[PATCH v5 2/5] tpm: Implement tpm2_load_null() rollback

[Stefan Berger]

On 9/21/24 8:08 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 at vger.kernel.org # v6.10+
> Fixes: eb24c9788cd9 ("tpm: disable the TPM if NULL name changes")
> Signed-off-by: Jarkko Sakkinen <jarkko at kernel.org>
> ---
> 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 0f09ac33ae99..a856adef18d3 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");

s/failedh/failed

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

return rc ? -ENODEV : 0;

>   }
>   
>   /**