On 09.05.26 17:18, Jarkko Sakkinen wrote:
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-Sup...
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.
The key point Benedikt tries to make here is that the TPM 2 spec forces any vendor to do something about persisting the last seen time at least every 70 min. If they didn't do that, then they would violate the space - arguably a bug. But, correct, it does not tell us anything about how this happens in a random firmware TPM implementation.
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.
I would still not recommend anyone relying on a firmware TPM to turn on CONFIG_HW_RANDOM_TPM if there are viable alternatives. In case of the open source stack with optee_os + optee_ftpm, we know that at least one exists: CONFIG_HW_RANDOM_OPTEE.
So, if there is no good place to document this in the kernel, maybe it is worth to document it in optee_ftpm instead.
Jan