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

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Date: Mon, 12 Feb 2024 22:05:12 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Lino Sanfilippo" <l.sanfilippo@...bus.com>, "Daniel P. Smith"
 <dpsmith@...rtussolutions.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 Fri Feb 2, 2024 at 5:08 AM EET, 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 control 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 integer
> >> 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.
> > 
>
> 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_counter from being increased
> to 1, see 
>
> 	ret = __tpm_tis_request_locality(chip, l);
> 	if (!ret) /* Counter not increased since ret == 1 */
> 		priv->locality_count++;
>
> in tpm_tis_request_locality().
>
> If now the locality is released the counter is decreased to below zero (resulting
> in an underflow since "locality_counter" is an unsigned int. 

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

BR, Jarkko