linux-kernel - Re: [PATCH v11 14/20] tpm: Protect against locality counter underflow

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Message-Id: <D5AQLQJ7LN9K.3F3RQZVUMH0GG@kernel.org>
Date: Fri, 01 Nov 2024 11:33:13 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Ross Philipson" <ross.philipson@...cle.com>,
 <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
 <linux-integrity@...r.kernel.org>, <linux-doc@...r.kernel.org>,
 <linux-crypto@...r.kernel.org>, <kexec@...ts.infradead.org>,
 <linux-efi@...r.kernel.org>, <iommu@...ts.linux-foundation.org>
Cc: <dpsmith@...rtussolutions.com>, <tglx@...utronix.de>,
 <mingo@...hat.com>, <bp@...en8.de>, <hpa@...or.com>,
 <dave.hansen@...ux.intel.com>, <ardb@...nel.org>, <mjg59@...f.ucam.org>,
 <James.Bottomley@...senpartnership.com>, <peterhuewe@....de>,
 <jgg@...pe.ca>, <luto@...capital.net>, <nivedita@...m.mit.edu>,
 <herbert@...dor.apana.org.au>, <davem@...emloft.net>, <corbet@....net>,
 <ebiederm@...ssion.com>, <dwmw2@...radead.org>, <baolu.lu@...ux.intel.com>,
 <kanth.ghatraju@...cle.com>, <andrew.cooper3@...rix.com>,
 <trenchboot-devel@...glegroups.com>
Subject: Re: [PATCH v11 14/20] tpm: Protect against locality counter
 underflow

On Fri Sep 13, 2024 at 11:05 PM EEST, Ross Philipson wrote:
> From: "Daniel P. Smith" <dpsmith@...rtussolutions.com>
>
> 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@...rtussolutions.com>
> Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
> Reported-by: Kanth Ghatraju <kanth.ghatraju@...cle.com>

Remove reported-by.

> ---
>  drivers/char/tpm/tpm_tis_core.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
> index fdef214b9f6b..a6967f312837 100644
> --- a/drivers/char/tpm/tpm_tis_core.c
> +++ b/drivers/char/tpm/tpm_tis_core.c
> @@ -180,7 +180,10 @@ 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--;
> +	else
> +		pr_info("Invalid: locality count dropped below zero\n");

Explain the context.

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

BR, Jarkko