The TCP/IP stack is already taken from Linux, of course without copyright assignments.

The driver situation is a bit different as the Hurd runs in user space, so it does not care about device drivers. Those are all in the underlying microkernel (GNU Mach currently), which has not been written by the FSF in the first place and also uses Linux drivers (though very outdated ones). So Linux code is being used were it makes sense, maybe not taking it to extremes though.

Michael