tpm2_load_null() has weak and broken error handling:

- The return value of tpm2_create_primary() is ignored.
- Leaks TPM return codes from tpm2_load_context() to the caller.
- If the key name comparison succeeds returns previous error
  instead of zero to the caller.

Implement a proper error rollback.

Cc: stable@vger.kernel.org # v6.10+
Fixes: eb24c9788cd9 ("tpm: disable the TPM if NULL name changes")
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
--
v6:
- Address Stefan's remark:
  https://lore.kernel.org/linux-integrity/def4ec2d-584b-405f-9d5e-99267013c3c0@linux.ibm.com/
v5:
- Fix the TPM error code leak from tpm2_load_context().
v4:
- No changes.
v3:
- Update log messages. Previously the log message incorrectly stated
  on load failure that integrity check had been failed, even tho the
  check is done *after* the load operation.
v2:
- Refined the commit message.
- Reverted tpm2_create_primary() changes. They are not required if
  tmp_null_key is used as the parameter.
---
 drivers/char/tpm/tpm2-sessions.c | 43 +++++++++++++++++---------------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
index 1e12e0b2492e..bdac11964b55 100644
--- a/drivers/char/tpm/tpm2-sessions.c
+++ b/drivers/char/tpm/tpm2-sessions.c
@@ -915,33 +915,36 @@ static int tpm2_parse_start_auth_session(struct tpm2_auth *auth,
 
 static int tpm2_load_null(struct tpm_chip *chip, u32 *null_key)
 {
-	int rc;
 	unsigned int offset = 0; /* dummy offset for null seed context */
 	u8 name[SHA256_DIGEST_SIZE + 2];
+	u32 tmp_null_key;
+	int rc;
 
 	rc = tpm2_load_context(chip, chip->null_key_context, &offset,
-			       null_key);
-	if (rc != -EINVAL)
-		return rc;
+			       &tmp_null_key);
+	if (rc != -EINVAL) {
+		if (!rc)
+			*null_key = tmp_null_key;
+		goto err;
+	}
 
-	/* an integrity failure may mean the TPM has been reset */
-	dev_err(&chip->dev, "NULL key integrity failure!\n");
-	/* check the null name against what we know */
-	tpm2_create_primary(chip, TPM2_RH_NULL, NULL, name);
-	if (memcmp(name, chip->null_key_name, sizeof(name)) == 0)
-		/* name unchanged, assume transient integrity failure */
-		return rc;
-	/*
-	 * Fatal TPM failure: the NULL seed has actually changed, so
-	 * the TPM must have been illegally reset.  All in-kernel TPM
-	 * operations will fail because the NULL primary can't be
-	 * loaded to salt the sessions, but disable the TPM anyway so
-	 * userspace programmes can't be compromised by it.
-	 */
-	dev_err(&chip->dev, "NULL name has changed, disabling TPM due to interference\n");
+	rc = tpm2_create_primary(chip, TPM2_RH_NULL, &tmp_null_key, name);
+	if (rc)
+		goto err;
+
+	/* Return the null key if the name has not been changed: */
+	if (memcmp(name, chip->null_key_name, sizeof(name)) == 0) {
+		*null_key = tmp_null_key;
+		return 0;
+	}
+
+	/* Deduce from the name change TPM interference: */
+	dev_err(&chip->dev, "the null key integrity check failedh\n");
+	tpm2_flush_context(chip, tmp_null_key);
 	chip->flags |= TPM_CHIP_FLAG_DISABLE;
 
-	return rc;
+err:
+	return rc ? -ENODEV : 0;
 }
 
 /**
-- 
2.47.0

On 10/21/24 1:39 AM, Jarkko Sakkinen wrote:
> tpm2_load_null() has weak and broken error handling:
> 
> - The return value of tpm2_create_primary() is ignored.
> - Leaks TPM return codes from tpm2_load_context() to the caller.
> - If the key name comparison succeeds returns previous error
>    instead of zero to the caller.
> 
> Implement a proper error rollback.
> 
> Cc: stable@vger.kernel.org # v6.10+
> Fixes: eb24c9788cd9 ("tpm: disable the TPM if NULL name changes")
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> --
> v6:
> - Address Stefan's remark:
>    https://lore.kernel.org/linux-integrity/def4ec2d-584b-405f-9d5e-99267013c3c0@linux.ibm.com/
> v5:
> - Fix the TPM error code leak from tpm2_load_context().
> v4:
> - No changes.
> v3:
> - Update log messages. Previously the log message incorrectly stated
>    on load failure that integrity check had been failed, even tho the
>    check is done *after* the load operation.
> v2:
> - Refined the commit message.
> - Reverted tpm2_create_primary() changes. They are not required if
>    tmp_null_key is used as the parameter.
> ---
>   drivers/char/tpm/tpm2-sessions.c | 43 +++++++++++++++++---------------
>   1 file changed, 23 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
> index 1e12e0b2492e..bdac11964b55 100644
> --- a/drivers/char/tpm/tpm2-sessions.c
> +++ b/drivers/char/tpm/tpm2-sessions.c
> @@ -915,33 +915,36 @@ static int tpm2_parse_start_auth_session(struct tpm2_auth *auth,
>   
>   static int tpm2_load_null(struct tpm_chip *chip, u32 *null_key)
>   {
> -	int rc;
>   	unsigned int offset = 0; /* dummy offset for null seed context */
>   	u8 name[SHA256_DIGEST_SIZE + 2];
> +	u32 tmp_null_key;
> +	int rc;
>   
>   	rc = tpm2_load_context(chip, chip->null_key_context, &offset,
> -			       null_key);
> -	if (rc != -EINVAL)
> -		return rc;
> +			       &tmp_null_key);
> +	if (rc != -EINVAL) {
> +		if (!rc)
> +			*null_key = tmp_null_key;
> +		goto err;
> +	}
>   
> -	/* an integrity failure may mean the TPM has been reset */
> -	dev_err(&chip->dev, "NULL key integrity failure!\n");
> -	/* check the null name against what we know */
> -	tpm2_create_primary(chip, TPM2_RH_NULL, NULL, name);
> -	if (memcmp(name, chip->null_key_name, sizeof(name)) == 0)
> -		/* name unchanged, assume transient integrity failure */
> -		return rc;
> -	/*
> -	 * Fatal TPM failure: the NULL seed has actually changed, so
> -	 * the TPM must have been illegally reset.  All in-kernel TPM
> -	 * operations will fail because the NULL primary can't be
> -	 * loaded to salt the sessions, but disable the TPM anyway so
> -	 * userspace programmes can't be compromised by it.
> -	 */
> -	dev_err(&chip->dev, "NULL name has changed, disabling TPM due to interference\n");
> +	rc = tpm2_create_primary(chip, TPM2_RH_NULL, &tmp_null_key, name);
> +	if (rc)
> +		goto err;
> +
> +	/* Return the null key if the name has not been changed: */
> +	if (memcmp(name, chip->null_key_name, sizeof(name)) == 0) {
> +		*null_key = tmp_null_key;
> +		return 0;
> +	}
> +
> +	/* Deduce from the name change TPM interference: */
> +	dev_err(&chip->dev, "the null key integrity check failedh\n");

stray 'h': s/failedh/failed

> +	tpm2_flush_context(chip, tmp_null_key);
>   	chip->flags |= TPM_CHIP_FLAG_DISABLE;
>   
> -	return rc;
> +err:
> +	return rc ? -ENODEV : 0;
>   }
>   
>   /**

Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>