linux-kernel - Re: [RFC PATCH v2 07/13] tpm: enable bank selection for PCR extend

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Message-ID: <Z-Bx_-EvcfCzWqr7@kernel.org>
Date: Sun, 23 Mar 2025 22:41:35 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Nicolai Stange <nstange@...e.de>
Cc: Mimi Zohar <zohar@...ux.ibm.com>,
	Roberto Sassu <roberto.sassu@...wei.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	Eric Snowberg <eric.snowberg@...cle.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 07/13] tpm: enable bank selection for PCR extend

On Sun, Mar 23, 2025 at 03:09:05PM +0100, Nicolai Stange wrote:
> The existing tpm_pcr_extend() extends all of a PCR's allocated banks with
> the corresponding digest from the provided digests[] argument.

Why not "just" tpm_pcr_extend(). We don't have a concept of
"non-existing tpm_pcr_extend()".

"tpm_pcr_extend() extends the allocated PCR banks ..."

or something.

> 
> An upcoming code change to IMA will introduce the need to skip over those

Don't talk about upcoming code changes. Just explain why IMA depends on
the change.

> banks it does not have a hash algorithm implementation available for.
> 
> Introduce tpm_pcr_extend_sel() to support this.
> 
> tpm_pcr_extend_sel() also expects a digests[] array, always being the
> number of allocated PCR banks in size, just as it's the case for the
> existing tpm_pcr_extend(). In addition to that however, it takes a
> 'banks_skip_mask', and will skip the extension of any bank having its
> corresponding bit set there.
> 
> Signed-off-by: Nicolai Stange <nstange@...e.de>
> ---
>  drivers/char/tpm/tpm-interface.c | 29 +++++++++++++++++++++++++++--
>  drivers/char/tpm/tpm.h           |  3 ++-
>  drivers/char/tpm/tpm2-cmd.c      | 29 +++++++++++++++++++++++++++--
>  include/linux/tpm.h              |  3 +++
>  4 files changed, 59 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index b1daa0d7b341..88b4496de1df 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -314,6 +314,26 @@ EXPORT_SYMBOL_GPL(tpm_pcr_read);
>   */
>  int tpm_pcr_extend(struct tpm_chip *chip, u32 pcr_idx,
>  		   struct tpm_digest *digests)
> +{
> +	return tpm_pcr_extend_sel(chip, pcr_idx, digests, 0);
> +}
> +EXPORT_SYMBOL_GPL(tpm_pcr_extend);

I'd add just an extra argument to tpm_pcr_extend().

BR, Jarkko