Re: [PATCH 2/2] tpm: Prevent hwrng from activating during resume

[Jerry Snitselaar]

On Wed, Apr 26, 2023 at 08:29:28PM +0300, Jarkko Sakkinen wrote:
> 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@xxxxxxxxxxxxxxx
> Fixes: 6e592a065d51 ("tpm: Move Linux RNG connection to hwrng")
> Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>


Reviewed-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx

> ---
>  drivers/char/tpm/tpm-chip.c      |  4 ++++
>  drivers/char/tpm/tpm-interface.c | 10 ++++++++++
>  include/linux/tpm.h              | 13 +++++++------
>  3 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index 6fdfa65a00c3..6f5ee27aeda1 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -572,6 +572,10 @@ 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 7e513b771832..0f941cb32eb1 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 4dc97b9f65fb..d7073dc45444 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -274,13 +274,14 @@ enum tpm2_cc_attrs {
>  #define TPM_VID_ATML     0x1114
>  
>  enum tpm_chip_flags {
> -	TPM_CHIP_FLAG_TPM2		= BIT(1),
> -	TPM_CHIP_FLAG_IRQ		= BIT(2),
> -	TPM_CHIP_FLAG_VIRTUAL		= BIT(3),
> -	TPM_CHIP_FLAG_HAVE_TIMEOUTS	= BIT(4),
> -	TPM_CHIP_FLAG_ALWAYS_POWERED	= BIT(5),
> +	TPM_CHIP_FLAG_SUSPENDED			= BIT(0),
> +	TPM_CHIP_FLAG_TPM2			= BIT(1),
> +	TPM_CHIP_FLAG_IRQ			= BIT(2),
> +	TPM_CHIP_FLAG_VIRTUAL			= BIT(3),
> +	TPM_CHIP_FLAG_HAVE_TIMEOUTS		= BIT(4),
> +	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_FIRMWARE_UPGRADE		= BIT(7),
>  };
>  
>  #define to_tpm_chip(d) container_of(d, struct tpm_chip, dev)
> -- 
> 2.39.2
>