RE: [tpmdd-devel] [PATCH RFC 1/3] tpm_crb: expand struct crb_control_area to struct crb_regs

[Winkler, Tomas]

> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgunthorpe@xxxxxxxxxxxxxxxxxxxx]
> Sent: Monday, October 10, 2016 02:08
> To: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> Cc: moderated list:TPM DEVICE DRIVER <tpmdd-devel@xxxxxxxxxxxxxxxxxxxxx>;
> open list <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Re: [tpmdd-devel] [PATCH RFC 1/3] tpm_crb: expand struct
> crb_control_area to struct crb_regs
> 
> 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.

I addition I believe it should be always on offset FED4_0xxxh by the Spec, so all this arithmetic is a bit of overkill.
Thanks
Tomas