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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Dec 15, 2025 at 08:09:39PM +0000, Eric Biggers wrote:
> 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.

I get also the argument of using TPM RNG here just for the sake of
matching the creation with fully internally generated TPM objects.

I'm a bit little in-between what to do with this patch.

I suggested a comment to James. Other alternative would be do this
change and update this patch with a comment:

/*
 * tpm_get_random() was used previously here as the RNG in order to match
 * rng with the objects generated internally inside the TPM. However, since
 * e.g., FIPS certification requires kernel crypto and rng to be FIPS
 * certified, formally kernel_get_random() is equally legit source for
 * the random numbers.
 */

It's longish but I think this fully covers the whole issue.

And if there is ever need to return to this, it's a good remainder of
the design choices.

> 
> - Eric

BR, Jarkko