Re: [PATCH 1/3] tpm: protect against locality counter underflow

[Jarkko Sakkinen]

On Fri Feb 23, 2024 at 3:57 AM EET, Daniel P. Smith wrote:
> On 2/21/24 14:43, Jarkko Sakkinen wrote:
> > On Wed Feb 21, 2024 at 12:37 PM UTC, James Bottomley wrote:
> >> On Tue, 2024-02-20 at 22:31 +0000, Jarkko Sakkinen wrote:
> >>>
> >>> 2. Because localities are not too useful these days given TPM2's
> >>>     policy mechanism
> >>
> >> Localitites are useful to the TPM2 policy mechanism.  When we get key
> >> policy in the kernel it will give us a way to create TPM wrapped keys
> >> that can only be unwrapped in the kernel if we run the kernel in a
> >> different locality from userspace (I already have demo patches doing
> >> this).
> > 
> > Let's keep this discussion in scope, please.
> > 
> > Removing useless code using registers that you might have some actually
> > useful use is not wrong thing to do. It is better to look at things from
> > clean slate when the time comes.
> > 
> >>>   I cannot recall out of top of my head can
> >>>     you have two localities open at same time.
> >>
> >> I think there's a misunderstanding about what localities are: they're
> >> effectively an additional platform supplied tag to a command.  Each
> >> command can therefore have one and only one locality.  The TPM doesn't
> > 
> > Actually this was not unclear at all. I even read the chapters from
> > Ariel Segall's yesterday as a refresher.
> > 
> > I was merely asking that if TPM_ACCESS_X is not properly cleared and you
> > se TPM_ACCESS_Y where Y < X how does the hardware react as the bug
> > report is pretty open ended and not very clear of the steps leading to
> > unwanted results.
> > 
> > With a quick check from [1] could not spot the conflict reaction but
> > it is probably there.
>
> The expected behavior is explained in the Informative Comment of section 
> 6.5.2.4 of the Client PTP spec[1]:
>
> "The purpose of this register is to allow the processes operating at the 
> various localities to share the TPM. The basic notion is that any 
> locality can request access to the TPM by setting the 
> TPM_ACCESS_x.requestUse field using its assigned TPM_ACCESS_x register 
> address. If there is no currently set locality, the TPM sets current 
> locality to the requesting one and allows operations only from that 
> locality. If the TPM is currently at another locality, the TPM keeps the 
> request pending until the currently executing locality frees the TPM. 

Right.

I'd think it would make sense to document the basic dance like this as
part of kdoc for request_locality:

* Setting TPM_ACCESS_x.requestUse:
*  1. No locality reserved => set locality.
*  2. Locality reserved => set pending.

I.e. easy reminder with enough granularity.

> Software relinquishes the TPM’s locality by writing a 1 to the 
> TPM_ACCESS_x.activeLocality field. Upon release, the TPM honors the 
> highest locality request pending. If there is no pending request, the 
> TPM enters the “free” state."

And this for relinquish_locality:

* Setting TPM_ACCESS_x.activeLocality:
*  1. No locality pending => free.
*  2. Localities pending => reserve for highest.

> >> submission).   I think the locality request/relinquish was modelled
> >> after some other HW, but I don't know what.
> > 
> > My wild guess: first implementation was made when TPM's became available
> > and there was no analytical thinking other than getting something that
> > runs :-)
>
> Actually, no that is not how it was done. IIRC, localities were designed 
> in conjunction with D-RTM when Intel and MS started the LeGrande effort 
> back in 2000. It was then generalized for the TPM 1.1b specification. My 

OK, thanks for this bit of information! I did not know this.

> first introduction to LeGrande/TXT wasn't until 2005 as part of an early 
> access program. So most of my historical understanding is from 
> discussions I luckily got to have with one of the architects and a few 
> of the original TCG committee members.

Thanks alot for sharing this.

>
> [1] 
> https://trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2p0-v1p05p_r14_pub.pdf
>
> v/r,
> dps


BR, Jarkko