linux-kernel - Re: [PATCH v2] Allow hwrng to initialize crng.

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Date:   Fri, 14 Dec 2018 11:12:51 +0800
From:   Louis Collard <louiscollard@...omium.org>
To:     ard.biesheuvel@...aro.org
Cc:     linux-crypto@...r.kernel.org, mpm@...enic.com,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Gary R Hook <gary.hook@....com>, Michael Buesch <m@...s.ch>,
        PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>,
        mst@...hat.com, linux-kernel@...r.kernel.org,
        Andrey Pronin <apronin@...omium.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        linux@...ewoehner.de, "David R. Bild" <david.bild@...tum.com>,
        tytso@....edu
Subject: Re: [PATCH v2] Allow hwrng to initialize crng.

On Thu, Dec 13, 2018 at 5:48 PM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Thu, 13 Dec 2018 at 10:18, Louis Collard <louiscollard@...omium.org> wrote:
> >
> > Some systems, for example embedded systems, do not generate
> > enough entropy on boot through interrupts, and boot may be blocked for
> > several minutes waiting for a call to getrandom to complete.
> >
> > Currently, random data is read from a hwrng when it is registered,
> > and is loaded into primary_crng. This data is treated in the same
> > way as data that is device-specific but otherwise unchanging, and
> > so primary_crng cannot become initialized with the data from the
> > hwrng.
> >
> > This change causes the data initially read from the hwrng to be
> > treated the same as subsequent data that is read from the hwrng if
> > it's quality score is non-zero.
> >
> > The implications of this are:
> >
> > The data read from hwrng can cause primary_crng to become
> > initialized, therefore avoiding problems of getrandom blocking
> > on boot.
> >
> > Calls to getrandom (with GRND_RANDOM) may be using entropy
> > exclusively (or in practise, almost exclusively) from the hwrng.
> >
> > Regarding the latter point; this behavior is the same as if a
> > user specified a quality score of 1 (bit of entropy per 1024 bits)
> > so hopefully this is not too scary a change to make.
> >
> > This change is the result of the discussion here:
> > https://patchwork.kernel.org/patch/10453893/
> >
> > Signed-off-by: Louis Collard <louiscollard@...omium.org>
> > Acked-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> > ---
> >  drivers/char/hw_random/core.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index 95be7228f327..7736e1a96321 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -24,6 +24,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/slab.h>
> >  #include <linux/uaccess.h>
> > +#include <crypto/chacha20.h>
> >
> >  #define RNG_MODULE_NAME                "hw_random"
> >
> > @@ -64,13 +65,19 @@ static size_t rng_buffer_size(void)
> >  static void add_early_randomness(struct hwrng *rng)
> >  {
> >         int bytes_read;
> > -       size_t size = min_t(size_t, 16, rng_buffer_size());
> > +       /* Read enough to initialize crng. */
> > +       size_t size = min_t(size_t,
> > +                           2*CHACHA20_KEY_SIZE,
>
> This should be as symbolic constant that retains its meaning even if
> we move away from ChaCha20 or modify the current implementation
>
> > +                           rng_buffer_size());
> >
> >         mutex_lock(&reading_mutex);
> >         bytes_read = rng_get_data(rng, rng_buffer, size, 1);
> >         mutex_unlock(&reading_mutex);
> >         if (bytes_read > 0)
> > -               add_device_randomness(rng_buffer, bytes_read);
> > +               /* Allow crng to become initialized, but do not add
> > +                * entropy to the pool.
> > +                */
> > +               add_hwgenerator_randomness(rng_buffer, bytes_read, 0);
> >  }
> >
> >  static inline void cleanup_rng(struct kref *kref)
> > --
> > 2.13.5
> >

Right, this should be [equal to] CRNG_INIT_CNT_THRESH from random.c -
I wasn't sure where/how to pull this out to though..