linux-kernel - [PATCH v2 2/2] tpm: Prevent hwrng from activating during resume

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Message-Id: <20230502011558.10743-3-jarkko@kernel.org>
Date:   Tue,  2 May 2023 04:15:58 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jarkko Sakkinen <jarkko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerry Snitselaar <jsnitsel@...hat.com>, stable@...r.kernel.org
Subject: [PATCH v2 2/2] tpm: Prevent hwrng from activating during resume

Set TPM_CHIP_FLAG_SUSPENDED in tpm_pm_suspend() and reset in
tpm_pm_resume(). While the flag is set, tpm_hwrng() gives back zero
bytes. This prevents hwrng from racing during resume.

Cc: stable@...r.kernel.org
Fixes: 6e592a065d51 ("tpm: Move Linux RNG connection to hwrng")
Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
---
 drivers/char/tpm/tpm-chip.c      |  4 ++++
 drivers/char/tpm/tpm-interface.c | 10 ++++++++++
 include/linux/tpm.h              |  1 +
 3 files changed, 15 insertions(+)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index c10a4aa97373..cd48033b804a 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -571,6 +571,10 @@ static int tpm_hwrng_read(struct hwrng *rng, void *data, size_t max, bool wait)
 {
 	struct tpm_chip *chip = container_of(rng, struct tpm_chip, hwrng);
 
+	/* Give back zero bytes, as TPM chip has not yet fully resumed: */
+	if (chip->flags & TPM_CHIP_FLAG_SUSPENDED)
+		return 0;
+
 	return tpm_get_random(chip, data, max);
 }
 
diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index 4463d0018290..586ca10b0d72 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -412,6 +412,8 @@ int tpm_pm_suspend(struct device *dev)
 	}
 
 suspended:
+	chip->flags |= TPM_CHIP_FLAG_SUSPENDED;
+
 	if (rc)
 		dev_err(dev, "Ignoring error %d while suspending\n", rc);
 	return 0;
@@ -429,6 +431,14 @@ int tpm_pm_resume(struct device *dev)
 	if (chip == NULL)
 		return -ENODEV;
 
+	chip->flags &= ~TPM_CHIP_FLAG_SUSPENDED;
+
+	/*
+	 * Guarantee that SUSPENDED is written last, so that hwrng does not
+	 * activate before the chip has been fully resumed.
+	 */
+	wmb();
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(tpm_pm_resume);
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 77693389c3f9..6a1e8f157255 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -282,6 +282,7 @@ enum tpm_chip_flags {
 	TPM_CHIP_FLAG_ALWAYS_POWERED		= BIT(5),
 	TPM_CHIP_FLAG_FIRMWARE_POWER_MANAGED	= BIT(6),
 	TPM_CHIP_FLAG_FIRMWARE_UPGRADE		= BIT(7),
+	TPM_CHIP_FLAG_SUSPENDED			= BIT(8),
 };
 
 #define to_tpm_chip(d) container_of(d, struct tpm_chip, dev)
-- 
2.39.2