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Date: Tue, 20 Feb 2024 21:04:12 +0200
Message-Id: <CZA51ZEVD7KW.3R3QBO7CGX1SC@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>
To: "Alexander Steffen" <Alexander.Steffen@infineon.com>, "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>
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>
In-Reply-To: <ae3fecc4-7b76-4607-8749-045e17941923@infineon.com>

On Tue Feb 20, 2024 at 8:42 PM EET, Alexander Steffen wrote:
> On 02.02.2024 04:08, 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 contr=
ol 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 int=
eger
> >>> 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 message.
> >>
> >=20
> > AFAIU this is:
> >=20
> > 1. We start with a locality_counter of 0 and then we call tpm_tis_reque=
st_locality()
> > for the first time, but since a locality is (unexpectedly) already acti=
ve
> > check_locality() and consequently __tpm_tis_request_locality() return "=
true".
>
> check_locality() returns true, but __tpm_tis_request_locality() returns=
=20
> the requested locality. Currently, this is always 0, so the check for=20
> !ret will always correctly indicate success and increment the=20
> locality_count.
>
> But since theoretically a locality !=3D 0 could be requested, the correct=
=20
> fix would be to check for something like ret >=3D 0 or ret =3D=3D l inste=
ad of=20
> !ret. Then the counter will also be incremented correctly for localities=
=20
> !=3D 0, and no underflow will happen later on. Therefore, explicitly=20
> checking for an underflow is unnecessary and hides the real problem.

Good point.

I think that the check should contain info-level klog message of the
event together with the check against the underflow. I think this is
very useful info for live systems.

BR, Jarkko