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

["Niedermayr, BENEDIKT" <benedikt.niedermayr@siemens.com>]

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.

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