Re: [PATCH] KEYS: trusted: Use get_random-fallback for TPM

[Eric Biggers <ebiggers@kernel.org>]

On Sun, Dec 14, 2025 at 11:32:36PM +0200, Jarkko Sakkinen wrote:
> 1. tpm2_get_random() is costly when TCG_TPM2_HMAC is enabled and thus its
>    use should be pooled rather than directly used. This both reduces
>    latency and improves its predictability.
> 
> 2. Linux is better off overall if every subsystem uses the same source for
>    the random bistream as the de-facto choice, unless *force majeure*
>    reasons point to some other direction.
> 
> In the case, of TPM there is no reason for trusted keys to invoke TPM
> directly.
> 
> Thus, unset '.get_random', which causes fallback to kernel_get_random().
> 
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> ---
>  security/keys/trusted-keys/trusted_tpm1.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/security/keys/trusted-keys/trusted_tpm1.c b/security/keys/trusted-keys/trusted_tpm1.c
> index 636acb66a4f6..33b7739741c3 100644
> --- a/security/keys/trusted-keys/trusted_tpm1.c
> +++ b/security/keys/trusted-keys/trusted_tpm1.c
> @@ -936,11 +936,6 @@ static int trusted_tpm_unseal(struct trusted_key_payload *p, char *datablob)
>  	return ret;
>  }
>  
> -static int trusted_tpm_get_random(unsigned char *key, size_t key_len)
> -{
> -	return tpm_get_random(chip, key, key_len);
> -}
> -
>  static int __init init_digests(void)
>  {
>  	int i;
> @@ -992,6 +987,5 @@ struct trusted_key_ops trusted_key_tpm_ops = {
>  	.init = trusted_tpm_init,
>  	.seal = trusted_tpm_seal,
>  	.unseal = trusted_tpm_unseal,
> -	.get_random = trusted_tpm_get_random,
>  	.exit = trusted_tpm_exit,
>  };

Reviewed-by: Eric Biggers <ebiggers@kernel.org>

Agreed that kernel code should prefer the standard Linux RNG whenever
possible.  Note that the standard Linux RNG already incorporates entropy
from hardware RNGs, when available.

- Eric