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

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Message-ID: <439b7663-233d-46c3-9925-9b830833aa4f@apertussolutions.com>
Date: Mon, 19 Feb 2024 12:54:16 -0500
From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 Lino Sanfilippo <l.sanfilippo@...bus.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 2/12/24 15:05, Jarkko Sakkinen wrote:
> 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?

ack

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dps