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

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Message-Id: <CYU3CFW08DAA.29DJY7SJYPJJZ@suppilovahvero>
Date: Fri, 02 Feb 2024 00:21:10 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Daniel P. Smith" <dpsmith@...rtussolutions.com>, "Jason Gunthorpe"
 <jgg@...pe.ca>, "Sasha Levin" <sashal@...nel.org>, "Lino Sanfilippo"
 <l.sanfilippo@...bus.com>, <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 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.

>
> Signed-off-by: Daniel P. Smith <dpsmith@...rtussolutions.com>
> Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
> Reported-by: Kanth Ghatraju <kanth.ghatraju@...cle.com>
> Fixes: 933bfc5ad213 ("tpm, tpm: Implement usage counter for locality")
> ---
>  drivers/char/tpm/tpm_tis_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
> index 1b350412d8a6..4176d3bd1f04 100644
> --- a/drivers/char/tpm/tpm_tis_core.c
> +++ b/drivers/char/tpm/tpm_tis_core.c
> @@ -180,7 +180,8 @@ static int tpm_tis_relinquish_locality(struct tpm_chip *chip, int l)
>  	struct tpm_tis_data *priv = dev_get_drvdata(&chip->dev);
>  
>  	mutex_lock(&priv->locality_count_mutex);
> -	priv->locality_count--;
> +	if (priv->locality_count > 0)
> +		priv->locality_count--;
>  	if (priv->locality_count == 0)
>  		__tpm_tis_relinquish_locality(priv, l);
>  	mutex_unlock(&priv->locality_count_mutex);

BR, Jarkko