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

[Jarkko Sakkinen]

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@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxx>
> Reported-by: Kanth Ghatraju <kanth.ghatraju@xxxxxxxxxx>
> 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