linux-kernel - Re: [PATCH 1/5] tpm: add functions to set and unset the tpm chips reset state

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Message-ID: <20220411114741.GA64706@ziepe.ca>
Date:   Mon, 11 Apr 2022 08:47:41 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org, =robh+dt@...nel.org, devicetree@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        stefanb@...ux.ibm.com, p.rosenberger@...bus.com
Subject: Re: [PATCH 1/5] tpm: add functions to set and unset the tpm chips
 reset state

On Sun, Apr 10, 2022 at 07:11:23PM +0200, Lukas Wunner wrote:
> On Thu, Apr 07, 2022 at 11:25:26AM -0300, Jason Gunthorpe wrote:
> > On Thu, Apr 07, 2022 at 01:18:45PM +0200, Lino Sanfilippo wrote:
> > > Currently it is not possible to set the tpm chips reset state from within
> > > the driver. This is problematic if the chip is still in reset after the
> > > system comes up. This may e.g. happen if the reset line is pulled into
> > > reset state by a pin configuration in the device tree.
> > 
> > This kind of system is badly misdesigned.
> > 
> > TPM PCRs fundementally cannot work if the TPM reset line is under
> > software control.
> 
> Not every system which incorporates a TPM wants to use or is even capable
> of measuring software state of any kind or perform secure boot.
> 
> Those systems may merely want to use the TPM to store key material.

Then maybe the TPM driver should make it clear somehow that the PCRs
don't work in these systems.

It is really dangerous to add capabilities like this that should
never, ever be used in sanely designed systems.

Jason