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

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Date: Tue, 20 Feb 2024 19:42:24 +0100
From: Alexander Steffen <Alexander.Steffen@...ineon.com>
To: Lino Sanfilippo <l.sanfilippo@...bus.com>, 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 02.02.2024 04:08, 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".

check_locality() returns true, but __tpm_tis_request_locality() returns 
the requested locality. Currently, this is always 0, so the check for 
!ret will always correctly indicate success and increment the 
locality_count.

But since theoretically a locality != 0 could be requested, the correct 
fix would be to check for something like ret >= 0 or ret == l instead of 
!ret. Then the counter will also be incremented correctly for localities 
!= 0, and no underflow will happen later on. Therefore, explicitly 
checking for an underflow is unnecessary and hides the real problem.

> 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.