Re: [PATCH v2] tpm: tpm_tis_spi_cr50: Add default RNG quality

[Jarkko Sakkinen]

On Sun, 2021-10-31 at 19:25 +0200, Jarkko Sakkinen wrote:
> On Fri, 2021-10-29 at 10:09 +0200, AngeloGioacchino Del Regno wrote:
> > To allow this device to fill the kernel's entropy pool at boot,
> > setup a default quality for the hwrng found in Cr50.
> > 
> > After some testing with rngtest and dieharder it was, in short,
> > discovered that the RNG produces fair quality randomness, giving
> > around 99.93% successes in rngtest FIPS140-2.
> > 
> > Notably, though, when testing with dieharder it was noticed that
> > we get 3 WEAK results over 114, which isn't optimal, and also
> > the p-values distribution wasn't uniform in all the cases, so a
> > conservative quality value was chosen by applying an arbitrary
> > penalty to the calculated values.
> > 
> > For reference, this is how the values were calculated:
> > 
> > The dieharder results were averaged, then normalized (0-1000)
> > and re-averaged with the rngtest result (where the result was
> > given a score of 99.93% of 1000, so 999.3), then aggregated
> > together and averaged again.
> > An arbitrary penalty of -100 was applied due to the retrieved
> > value, which brings us finally to 700.
> > 
> > Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
> > ---
> >  drivers/char/tpm/tpm_tis_spi_cr50.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/char/tpm/tpm_tis_spi_cr50.c b/drivers/char/tpm/tpm_tis_spi_cr50.c
> > index ea759af25634..dae98dbeeeac 100644
> > --- a/drivers/char/tpm/tpm_tis_spi_cr50.c
> > +++ b/drivers/char/tpm/tpm_tis_spi_cr50.c
> > @@ -36,6 +36,9 @@
> >  #define TPM_CR50_FW_VER(l)                     (0x0f90 | ((l) << 12))
> >  #define TPM_CR50_MAX_FW_VER_LEN                        64
> >  
> > +/* Default quality for hwrng. */
> > +#define TPM_CR50_DEFAULT_RNG_QUALITY           700
> > +
> >  struct cr50_spi_phy {
> >         struct tpm_tis_spi_phy spi_phy;
> >  
> > @@ -264,6 +267,7 @@ int cr50_spi_probe(struct spi_device *spi)
> >         phy = &cr50_phy->spi_phy;
> >         phy->flow_control = cr50_spi_flow_control;
> >         phy->wake_after = jiffies;
> > +       phy->priv.rng_quality = TPM_CR50_DEFAULT_RNG_QUALITY;
> >         init_completion(&phy->ready);
> >  
> >         cr50_phy->access_delay = CR50_NOIRQ_ACCESS_DELAY;
> 
> Thank you.
> 
> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxx>

Oops, a typo.

Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

/Jarkko