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

Chris Fenner
2026-03-18T17:36:22Z

Apologies if my long message derailed this discussion. I meant to
support Mimi's concern here and project a future vision where
TCG_TPM2_HMAC doesn't conflict with other features.

More concisely, I think that:

> tpm2_get_random() is costly when TCG_TPM2_HMAC is enabled

is not a compelling argument for removing TPM as an RNG source,
because TCG_TPM2_HMAC is known to have poor performance already
anyway.

Thanks
Chris

On Thu, Mar 5, 2026 at 7:37 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Tue, 2026-03-03 at 23:32 +0200, Jarkko Sakkinen wrote:
> > 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.
>
> Jarkko, you totally ignored my comments below. I object to your removing the
> TPM trusted-keys RNG support.
>
> Mimi
>
> >
> > 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