qmp_output_visitor_cleanup()

Stack entries in QmpOutputVisitor are navigation links (weak references),
except the bottom (ie. least recently added) entry, which owns the root
QObject [1]. Make qmp_output_visitor_cleanup() drop the stack entries,
then release the QObject tree by the root.

Attempting to serialize an invalid enum inside a dictionary is an example
for triggering the double free.

[1] http://lists.nongnu.org/archive/html/qemu-devel/2012-03/msg03276.html

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index e0697b0d0f..2bce9d5db1 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -199,14 +199,16 @@ void qmp_output_visitor_cleanup(QmpOutputVisitor *v)
 {
     QStackEntry *e, *tmp;
 
+    /* The bottom QStackEntry, if any, owns the root QObject. See the
+     * qmp_output_push_obj() invocations in qmp_output_add_obj(). */
+    QObject *root = QTAILQ_EMPTY(&v->stack) ? NULL : qmp_output_first(v);
+
     QTAILQ_FOREACH_SAFE(e, &v->stack, node, tmp) {
         QTAILQ_REMOVE(&v->stack, e, node);
-        if (e->value) {
-            qobject_decref(e->value);
-        }
         g_free(e);
     }
 
+    qobject_decref(root);
     g_free(v);
 }