linux-kernel - Re: [PATCH v2 0/5] Streamline TPM2 HMAC sessions

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Message-ID: <aThDeSWFhbI6GRgh@kernel.org>
Date: Tue, 9 Dec 2025 17:42:49 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: linux-integrity@...r.kernel.org
Cc: Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] Streamline TPM2 HMAC sessions

On Tue, Dec 09, 2025 at 12:05:23PM +0200, Jarkko Sakkinen wrote:
> Since we cannot at this point cache names of the keys given limitations
> of the ASN.1 file format, I'll start a fresh patch set. Let's fixup what
> we can right now.
> 
> This patch set addresses two major issues in the feature:
> 
> 1. Dynamic resolution without gain. All kernel sites have at most single
>    handle to authorize. Even if this changes some day this is how it is
>    as of today and we definitely do not want to dictate the future but
>    instead downscale code to the metrics that we have as of today.
> 2. Eliminate at least one unnnecessary tpm2_read_public() call.
> 
> Jarkko Sakkinen (5):
>   KEYS: trusted: Remove dead branch from tpm2_unseal_cmd
>   tpm2-sessions: Define TPM2_NAME_MAX_SIZE
>   KEYS: trusted: Re-orchestrate tpm2_read_public() calls
>   tpm2-sessions: Remove AUTH_MAX_NAMES
>   tpm-buf: Remove tpm_buf_append_handle
> 
>  drivers/char/tpm/tpm-buf.c                |  25 ----
>  drivers/char/tpm/tpm-sysfs.c              |   2 +-
>  drivers/char/tpm/tpm2-cmd.c               |   9 +-
>  drivers/char/tpm/tpm2-sessions.c          | 130 ++++++---------------
>  include/linux/tpm.h                       |  49 +++++---
>  security/keys/trusted-keys/trusted_tpm2.c | 134 +++++++++++++---------
>  6 files changed, 155 insertions(+), 194 deletions(-)
> 
> -- 
> 2.52.0
> 

For hwrng we can e.g., calculate factor by timing tpm2_get_random() with
and without HMAC encryption. Then we can use this as frequency how often 
data is pulled.

The other angle to combine this is to maintain largeish FIFO of random 
bytes and fill this when it goes below a treshold. 

Probably some combination of these will provide answer to performance
and latency problem with hwrng when HMAC encryption is turned on:

1. The first amortizes the overall quota.
2. Second provides constant latency without major spikes.

BR, Jarkko