qcow2: Check backing_file_offset (CVE-2014-0144) Header, header extension and the backing file name must all be stored in the first cluster. Setting the backing file to a much higher value allowed header extensions to become much bigger than we want them to be (unbounded allocation). Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>