linux-kernel - [PATCH v10 14/20] tpm: Protect against locality counter underflow

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Message-Id: <20240826223835.3928819-15-ross.philipson@oracle.com>
Date: Mon, 26 Aug 2024 15:38:29 -0700
From: Ross Philipson <ross.philipson@...cle.com>
To: linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
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Cc: ross.philipson@...cle.com, dpsmith@...rtussolutions.com,
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        trenchboot-devel@...glegroups.com
Subject: [PATCH v10 14/20] tpm: Protect against locality counter underflow

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>
---
 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");
 	if (priv->locality_count == 0)
 		__tpm_tis_relinquish_locality(priv, l);
 	mutex_unlock(&priv->locality_count_mutex);
-- 
2.39.3