linux-kernel - Re: [PATCH] tpm: avoid accessing cleared ops during shutdown

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Message-ID: <20200710114000.GD2614@linux.intel.com>
Date:   Fri, 10 Jul 2020 14:40:00 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Andrey Pronin <apronin@...omium.org>
Cc:     peterhuewe@....de, jgg@...pe.ca, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org, groeck@...omium.org
Subject: Re: [PATCH] tpm: avoid accessing cleared ops during shutdown

On Thu, Jul 09, 2020 at 05:22:09PM -0700, Andrey Pronin wrote:
> This patch prevents NULL dereferencing when using chip->ops while
> sending TPM2_Shutdown command if both tpm_class_shutdown handler and
> tpm_del_char_device are called during system shutdown.
> 
> Both these handlers set chip->ops to NULL but don't check if it's
> already NULL when they are called before using it.
> 
> This issue was revealed in Chrome OS after a recent set of changes
> to the unregister order for spi controllers, such as:
>   b4c6230bb0ba spi: Fix controller unregister order
>   f40913d2dca1 spi: pxa2xx: Fix controller unregister order
> and similar for other controllers.

I'm not sure I fully understand the scenario. When does thi happen?
Why does not tpm_del_char_device need this? The changes listed tell
me nothing. Why they have this effect?

I'm just trying to understand whether this could be a regression or
not.

I neither understand what you mean by "and similar for other
controllers."

NAK for the reason that I don't understand what I'm merging.

/Jarkko

> 
> Signed-off-by: Andrey Pronin <apronin@...omium.org>
> ---
>  drivers/char/tpm/tpm-chip.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index 8c77e88012e9..a410ca40a3c5 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -296,7 +296,7 @@ static int tpm_class_shutdown(struct device *dev)
>  	struct tpm_chip *chip = container_of(dev, struct tpm_chip, dev);
>  
>  	down_write(&chip->ops_sem);
> -	if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> +	if (chip->ops && (chip->flags & TPM_CHIP_FLAG_TPM2)) {
>  		if (!tpm_chip_start(chip)) {
>  			tpm2_shutdown(chip, TPM2_SU_CLEAR);
>  			tpm_chip_stop(chip);
> @@ -479,7 +479,7 @@ static void tpm_del_char_device(struct tpm_chip *chip)
>  
>  	/* Make the driver uncallable. */
>  	down_write(&chip->ops_sem);
> -	if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> +	if (chip->ops && (chip->flags & TPM_CHIP_FLAG_TPM2)) {
>  		if (!tpm_chip_start(chip)) {
>  			tpm2_shutdown(chip, TPM2_SU_CLEAR);
>  			tpm_chip_stop(chip);
> -- 
> 2.25.1
>