Also, it is not bullet proof because apparently there are lots of info leaks in linux and I think linux also does not reboot after a panic (http://www.cyberciti.biz/tips/reboot-linux-box-after-a-kerne...) so if the entropy for KASLR is small enough you can retry very aggressively. Though in this particular instance if you have to wait 30 minutes between each try that would kill brute forcing.