linux-kernel - Re: [PATCH] tpm, tpm_tis: Fix timeout handling when waiting for TPM status

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Message-ID: <Z8okyZetPHrzkwHV@kernel.org>
Date: Fri, 7 Mar 2025 00:42:17 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Jonathan McDowell <noodles@...th.li>
Cc: Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Lino Sanfilippo <l.sanfilippo@...bus.com>,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm, tpm_tis: Fix timeout handling when waiting for TPM
 status

On Wed, Mar 05, 2025 at 09:45:15AM +0000, Jonathan McDowell wrote:
> From: Jonathan McDowell <noodles@...a.com>
> 
> The change to only use interrupts to handle supported status changes,
> then switch to polling for the rest, inverted the status test and sleep
> such that we can end up sleeping beyond our timeout and not actually
> checking the status. This can result in spurious TPM timeouts,

I *really* have hard time understanding what I'm reading the first
sentence *but* I do understand the code change. Maybe you could try
to be a bit more punctual there...

> especially on a more loaded system. Fix by switching the order back so
> we sleep *then* check. We've done a up front check when we enter the
> function so this won't cause an additional delay when the status is
> already what we're looking for.

Remove the use of we-pronoun: it is best for science papers. Also, e.g.
why not just say "switch" instead of "fix by switching" ? :-)

I'd rewrite this tail part like:

"Switch the order back where it was i.e., sleep before check."

Instead of "upfront check" it'd nice to be a bit more specific...

> 
> Cc: stable@...r.kernel.org # v6.4+
> Fixes: e87fcf0dc2b4 ("tpm, tpm_tis: Only handle supported interrupts")
> Signed-off-by: Jonathan McDowell <noodles@...a.com>
> Reviewed-by: Michal Suchánek <msuchanek@...e.de>
> ---
>  drivers/char/tpm/tpm_tis_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
> index fdef214b9f6b..167d71747666 100644
> --- a/drivers/char/tpm/tpm_tis_core.c
> +++ b/drivers/char/tpm/tpm_tis_core.c
> @@ -114,11 +114,11 @@ static int wait_for_tpm_stat(struct tpm_chip *chip, u8 mask,
>  		return 0;
>  	/* process status changes without irq support */
>  	do {
> +		usleep_range(priv->timeout_min,
> +			     priv->timeout_max);
>  		status = chip->ops->status(chip);
>  		if ((status & mask) == mask)
>  			return 0;
> -		usleep_range(priv->timeout_min,
> -			     priv->timeout_max);
>  	} while (time_before(jiffies, stop));
>  	return -ETIME;
>  }
> -- 
> 2.48.1
> 
> 

The fix itself looks legit although you could just as well put it into a
single line.

BR, Jarkko