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Date: Fri, 23 Feb 2024 22:44:48 +0200
Message-Id: <CZCR2N9XKXJQ.32FMAU50UOLT2@suppilovahvero>
Cc: "Ross Philipson" <ross.philipson@oracle.com>, "Kanth Ghatraju"
 <kanth.ghatraju@oracle.com>, "Peter Huewe" <peterhuewe@gmx.de>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow
From: "Jarkko Sakkinen" <jarkko@kernel.org>
To: "Daniel P. Smith" <dpsmith@apertussolutions.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>
References: <20240131170824.6183-1-dpsmith@apertussolutions.com>
 <20240131170824.6183-2-dpsmith@apertussolutions.com>
 <CYU3CFW08DAA.29DJY7SJYPJJZ@suppilovahvero>
 <2ba9a96e-f93b-48e2-9ca0-48318af7f9b1@kunbus.com>
 <ae3fecc4-7b76-4607-8749-045e17941923@infineon.com>
 <91f600ef-867b-4523-89be-1c0ba34f8a4c@kunbus.com>
 <8692fcf6-2e67-45b4-b809-7723f30736a2@apertussolutions.com>
In-Reply-To: <8692fcf6-2e67-45b4-b809-7723f30736a2@apertussolutions.com>

On Fri Feb 23, 2024 at 3:56 AM EET, Daniel P. Smith wrote:
> On 2/20/24 15:54, Lino Sanfilippo wrote:
> > Hi,
> >=20
> > On 20.02.24 19:42, Alexander Steffen wrote:
> >> ATTENTION: This e-mail is from an external sender. Please check attach=
ments and links before opening e.g. with mouseover.
> >>
> >>
> >> On 02.02.2024 04:08, Lino Sanfilippo wrote:
> >>> On 01.02.24 23:21, Jarkko Sakkinen wrote:
> >>>
> >>>>
> >>>> On Wed Jan 31, 2024 at 7:08 PM EET, Daniel P. Smith wrote:
> >>>>> Commit 933bfc5ad213 introduced the use of a locality counter to con=
trol when a
> >>>>> locality request is allowed to be sent to the TPM. In the commit, t=
he counter
> >>>>> is indiscriminately decremented. Thus creating a situation for an i=
nteger
> >>>>> underflow of the counter.
> >>>>
> >>>> What is the sequence of events that leads to this triggering the
> >>>> underflow? This information should be represent in the commit messag=
e.
> >>>>
> >>>
> >>> AFAIU this is:
> >>>
> >>> 1. We start with a locality_counter of 0 and then we call tpm_tis_req=
uest_locality()
> >>> for the first time, but since a locality is (unexpectedly) already ac=
tive
> >>> check_locality() and consequently __tpm_tis_request_locality() return=
 "true".
> >>
> >> check_locality() returns true, but __tpm_tis_request_locality() return=
s
> >> the requested locality. Currently, this is always 0, so the check for
> >> !ret will always correctly indicate success and increment the
> >> locality_count.
> >>
> >=20
> > Will the TPM TIS CORE ever (have to) request another locality than 0? M=
aybe the best would
> > be to hardcode TPM_ACCESS(0) and get rid of all the locality parameters=
 that are
> > passed from one function to another.
> > But this is rather code optimization and not really required to fix the=
 reported bug.
>
> Actually, doing so will break the TPM API. The function=20
> tpm_tis_request_locality() is registered as the locality handler,=20
>   int (*request_locality)(struct tpm_chip *chip, int loc), in the tis=20
> instance of struct tpm_class_ops{}. This is the API used by the Secure=20
> Launch series to open Locality2 for the measurements it must record.

OK, based on James' earlier feedback on possibility to have kernel
specific locality and this , and some reconsideration of my position on
the topic, and reading all these great and informative responses, I
think I went too far with this :-)

>
> v/r,
> dps

BR, Jarkko