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

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Message-ID: <aRAb7KEPmPmoyQbm@kernel.org>
Date: Sun, 9 Nov 2025 06:43:24 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Jan Kiszka <jan.kiszka@...mens.com>
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 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.

> 
> Jan
> 
> -- 
> Siemens AG, Foundational Technologies
> Linux Expert Center

BR, Jarkko