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

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Message-ID: <aGeYqQG15lb2_NaU@earth.li>
Date: Fri, 4 Jul 2025 10:02:33 +0100
From: Jonathan McDowell <noodles@...th.li>
To: Jarkko Sakkinen <jarkko@...nel.org>
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 07:43:07PM +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():

Or just take the simple approach and make the check after the while 
loop:

	if ((tpm_chip_status(chip) & chip->ops->req_complete_mask) ==
	    chip->ops->req_complete_val)
		goto out_recv;

There might be potential for a longer term cleanup using chip->status to 
cache things, but I'm little concerned that's going to open paths where 
we might not correctly update it, so I think it should be a separate 
piece.

(I'm motivated by the fact we've started to see the "Operation Canceled" 
error and I'd like us to close on the best way to fix it. :) )

J.

-- 
I am afraid of the dark.