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Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     James Bottomley <James.Bottomley@HansenPartnership.com>,
        Jerry Snitselaar <jsnitsel@redhat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        David Safford <david.safford@ge.com>,
        linux-integrity@vger.kernel.org, stable@vger.kernel.org,
        David Howells <dhowells@redhat.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        "open list:ASYMMETRIC KEYS" <keyrings@vger.kernel.org>,
        "open list:CRYPTO API" <linux-crypto@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Date:   Sat, 05 Oct 2019 20:38:53 -0400
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On Fri, 2019-10-04 at 15:11 -0700, James Bottomley wrote:

> +
> +/**
> + * tpm_get_random() - get random bytes influenced by the TPM's RNG
> + * @chip:	a &struct tpm_chip instance, %NULL for the default chip
> + * @out:	destination buffer for the random bytes
> + * @max:	the max number of bytes to write to @out
> + *
> + * Uses the TPM as a source of input to the kernel random number
> + * generator and then takes @max bytes directly from the kernel.  In
> + * the worst (no other entropy) case, this will return the pure TPM
> + * random number, but if the kernel RNG has any entropy at all it will
> + * return a mixed entropy output which doesn't rely on a single
> + * source.
> + *
> + * Return: number of random bytes read or a negative error value.
> + */
> +int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
> +{
> +	int rc;
> +
> +	rc = __tpm_get_random(chip, out, max);
> +	if (rc <= 0)
> +		return rc;
> +	/*
> +	 * assume the TPM produces pure randomness, so the amount of
> +	 * entropy is the number of bits returned
> +	 */
> +	add_hwgenerator_randomness(out, rc, rc * 8);
> +	get_random_bytes(out, rc);

Using the TPM as a source of input to the kernel random number
generator is fine, but please don't change the meaning of trusted
keys.  The trusted-encrypted keys documentation clearly states
"Trusted Keys use a TPM both to generate and to seal the keys."

If you really want to use a different random number source instead of
the TPM, then define a new trusted key option (eg. rng=kernel), with
the default being the TPM.

Mimi


> +
> +	return rc;
> +}
>  EXPORT_SYMBOL_GPL(tpm_get_random);