linux-kernel - Re: [PATCH v3] tpm: Check for completion after timeout

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Message-ID: <aIAcWFGjWEViwwh6@kernel.org>
Date: Wed, 23 Jul 2025 02:18:48 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Ivan Orlov <ivan.orlov0322@...il.com>
Cc: peterhuewe@....de, iorlov@...zon.co.uk, jgg@...pe.ca,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
	dwmw@...zon.co.uk, noodles@...th.li
Subject: Re: [PATCH v3] tpm: Check for completion after timeout

On Sat, Jul 19, 2025 at 08:13:39PM +0000, Ivan Orlov wrote:
> The current implementation of timeout detection works in the following
> way:
> 
> 1. Read completion status. If completed, return the data
> 2. Sleep for some time (usleep_range)
> 3. Check for timeout using current jiffies value. Return an error if
>    timed out
> 4. Goto 1
> 
> usleep_range doesn't guarantee it's always going to wake up strictly in
> (min, max) range, so such a situation is possible:
> 
> 1. Driver reads completion status. No completion yet
> 2. Process sleeps indefinitely. In the meantime, TPM responds
> 3. We check for timeout without checking for the completion again.
>    Result is lost.
> 
> Such a situation also happens for the guest VMs: if vCPU goes to sleep
> and doesn't get scheduled for some time, the guest TPM driver will
> timeout instantly after waking up without checking for the completion
> (which may already be in place).
> 
> Perform the completion check once again after exiting the busy loop in
> order to give the device the last chance to send us some data.
> 
> Since now we check for completion in two places, extract this check into
> a separate function.
> 
> Signed-off-by: Ivan Orlov <ivan.orlov0322@...il.com>
> ---
> V1 -> V2:
> - Exclude the jiffies -> ktime change from the patch
> - Instead of recording the time before checking for completion, check
>   for completion once again after leaving the loop
> V2 -> V3:
> - Avoid reading the chip status twice in the inner loop by passing
>   status into tpm_transmit_completed
> 
>  drivers/char/tpm/tpm-interface.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index 8d7e4da6ed53..8d18b33aa62d 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -82,6 +82,13 @@ static bool tpm_chip_req_canceled(struct tpm_chip *chip, u8 status)
>  	return chip->ops->req_canceled(chip, status);
>  }
>  
> +static bool tpm_transmit_completed(u8 status, struct tpm_chip *chip)
> +{
> +	u8 status_masked = status & chip->ops->req_complete_mask;
> +
> +	return status_masked == chip->ops->req_complete_val;
> +}
> +
>  static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
>  {
>  	struct tpm_header *header = buf;
> @@ -129,8 +136,7 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
>  	stop = jiffies + tpm_calc_ordinal_duration(chip, ordinal);
>  	do {
>  		u8 status = tpm_chip_status(chip);
> -		if ((status & chip->ops->req_complete_mask) ==
> -		    chip->ops->req_complete_val)
> +		if (tpm_transmit_completed(status, chip))
>  			goto out_recv;
>  
>  		if (tpm_chip_req_canceled(chip, status)) {
> @@ -142,6 +148,13 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
>  		rmb();
>  	} while (time_before(jiffies, stop));
>  
> +	/*
> +	 * Check for completion one more time, just in case the device reported
> +	 * it while the driver was sleeping in the busy loop above.
> +	 */
> +	if (tpm_transmit_completed(tpm_chip_status(chip), chip))
> +		goto out_recv;
> +
>  	tpm_chip_cancel(chip);
>  	dev_err(&chip->dev, "Operation Timed out\n");
>  	return -ETIME;
> -- 
> 2.43.0
> 

I guess this is completed too by now ...

Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>

BR, Jarkko