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

[Jarkko Sakkinen]

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>

/Jarkko