Re: [PATCH v3] KEYS: trusted: allow module init if TPM is inactive or deactivated

From: Jarkko Sakkinen
Date: Mon Aug 05 2019 - 18:12:56 EST

On Mon, Aug 05, 2019 at 06:44:27PM +0200, Roberto Sassu wrote:
> Commit c78719203fc6 ("KEYS: trusted: allow trusted.ko to initialize w/o a
> TPM") allows the trusted module to be loaded even if a TPM is not found, to
> avoid module dependency problems.
> However, trusted module initialization can still fail if the TPM is
> inactive or deactivated. tpm_get_random() returns an error.
> This patch removes the call to tpm_get_random() and instead extends the PCR
> specified by the user with zeros. The security of this alternative is
> equivalent to the previous one, as either option prevents with a PCR update
> unsealing and misuse of sealed data by a user space process.
> Even if a PCR is extended with zeros, instead of random data, it is still
> computationally infeasible to find a value as input for a new PCR extend
> operation, to obtain again the PCR value that would allow unsealing.
> Fixes: 240730437deb ("KEYS: trusted: explicitly use tpm_chip structure...")
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Reviewed-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
> Suggested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>