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

From: "Jarkko Sakkinen" <jarkko@kernel.org>
To: "Jarkko Sakkinen" <jarkko@kernel.org>,
	"Daniel P. Smith" <dpsmith@apertussolutions.com>,
	"James Bottomley" <James.Bottomley@HansenPartnership.com>,
	"Lino Sanfilippo" <l.sanfilippo@kunbus.com>,
	"Alexander Steffen" <Alexander.Steffen@infineon.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Sasha Levin" <sashal@kernel.org>,
	<linux-integrity@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Cc: "Ross Philipson" <ross.philipson@oracle.com>,
	"Kanth Ghatraju" <kanth.ghatraju@oracle.com>,
	"Peter Huewe" <peterhuewe@gmx.dekkjkj>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow
Date: Fri, 23 Feb 2024 22:42:23 +0200	[thread overview]
Message-ID: <CZCR0STNHQZ7.WQ6RUYVPYKZI@suppilovahvero> (raw)
In-Reply-To: <CZCQZ5FTCCB5.GIN1NU7G5S0@suppilovahvero>

On Fri Feb 23, 2024 at 10:40 PM EET, Jarkko Sakkinen wrote:
> 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.

Also for any non-TPM kernel developer this should be enough to get the
basic gist of the mechanism without spending too much time reading.

BR, Jarkko