Interestingly on this vendor list published on cert.org dd-wrt is marked as not affected while openwrt isn't mentioned. Not being mentioned perhaps an official cert request wasn't sent or didn't arrived the right person / wasn't answered?
Perhaps simply it doesn't use one of the affected TCP/IP stacks?
That embedded TCP/IP stacks are built for microcontrollers or embedded processors that mostly can't run Linux due some hardware limitations. Limitations can be RAM, ROM, MMU or even processor power.
But i can't see Linux built-in TCP/IP networking stack on this CERT vulnerability list. Therefore Linux (and probably BSD) isn't affected.