Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

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);