How secure is Linux's random number generator?
Just because something is closed source doesn't mean it's insecure.  RdRand meets various standards for RNGs and the dieharder tests don't show anything of concern.  While you can't be 100 percent sure of the reliability of RdRand because you can't audit it, I feel safe trusting it for all but the most critical of applications.  Here's a blog post describing testing RdRand with dieharder: <a href="http://smackerelofopinion.blogspot.com/2012/10/intel-rdrand-instruction-revisited.html" rel="nofollow">http://smackerelofopinion.blogspot.com/2012/10/intel-rdrand-...</a>