linux-kernel - Re: [PATCH] tpm: Don't make vendor check required for probe

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Date:   Fri, 18 Aug 2023 22:07:28 +0000
From:   "Jarkko Sakkinen" <jarkko@...nel.org>
To:     "Mario Limonciello" <mario.limonciello@....com>
Cc:     <linux-integrity@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Thorsten Leemhuis" <regressions@...mhuis.info>,
        <charles.d.prestopine@...el.com>, <rafael.j.wysocki@...el.com>,
        <len.brown@...el.com>, <stable@...r.kernel.org>,
        "Todd Brandt" <todd.e.brandt@...el.com>
Subject: Re: [PATCH] tpm: Don't make vendor check required for probe

On Fri Aug 18, 2023 at 6:15 PM UTC, Mario Limonciello wrote:
> The vendor check introduced by commit 554b841d4703 ("tpm: Disable RNG for
> all AMD fTPMs") doesn't work properly on Intel fTPM.  The TPM doesn't reply
> at bootup and returns back the command code.

Is this reproducible with any production hardware? You are stating it
as it was reproducible categorically with any Intel fTPM.

> As this isn't crucial for anything but AMD fTPM and AMD fTPM works, throw
> away the error code to let Intel fTPM continue to work.
>
> Cc: stable@...r.kernel.org
> Fixes: 554b841d4703 ("tpm: Disable RNG for all AMD fTPMs")

It does make sense not to exercise this outside of AMD CPus but since
there is no production hardware failing, it cannot be categorized as a
bug fix.

> Reported-by: Todd Brandt <todd.e.brandt@...el.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217804
> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
> ---
>  drivers/char/tpm/tpm_crb.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c
> index 9eb1a18590123..b0e9931fe436c 100644
> --- a/drivers/char/tpm/tpm_crb.c
> +++ b/drivers/char/tpm/tpm_crb.c
> @@ -472,8 +472,7 @@ static int crb_check_flags(struct tpm_chip *chip)
>  	if (ret)
>  		return ret;
>  
> -	ret = tpm2_get_tpm_pt(chip, TPM2_PT_MANUFACTURER, &val, NULL);
> -	if (ret)
> +	if (tpm2_get_tpm_pt(chip, TPM2_PT_MANUFACTURER, &val, NULL))
>  		goto release;

It would be better not to exercise a potentially failing code path at
all. This initiates full transaction with the TPM.

>  
>  	if (val == 0x414D4400U /* AMD */)
> -- 
> 2.34.1

BR, Jarkko