linux-kernel - Re: [PATCH RFC 1/3] tpm_crb: expand struct crb_control_area to struct crb

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Date:   Mon, 10 Oct 2016 07:45:08 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Peter Huewe <peterhuewe@....de>,
        Marcel Selhorst <tpmdd@...horst.net>,
        "moderated list:TPM DEVICE DRIVER" 
        <tpmdd-devel@...ts.sourceforge.net>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/3] tpm_crb: expand struct crb_control_area to
 struct crb_regs

On Sun, Oct 09, 2016 at 05:07:37PM -0600, Jason Gunthorpe wrote:
> On Sun, Oct 09, 2016 at 09:33:58PM +0300, Jarkko Sakkinen wrote:
> 
> > > Sorry I missed this part.
> > > 
> > > Here are the constraints for existing hardware:
> > > 
> > > 1. All the existing CRB start only hardware has the iomem covering the
> > >    control area and registers for multiple localities.
> > > 2. All the existing ACPI start hardware has only the control area.
> > > 
> > > If you assume that SSDT does not have malicous behavior caused by either
> > > a BIOS bug or maybe a rootkit, then the current patch works for all the
> > > existing hardware.
> > > 
> > > To counter-measure for unexpected behavior in non-existing hardware and
> > > buggy or malicious firmware it probably make sense to use crb_map_res to
> > > validate the part of the CRB registers that is not part of the control
> > > area.
> 
> I don't know how much I'd assume BIOS authors do what you think - the
> spec I saw for this seems very vauge.
> 
> Certainly checking that locality region falls within the acpi mapping
> seems essential.
> 
> > > Doing it in the way you proposed does not work for ACPI start devices.
> > > 
> > > For them it should be done in the same way as I'm doing in the existing
> > > patch as for ACPI start devices the address below the control area are
> > > never accessed. Having a separate crb_map_res for CRB start only devices
> > > is sane thing to do for validation.
> > 
> > Alternative is to do two structures crb_regs_head and crb_regs_tail,
> > which might be cleaner. I'm fine with going either route.
> 
> Since the iomem doesn't actually exist for a configuration having two
> pointers is the better choice. Make sure one is null for the
> configuration that does not support it.
> 
> The negative offset thing is way too subtle.

Yeah, I do agree with you on this. Even if it was functionalliy correct,
it is hard to understand if you don't proactively work on the driver.

> Jason

/Jarkko