Re: [PATCH v8 04/12] tpm: Change tpm_get_random() opportunistic

[Jonathan McDowell]

On Tue, Dec 16, 2025 at 11:21:38AM +0200, Jarkko Sakkinen wrote:
> hwrng framework does not have a requirement that the all bytes requested
> need to be provided. By enforcing such a requirement internally, TPM driver
> can cause unpredictability in latency, as a single tpm_get_random() call
> can result multiple TPM commands.
>
> Especially, when TCG_TPM2_HMAC is enabled, extra roundtrips could have
> significant effect to the system latency.
>
> Thus, send TPM command only once and return bytes received instead of
> committing to the number of requested bytes.

Function comment for tpm_get_random needs updated as well, as it 
currently says "until all of the @max bytes have been received", which 
is no longer true with this patch. With that:

Reviewed-by: Jonathan McDowell <noodles@xxxxxxxx>

> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> ---
> v7:
> - Given that hwrng is now only caller for tpm_get_random(), remove the
>  wait parameter.
> v4:
> - Fixed grammar mistakes.
> ---
> drivers/char/tpm/tpm-interface.c | 28 +++++-----------------------
> 1 file changed, 5 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index d157be738612..677dcef05dfb 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -626,10 +626,6 @@ static int tpm2_get_random(struct tpm_chip *chip, u8 *out, size_t max)
>  */
> int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
> {
> -	u32 num_bytes = max;
> -	u8 *out_ptr = out;
> -	int retries = 5;
> -	int total = 0;
> 	int rc;
>
> 	if (!out || !max || max > TPM_MAX_RNG_DATA)
> @@ -646,28 +642,14 @@ int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
> 		rc = tpm2_start_auth_session(chip);
> 		if (rc)
> 			return rc;
> -	}
> -
> -	do {
> -		if (chip->flags & TPM_CHIP_FLAG_TPM2)
> -			rc = tpm2_get_random(chip, out_ptr, num_bytes);
> -		else
> -			rc = tpm1_get_random(chip, out_ptr, num_bytes);
> -
> -		if (rc < 0)
> -			goto err;
> -
> -		out_ptr += rc;
> -		total += rc;
> -		num_bytes -= rc;
> -	} while (retries-- && total < max);
>
> -	tpm_put_ops(chip);
> -	return total ? total : -EIO;
> +		rc = tpm2_get_random(chip, out, max);
> +	} else {
> +		rc = tpm1_get_random(chip, out, max);
> +	}
>
> -err:
> 	tpm_put_ops(chip);
> -	return rc;
> +	return rc != 0 ? rc : -EIO;
> }
> EXPORT_SYMBOL_GPL(tpm_get_random);
>
> --
> 2.39.5

J.

--
... We are talking one charming motherf**king pig.