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 <[email protected]> >> >> 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 <[email protected]> > > 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. 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? 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
