linux-kernel - Re: [PATCH v2] tpm: Fix the timeout & use ktime

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Message-ID: <aFwomWU28ijZj3CZ@kernel.org>
Date: Wed, 25 Jun 2025 19:49:29 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Jonathan McDowell <noodles@...th.li>
Cc: "Orlov, Ivan" <iorlov@...zon.co.uk>,
	"peterhuewe@....de" <peterhuewe@....de>,
	"jgg@...pe.ca" <jgg@...pe.ca>,
	"linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [PATCH v2] tpm: Fix the timeout & use ktime

On Wed, Jun 25, 2025 at 07:43:11PM +0300, Jarkko Sakkinen wrote:
> On Sun, Jun 22, 2025 at 09:52:58PM +0100, Jonathan McDowell wrote:
> > On Fri, Jun 20, 2025 at 06:08:31PM +0000, Orlov, Ivan 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 <iorlov@...zon.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
> > > 
> > > 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..6960ee2798e1 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(struct tpm_chip *chip)
> > > +{
> > > +	u8 status_masked = tpm_chip_status(chip) & 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(chip))
> > > 			goto out_recv;
> > 
> > The only thing I'd point out here is we end up doing a double status read
> > one after the other (once here, once in tpm_transmit_completed), and I'm
> > pretty sure I've seen instances where that caused a problem.
> 
> It would be easy to to prevent at least double reads after completion
> e.g., in tpm_chip_status():
> 
> /*
>  * Read the chip status bitmask. After completion, the returned will mask will
>  * return value cached at the point of completion up until the next transmit.
>  */
> static u8 tpm_chip_status(struct tpm_chip *chip)
> {
> 	u8 status_masked = chip->status & chip->ops_req_complete_mask;
> 
> 	if (status_masked == chip->ops->req_complete_val)
> 		return chip->status;
> 
> 	chip->status = tpm_chip_status(chip);

OOPS:

	chip->status = chip->ops->status(chip);

Couple of additional constraints:

1. tpm_chip_alloc() should initialize chip->status to chip->ops->req_complete_mask.
2. tpm_try_transmit() should reset the value to chip->ops->req_complete_mask on
   failure paths.

With these constraints there is framework where chip->ops->status() is
only invoked when it actually makes sense i.e., only during the course
of tpm_try_trasmit().

BR, Jarkko