Re: [PATCH v9 01/11] KEYS: trusted: Use get_random-fallback for TPM

Jarkko Sakkinen
2026-03-03T21:32:55Z

On Fri, Feb 20, 2026 at 10:30:26AM -0800, Chris Fenner wrote:
> My conclusion about TCG_TPM2_HMAC after [1] and [2] was that
> TPM2_TCG_HMAC doesn't (or didn't at the time) actually solve the
> threat model it claims to (active interposer adversaries), while
> dramatically increasing the cost of many kernel TPM activities beyond
> the amount that would have been required to just solve for
> passive/bus-sniffer interposer adversaries. The added symmetric crypto
> required to secure a TPM transaction is almost not noticeable; the big
> performance problem is the re-bootstrapping of the session with ECDH
> for every command.
>
> My primary concern at that time was, essentially, that TCG_TPM2_HMAC
> punts on checking that the key that was used to secure the session was
> actually resident in a real TPM and not just an interposer adversary.
> I wrote up my understanding at
> https://www.dlp.rip/decorative-cryptography, for anyone who wants a
> long-form opinionated take :).
>
> Unless I'm wrong, or TCG_TPM2_HMAC has changed dramatically since
> August, I don't think "TPM2_TCG_HMAC makes this too costly" is a
> compelling reason to make a security decision. (There could be other
> reasons to make choices about whether to use the TPM as a source of
> randomness in the kernel! This just isn't one IMHO.)
>
> The version of TCG_TPM2_HMAC that I'd like to see someday would be one
> that fully admits that its threat model is only passive interposers,
> and sets up one session upon startup and ContextSaves/ContextLoads it
> back into the TPM as needed in order to secure parameter encryption
> for e.g., GetRandom() and Unseal() calls.

Neither agreeing nor disagreeing but this patch set clearly does not
move forward and I spent already enough energy for this. For better
ideas the patches are available in queue branch.

High-level takes don't move anything forward (or backward), sorry.

>
> [1]: https://lore.kernel.org/linux-integrity/CAMigqh2nwuRRxaLyOJ+QaTJ+XGmkQj=rMj5K9GP1bCcXp2OsBQ@mail.gmail.com/
> [2]: https://lore.kernel.org/linux-integrity/20250825203223.629515-1-jarkko@kernel.org/
>
> Thanks
> Chris
>
> On Fri, Feb 20, 2026 at 10:04 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
> >
> > [Cc: Chris Fenner, Jonathan McDowell, Roberto]
> >
> > On Sun, 2026-01-25 at 21:25 +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.
> >
> > If the concern is the latency of encrypting the bus session, please remember
> > that:
> >
> > - Not all environments expose the TPM bus to sniffing.
> > - The current TPM trusted keys design is based on TPM RNG, but already allows it
> > to be replaced with the kernel RNG via the "trusted_rng=kernel" boot command
> > line option.
> > - The proposed patch removes that possibility for no reason.
> >
> > Mimi & Elaine
> >
> >

BR, Jarkko