Re: [PATCH v9 13/19] tpm: Protect against locality counter underflow

[Jarkko Sakkinen]

On Fri May 31, 2024 at 4:03 AM EEST, Ross Philipson wrote:
> From: "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>
>
> 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.
>
> 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")

Not sure if we have practical use for fixes tag here but open for
argument ofc. I.e. I'm not sure what is the practical scenario to
worry about if Trenchboot did not exist.

> ---
>  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 176cd8dbf1db..7c1761bd6000 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--;

I'd signal the situation with pr_info() in else branch.

>  	if (priv->locality_count == 0)
>  		__tpm_tis_relinquish_locality(priv, l);
>  	mutex_unlock(&priv->locality_count_mutex);

BR, Jarkko