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tpm: tpm_tis_spi_cr50: Add default RNG quality

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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@collabora.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
This commit is contained in:
AngeloGioacchino Del Regno 2021-10-29 10:09:15 +02:00 committed by Jarkko Sakkinen
parent f04510f26f
commit d2704808f2
1 changed files with 4 additions and 0 deletions
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4
drivers/char/tpm/tpm_tis_spi_cr50.c
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@ -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;