Re: [PATCH] hwrng: tpm: Do not enable by default

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Apr 29, 2026 at 02:33:20PM +0000, Niedermayr, BENEDIKT wrote:
> On 10/27/25 20:51, Jarkko Sakkinen wrote:
> > On Tue, Oct 21, 2025 at 02:46:15PM +0200, Jan Kiszka wrote:
> >> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>
> >> As seen with optee_ftpm, which uses ms-tpm-20-ref [1], a TPM may write
> >> the current time epoch to its NV storage every 4 seconds if there are
> >> commands sent to it. The 60 seconds periodic update of the entropy pool
> >> that the hwrng kthread does triggers this, causing about 4 writes per
> >> requests. Makes 2 millions per year for a 24/7 device, and that is a lot
> >> for its backing NV storage.
> >>
> >> It is therefore better to make the user intentionally enable this,
> >> providing a chance to read the warning.
> >>
> >> [1] https://github.com/Microsoft/ms-tpm-20-ref
> >>
> >> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> > 
> > Looking at DRBG_* from [1] I don't see anything you describe. If OPTEE
> > writes NVRAM,  then the implementation is broken.
> > 
> > Also AFAIK, it is pre-seeded per power cycle. There's nothing that even
> > distantly relates on using NVRAM.
> > 
> > [1] https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-4-Supporting-Routines-Code.pdf
> 
> Hi all,
> 
> we recently also stumbled over this issue which led me here to this 
> thread and maybe adding our observations helps to clarify things here a 
> bit (hopefully) or at least augments the information related to firmware 
> TPM based implementation based on ms-tpm-20-ref.
> 
> Based on the optee_ftpm repo, as Jan already described, which currently 
> references commit 98b60a44aba7 of [1] suffers this exact issue because 
> of the NV_CLOCK_UPDATE_INTERVAL [2] which is set to "12" and issues a 
> write for each command after ~4 seconds have passed.
> 
> This config has been changed to "22" (on current master branch [3]) 
> which is the allowed maximum when following the TPM spec (chapter 36.3.2 
> in [4]) which leads to round about 70 minutes, but optee_ftpm didn't 
> move ahead to this commit, yet.
> This config exists for being able to adapt the write cycles to the 
> specific wear conditions of the hardware.
> 
> Moreover the ms-tpm-20-ref repo seems to not be maintained anymore and 
> one should rather switch to [6].
> 
> So there are currently firmware TPM implementations out there that lead 
> to these frequent writes.

Really this would need a product and official bug bulletin for it to
even consider a workaround. Speculation does not count.

> 
> AFAIK since the tpm-20-ref implementation basically only supports a file 
> on disk or RAM backing storage, the optee_ftpm repo [5] provides it's 
> own _plat_NV* implementations that replace the default ones and finally 
> call OP-TEE's TEE_* secure storage API, which then routes to whatever 
> backend OP-TEE is configured with (REE-FS or RPMB) – In our case the RPMB.
> 
> Because there are currently implementations out there (e.g. start using 
> optee_ftpm) it may make sense to add this information to the kernel 
> config's help text at least?

Your first forum to report such issues is the TPM vendor.

> 
> We are currently trying to bump optee_ftpm to use the more recent v1.84, 
> but since we're no TCG member the PRs on github could get a bit 
> adventurous (PR's not upstream, yet).
> Until then this is a valid issue that exists...
> 
> 
> [2] 
> https://github.com/microsoft/ms-tpm-20-ref/blob/98b60a44aba79b15fcce1c0d1e46cf5918400f6a/TPMCmd/tpm/include/TpmProfile.h#L199 
> 
> 
> [3] 
> https://github.com/microsoft/ms-tpm-20-ref/blob/98b60a44aba79b15fcce1c0d1e46cf5918400f6a/TPMCmd/tpm/include/TpmProfile.h#L200
> 
> [4] 
> https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf
> 
> [5] https://github.com/OP-TEE/optee_ftpm
> 
> [6] https://github.com/TrustedComputingGroup/TPM
> 
> BR,
> Benedikt
> 

BR, Jarkko