1. A method, within a computer system, for securing direct access, by a non-trusted processing entity, to a resource through a system bus, the method comprising: providing a device that includes access-authorization logic and that interconnects the non-trusted processing entity with the system bus and, through the system bus, to the resource; upon receiving, by the device, a resource-access request from the non-trusted processing entity specifying a portion of the resource, using the access-authorization logic to determine whether or not the access request is authorized; when device determines that the non-trusted processing entity is authorized to access the portion of the resource, providing access to the portion of the resource to the non-trusted processing entity; and when the device determines that the non-trusted processing entity is not authorized to access the portion of the resource, returning to the non-trusted processing entity an access error.