From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH v4 2/5] random: Add and use arch_get_rng_seed Date: Tue, 22 Jul 2014 14:16:43 -0700 Message-ID: <53CED4BB.3080601@zytor.com> References: <9c2a0549519b4eb5eee2d5d480f8e83a574273df.1405620944.git.luto@amacapital.net> <20140722135915.GB25291@thunk.org> <53CED031.9090907@zytor.com> <53CED2B2.1040401@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: "Theodore Ts'o" , kvm list , "linux-kernel@vger.kernel.org" , Kees Cook , X86 ML , Daniel Borkmann , Srivatsa Vaddagiri , Raghavendra K T , Gleb Natapov , Paolo Bonzini , Bandan Das , Andrew Honig To: Andy Lutomirski Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 07/22/2014 02:10 PM, Andy Lutomirski wrote: > On Tue, Jul 22, 2014 at 2:08 PM, H. Peter Anvin wrote: >> On 07/22/2014 02:04 PM, Andy Lutomirski wrote: >>> >>> Just to check: do you mean the RDRAND is very likely to work (i.e. >>> arch_get_random_long will return true) or that RDRAND will actually >>> reseed several times during initialization? >>> >> >> I mean that RDRAND will actually reseed several times during >> initialization. The documented architectural limit is actually >> extremely conservative. >> >> Either way, it isn't really different from seeding from a VM hosts >> /dev/urandom... > > Sure it is. The VM host's /dev/urandom makes no guarantee (or AFAIK > even any particular effort) to reseed such that the output has some > minimum entropy per bit, so there would be no point to reading extra > data from it. Depends on what you define as "extra data". If the data pulled is less than the size of the output pool, it *may* be fully entropic. (Fun fact: it may even have been fully entropic at the time you pull it, but then turn out not to be later because *another* process consumed data from /dev/urandom without adequate reseeding.) > Anyway, I'd be willing to drop the conservative RDRAND logic, but I > *still* think that arch_get_rng_seed is a much better interface than > arch_get_slow_rng_u64. That I will leave up to you and Ted. -hpa