Re: [PATCH v1 1/2] random-bits: Factor out entropy generating function

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Mon, Apr 4, 2022 at 12:42 PM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 01/04/2022 15:01, Cristian Rodríguez wrote:
> > On Thu, Mar 31, 2022 at 8:05 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> >> AFAIK our goal is entropy more so than security. For example
> >> if this is used to generate jiffies to stagger threads its not a security
> >> issue in any sense, it's just not ideal for performance.
> >
> > In any case this should be more than fast enough for the other use
> > cases of random_bits() .. maybe one new random_bits_fast() function
> > foe edge cases where even this is too slow?
>
> I think we are bike-shedding in the same issue OpenBSD guys stumbled
> and which they have solved 10 years ago [1].  Essentially, we need to
> come  up with a internal PRNG interface that can be used internally and
> externally, instead of reinventing cleaver ways to use the timer as
> entropy source.
>
> The issue is not really the cypher used, ideally it could be replace if
> we find out that it does not fit.  The main issue is to glue together
> all the requirements to have a concise internal interface, taking in
> consideration the glibc constrains to work with multiple kernel version
> and environments (where we can't assume we have access to a source or
> reliable entropy like getrandom syscall).
>
> My plan to rehearse Florian arc4random proposal to have some simpler
> to where we might improve upon (a simpler fork detection for kernels
> without MADV_WIPEONFORK that just issue a atfork handle, maybe using
> ChaCha20 as virtually all other systems do, no per-thread state).

Why no per-thread / per-cpu? It seems otherwise there will need to be
some explicit synchronization on the stream.


Regarding this patch, do we want to skip it and just wait on arc4random
interface in kernel/glibc or should I go forward with it and some arch
specific entropy functions in the mean-time?

>
> [1] https://www.youtube.com/watch?v=gp_90-3R0pE