Re: random(4) changes

Stephan Mueller
2016-04-23T07:53:04Z

Am Freitag, 22. April 2016, 18:27:48 schrieb Sandy Harris:

Hi Sandy,

> Stephan has recently proposed some extensive changes to this driver,
> and I proposed a quite different set earlier. My set can be found at:
> https://github.com/sandy-harris
>
> This post tries to find the bits of both proposals that seem clearly
> worth doing and entail neither large implementation problems nor large
> risk of throwing out any babies with the bathwater.
>
> Unfortunately, nothing here deals with the elephant in the room -- the
> distinctly hard problem of making sure the driver is initialised well
> enough & early enough. That needs a separate post, probably a separate
> thread. I do not find Stepan's solution to this problem plausible and
> my stuff does not claim to deal with it, though it includes some
> things that might help.

Interesting, I thought I solved the issue. But if you think it is not solved,
let us cover that in a separate thread.
>
> I really like Stephan's idea of simplifying the interrupt handling,
> replacing the multiple entropy-gathering calls in the current driver
> with one routine called for all interrupts. See section 1.2 of his
> doc. That seems to me a much cleaner design, easier both to analyse
> and to optimise as a fast interrupt handler. I also find Stephan's
> arguments that this will work better on modern systems -- VMs,
> machines with SSDs, etc. -- quite plausible.
>
> Note, though, that I am only talking about the actual interrupt
> handling, not the rest of Stephan's input handling code: the parity
> calculation and XORing the resulting single bit into the entropy pool.
> I'd be happier, at least initially, with a patch that only implemented
> a single-source interrupt handler that gave 32 or 64 bits to existing
> input-handling code.
>
> Stephan: would you want to provide such a patch?

Sure, if this is the will if the council, I will see it done.

> Ted: would you be inclined to accept it?
>
> I also quite like Stephan's idea of replacing the two output pools
> with a NIST-approved DBRG, mainly because this would probably make
> getting various certifications easier. I also like the idea of using
> crypto lib code for that since it makes both testing & maintenance
> easier. This strikes me, though, as a do-when-convenient sort of
> cleanup task, not at all urgent unless there are specific
> certifications we need soon.
>
> As for my proposals, I of course think they are full of good ideas,
> but there's only one I think is really important.
>
> In the current driver -- and I think in Stephan's, though I have not
> looked at his code in any detail, only his paper -- heavy use of
> /dev/urandom or the kernel get_random_bytes() call can deplete the
> entropy available to /dev/random. That can be a serious problem in
> some circumstances, but I think I have a fix.

To quote from my paper:

"""
When the secondary DRBG requests a reseeding from the primary DRBG and
the primary DRBG pulls from the entropy pool, an emergency entropy level
of 512 bits of entropy is left in the entropy pool. This emergency entropy is
provided to serve /dev/random even while /dev/urandom is stressed.
"""

Note, the 512 bits are chosen arbitrarily and can be set at compile time to
any other value with LRNG_EMERG_POOLSIZE. If needed, we can even make this
runtime-configurable.
>
> You have an input pool (I) plus a blocking pool (B) & a non-blocking
> pool (NB). The problem is what to do when NB must produce a lot of
> output but you do not want to deplete I too much. B & NB might be
> replaced by DBRGs and the problem would not change.
>
> B must be reseeded before very /dev/random output, NB after some
> number of output blocks. I used #define SAFE_OUT 503 but some other
> number might be better depending how NB is implemented & how
> paranoid/conservative one feels.
>
> B can only produce one full-entropy output, suitable for /dev/random,
> per reseed but B and NB are basically the same design so B can also
> produce SAFE_OUT reasonably good random numbers per reseed. Use those
> to reseed NB.and you reduce the load on I for reseeding NB from
> SAFE_OUT (use I every time NB is reseeded) to SAFE_OUT*SAFE_OUT (use I
> only to reseed B).
>
> This does need analysis by cryptographers, but at a minimum it is
> basically plausible and, even with some fairly small value for
> SAFE_OUT, it greatly alleviates the problem.

Ciao
Stephan