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

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Message-ID: <f1a54774-9a44-4400-91e2-358facc12191@apertussolutions.com>
Date: Thu, 22 Feb 2024 20:57:30 -0500
From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 James Bottomley <James.Bottomley@...senPartnership.com>,
 Lino Sanfilippo <l.sanfilippo@...bus.com>,
 Alexander Steffen <Alexander.Steffen@...ineon.com>,
 Jason Gunthorpe <jgg@...pe.ca>, Sasha Levin <sashal@...nel.org>,
 linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Ross Philipson <ross.philipson@...cle.com>,
 Kanth Ghatraju <kanth.ghatraju@...cle.com>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow

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. 
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."

>> 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 
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.

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

v/r,
dps