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

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Message-ID: <a10203a2-56b5-40f7-ac46-8ad586cf52f2@siemens.com>
Date: Sun, 9 Nov 2025 11:04:27 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Peter Huewe <peterhuewe@....de>, linux-integrity@...r.kernel.org,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Jens Wiklander <jens.wiklander@...aro.org>,
 OP-TEE TrustedFirmware <op-tee@...ts.trustedfirmware.org>,
 linux-crypto@...r.kernel.org
Subject: Re: [PATCH] hwrng: tpm: Do not enable by default

On 09.11.25 05:43, Jarkko Sakkinen wrote:
> On Tue, Oct 28, 2025 at 06:46:39AM +0100, Jan Kiszka wrote:
>> On 27.10.25 20:51, Jarkko Sakkinen wrote:
>>> On Tue, Oct 21, 2025 at 02:46:15PM +0200, Jan Kiszka wrote:
>>>> From: Jan Kiszka <jan.kiszka@...mens.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@...mens.com>
>>>
>>> Looking at DRBG_* from [1] I don't see anything you describe. If OPTEE
>>> writes NVRAM,  then the implementation is broken.
>>
>> It's not OP-TEE, but it might be indeed an artifact of the reference
>> implementation that the fTPM is using because it is also targeting
>> simulation:
>>
>> https://github.com/microsoft/ms-tpm-20-ref/blob/ee21db0a941decd3cac67925ea3310873af60ab3/TPMCmd/tpm/src/main/ExecCommand.c#L99
>> (Page 942 in [1])
>>
>> -> ... ->
>>
>> https://github.com/microsoft/ms-tpm-20-ref/blob/main/TPMCmd/tpm/src/subsystem/Time.c#L68
>> (Page 1075 in [1])
>>
>>>
>>> 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
>>>
>>> BR, Jarkko
>>
>> Given how detailed [1] is, we likely need to address that directly there
>> to avoid spreading this issue into fTPMs. Fact is, that there firmware
>> implementations out there which exactly do what [1] suggests: writing to
>> NV every 4 seconds on every command.
> 
> We don't reference code as a certified hardware product, sorry.
> 

Means what?

Jan

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