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

Jarkko Sakkinen
2026-03-23T05:24:50Z

On Thu, Mar 05, 2026 at 10:37:08AM -0500, Mimi Zohar 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.

It has not been removed but I can keep the patches still backed
up in a branch, can't I?

BR, Jarkko