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

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Date: Fri, 2 Feb 2024 04:08:07 +0100
From: Lino Sanfilippo <l.sanfilippo@...bus.com>
To: Jarkko Sakkinen <jarkko@...nel.org>,
 "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 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.