linux-kernel - Re: [PATCH v2] KEYS: trusted: Use get

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Message-ID: <aUCY0oF3Ts0V7H1h@kernel.org>
Date: Tue, 16 Dec 2025 01:25:06 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: linux-integrity@...r.kernel.org
Cc: Eric Biggers <ebiggers@...nel.org>, David Howells <dhowells@...hat.com>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Mimi Zohar <zohar@...ux.ibm.com>,
	"open list:KEYS/KEYRINGS" <keyrings@...r.kernel.org>,
	"open list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KEYS: trusted: Use get_random-fallback for TPM

On Tue, Dec 16, 2025 at 01:14:38AM +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
>    generating the random numbers required.
> 
> Thus, unset '.get_random', which causes fallback to kernel_get_random().
> 
> One might argue that TPM RNG should be used so that generated trusted keys
> have the matching entropy with the TPM internally generated objects.
> 
> This argument does some weight into it but as far cryptography goes, FIPS
> certification sets the exact bar, not which exact FIPS certified RNG will
> be used. Thus, the rational choice is obviously to pick the lowest latency
> path.
> 
> Finally, there also some actual defence in depth benefits on using kernel
> RNG. E.g., it helps to mitigate TPM firmware bugs concerning RNG
> implementation, which do happen in the wild occasionally.
> 
> Reviewed-by: Eric Biggers <ebiggers@...nel.org>
> Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>

I noticed also some typos in the commit message.

I think I will also supplement this with a patch that unexports
tpm_get_random(), as the patch zeros the external call sites.

Full encapsulation to the driver is exactly should aim for in order to
make hwrng easier target for further optimizations.

BR, Jarkko