Re: [PATCH] tpm: Fix barriers to prevent hwrng from activating during resume

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Aug 03, 2026 at 11:13:24AM +0200, Thomas Fourier wrote:
> The memory barrier is placed after clearing the suspened flag so
> memory reordering between the wake up and clearing the flag is allowed.
> The reader of the flag is missing the corresponding barrier.
> 
> Clear the flag after the memory barrier and add a memory barrier in
> the reader.
> 
> Fixes: 99d464506255 ("tpm: Prevent hwrng from activating during resume")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>

Thanks I investigate this properly before answering. It's my first day
after holiday so might take until Tue/Wed.

PS. And if someone noticed, sorry for copy-pasting the same response to
two bug reports. I just cherry-picked critical looking stuff from my
inbox :-)

BR, Jarkko

> ---
>  drivers/char/tpm/tpm-chip.c      | 3 +++
>  drivers/char/tpm/tpm-interface.c | 3 +--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index 12b7394b34bd..7f500797b7a7 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -173,6 +173,9 @@ int tpm_try_get_ops(struct tpm_chip *chip)
>  	if (chip->flags & TPM_CHIP_FLAG_SUSPENDED)
>  		goto out_lock;
>  
> +	/* Ensure that device is fully resumed */
> +	rmb();
> +
>  	rc = tpm_chip_start(chip);
>  	if (rc)
>  		goto out_lock;
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index f745a098908b..2de12b02f62b 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -474,13 +474,12 @@ 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();
> +	chip->flags &= ~TPM_CHIP_FLAG_SUSPENDED;
>  
>  	return 0;
>  }
> -- 
> 2.43.0
>