usb-mtp: outlaw slashes in filenames

Slash is unix directory separator, so they are not allowed in filenames.
Note this also stops the classic escape via "../".

Fixes: CVE-2018-16867
Reported-by: NMichael Hanselmann <public@hansmi.ch>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181203101045.27976-3-kraxel@redhat.com

hw/usb/dev-mtp.c

@@ -1719,6 +1719,12 @@ static void usb_mtp_write_metadata(MTPState *s)
 
     filename = utf16_to_str(dataset->length, dataset->filename);
 
+    if (strchr(filename, '/')) {
+        usb_mtp_queue_result(s, RES_PARAMETER_NOT_SUPPORTED, d->trans,
+                             0, 0, 0, 0);
+        return;
+    }
+
     o = usb_mtp_object_lookup_name(p, filename, dataset->length);
     if (o != NULL) {
         next_handle = o->handle;