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

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Message-ID: <aFVhDDewVHneFXnO@earth.li>
Date: Fri, 20 Jun 2025 14:24:28 +0100
From: Jonathan McDowell <noodles@...th.li>
To: "Orlov, Ivan" <iorlov@...zon.co.uk>
Cc: "peterhuewe@....de" <peterhuewe@....de>,
	"jarkko@...nel.org" <jarkko@...nel.org>,
	"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] tpm: Fix the timeout & use ktime

On Wed, Jun 11, 2025 at 04:25:24PM +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.

This looks similar to the issue I fixed in 7146dffa875c ('Fix timeout
handling when waiting for TPM status'), I assume you're actually seeing
it in your systems? I think we're starting to see it (rarely) now the
other issues are fixed in our builds. As a similar approach does the
following work?

diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index 8d7e4da6ed53..18ae0767fa60 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -127,7 +127,7 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
  		goto out_recv;
  
  	stop = jiffies + tpm_calc_ordinal_duration(chip, ordinal);
-	do {
+	while (true) {
  		u8 status = tpm_chip_status(chip);
  		if ((status & chip->ops->req_complete_mask) ==
  		    chip->ops->req_complete_val)
@@ -138,9 +138,12 @@ static ssize_t tpm_try_transmit(struct tpm_chip *chip, void *buf, size_t bufsiz)
  			return -ECANCELED;
  		}
  
+		if (time_after(jiffies, stop))
+			break;
+
  		tpm_msleep(TPM_TIMEOUT_POLL);
  		rmb();
-	} while (time_before(jiffies, stop));
+	}
  
  	tpm_chip_cancel(chip);
  	dev_err(&chip->dev, "Operation Timed out\n");