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

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Message-ID: <9cbee028-81a7-4be6-aa31-907c7cc683e3@siemens.com>
Date: Tue, 28 Oct 2025 06:46:39 +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 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.

Jan

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