On Wed, Feb 05, 2020 at 05:08:55PM -0800, Andy Lutomirski wrote: > > > > On Feb 5, 2020, at 2:39 PM, Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx> wrote: > > > > From: Kees Cook <keescook@xxxxxxxxxxxx> > > > > Under earlyprintk, each RNG call produces a debug report line. When > > shuffling hundreds of functions, this is not useful information (each > > line is identical and tells us nothing new). Instead, allow for a NULL > > "purpose" to suppress the debug reporting. > > Have you counted how many RDRAND calls this causes? RDRAND is > exceedingly slow on all CPUs I’ve looked at. The whole “RDRAND > has great bandwidth” marketing BS actually means that it has decent > bandwidth if all CPUs hammer it at the same time. The latency is abysmal. > I have asked Intel to improve this, but the latency of that request will > be quadrillions of cycles :) In an earlier version of this series, it was called once per function section (so, about 50,000 times). The (lack of) speed was quite measurable. > I would suggest adding a little ChaCha20 DRBG or similar to the KASLR > environment instead. What crypto primitives are available there? Agreed. The simple PRNG in the next patch was most just a POC initially, but Kristen kept it due to its debugging properties (specifying an external seed). Pulling in ChaCha20 seems like a good approach. -- Kees Cook
