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Date: Mon, 12 Feb 2024 22:05:12 +0200
Message-Id: <CZ3DCC8JHNLK.3MGE70MQJT5XM@kernel.org>
To: "Lino Sanfilippo" <l.sanfilippo@kunbus.com>, "Daniel P. Smith"
 <dpsmith@apertussolutions.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.de>
Subject: Re: [PATCH 1/3] tpm: protect against locality counter underflow
From: "Jarkko Sakkinen" <jarkko@kernel.org>
X-Mailer: aerc 0.16.0
References: <20240131170824.6183-1-dpsmith@apertussolutions.com>
 <20240131170824.6183-2-dpsmith@apertussolutions.com>
 <CYU3CFW08DAA.29DJY7SJYPJJZ@suppilovahvero>
 <2ba9a96e-f93b-48e2-9ca0-48318af7f9b1@kunbus.com>
In-Reply-To: <2ba9a96e-f93b-48e2-9ca0-48318af7f9b1@kunbus.com>

On Fri Feb 2, 2024 at 5:08 AM EET, Lino Sanfilippo wrote:
>
>
> On 01.02.24 23:21, Jarkko Sakkinen wrote:
>
> >=20
> > On Wed Jan 31, 2024 at 7:08 PM EET, Daniel P. Smith wrote:
> >> Commit 933bfc5ad213 introduced the use of a locality counter to contro=
l when a
> >> locality request is allowed to be sent to the TPM. In the commit, the =
counter
> >> is indiscriminately decremented. Thus creating a situation for an inte=
ger
> >> underflow of the counter.
> >=20
> > What is the sequence of events that leads to this triggering the
> > underflow? This information should be represent in the commit message.
> >=20
>
> AFAIU this is:
>
> 1. We start with a locality_counter of 0 and then we call tpm_tis_request=
_locality()
> for the first time, but since a locality is (unexpectedly) already active=
 check_locality() and consequently
> __tpm_tis_request_locality() return "true". This prevents the locality_co=
unter from being increased
> to 1, see=20
>
> 	ret =3D __tpm_tis_request_locality(chip, l);
> 	if (!ret) /* Counter not increased since ret =3D=3D 1 */
> 		priv->locality_count++;
>
> in tpm_tis_request_locality().
>
> If now the locality is released the counter is decreased to below zero (r=
esulting
> in an underflow since "locality_counter" is an unsigned int.=20

Thanks, Daniel, can you transcript this to the commit message?

BR, Jarkko