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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Dec 09, 2025 at 05:42:55PM +0200, Jarkko Sakkinen wrote:
> 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.

Another perhaps more unorthodox idea:

1. Calculate factor as said above. Let's call it N.
2. Every Nth step seed a pseudo rng from TPM.
3. On steps not divisible by N, pull from pseud rng.

I'm not a cryptographer but would randomness suffer from this? Then the
seeds that woud come from TPM are HMAC encrypted at least without major 
hits on performance.

Just enumerated this. Not sure about this idea yet by any means but I do 
like simplicy of it (and that also makes me concerned about missed
details).

BR, Jarkko