linux-kernel - Re: [PATCH v2, RESEND 0/3] tpm: retrieve digest size of unknown algorithms from TPM

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Date:   Wed, 26 Sep 2018 10:40:40 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        jarkko.sakkinen@...ux.intel.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2, RESEND 0/3] tpm: retrieve digest size of unknown
 algorithms from TPM

On Mon, 2018-09-17 at 11:38 +0200, Roberto Sassu wrote:
> Resending to maintainer with correct mailing lists in CC.
> 
> The TPM driver currently relies on the crypto subsystem to determine the
> digest size of supported TPM algorithms. In the future, TPM vendors might
> implement new algorithms in their chips, and those algorithms might not
> be supported by the crypto subsystem.
> 
> Usually, vendors provide patches for the new hardware, and likely
> the crypto subsystem will be updated before the new algorithm is
> introduced. However, old kernels might be updated later, after patches
> are included in the mainline kernel. This would leave the opportunity
> for attackers to misuse PCRs, as PCR banks with an unknown algorithm
> are not extended.
> 
> This patch set provides a long term solution for this issue. If a TPM
> algorithm is not known by the crypto subsystem, the TPM driver retrieves
> the digest size from the TPM with a PCR read. All the PCR banks are
> extended, even if the algorithm is not yet supported by the crypto
> subsystem.

Other than checking the digest size before copying the pcrread buffer,
the patches look good.  Please add my Ack on all 3 patches.

(New address) Acked-by: Mimi Zohar <zohar@...ux.ibm.com>

Thanks!

Mimi