linux-kernel - Re: [PATCH v2] tpm: fix potential NULL pointer access in tpm_del_char

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Message-ID: <af847879-0f29-08e7-7609-da3b27381d3a@linux.ibm.com>
Date:   Tue, 21 Dec 2021 23:53:05 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org, jgg@...pe.ca
Cc:     p.rosenberger@...bus.com, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] tpm: fix potential NULL pointer access in
 tpm_del_char_device


On 12/20/21 10:06, Lino Sanfilippo wrote:
> Some SPI controller drivers unregister the controller in the shutdown
> handler (e.g. BCM2835). If such a controller is used with a TPM 2 slave
> chip->ops may be accessed when it is already NULL:
>
> At system shutdown the pre-shutdown handler tpm_class_shutdown() shuts down
> TPM 2 and sets chip->ops to NULL. Then at SPI controller unregistration
> tpm_tis_spi_remove() is called and eventually calls tpm_del_char_device()
> which tries to shut down TPM 2 again. Thereby it accesses chip->ops again:
> (tpm_del_char_device calls tpm_chip_start which calls tpm_clk_enable which
> calls chip->ops->clk_enable).
>
> Avoid the NULL pointer access by testing if chip->ops is valid and skipping
> the TPM 2 shutdown procedure in case it is NULL.
>
> Fixes: dcbeab1946454 ("tpm: fix crash in tpm_tis deinitialization")
> Cc: stable@...r.kernel.org
> Signed-off-by: Lino Sanfilippo <LinoSanfilippo@....de>
> ---
>
> Changes to v2:
> - rephrased the commit message to clarify the circumstances under which
>    this bug triggers (as requested by Jarkko)
>
>
> I was able to reproduce this issue with a SLB 9670 TPM chip controlled by
> a BCM2835 SPI controller.
>
> The approach to fix this issue in the BCM2835 driver was rejected after a
> discussion on the mailing list:
>
> https://marc.info/?l=linux-integrity&m=163285906725367&w=2
>
> The reason for the rejection was the realization, that this issue should rather
> be fixed in the TPM code:
>
> https://marc.info/?l=linux-spi&m=163311087423271&w=2
>
> So this is the reworked version of a patch that is supposed to do that.
>
>
>   drivers/char/tpm/tpm-chip.c | 16 +++++++++++-----
>   1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> index ddaeceb7e109..7960da490e72 100644
> --- a/drivers/char/tpm/tpm-chip.c
> +++ b/drivers/char/tpm/tpm-chip.c
> @@ -474,13 +474,19 @@ 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 (!tpm_chip_start(chip)) {
> -			tpm2_shutdown(chip, TPM2_SU_CLEAR);
> -			tpm_chip_stop(chip);
> +	/* Check if chip->ops is still valid: In case that the controller
> +	 * drivers shutdown handler unregisters the controller in its
> +	 * shutdown handler we are called twice and chip->ops to NULL.
> +	 */
> +	if (chip->ops) {
> +		if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> +			if (!tpm_chip_start(chip)) {
> +				tpm2_shutdown(chip, TPM2_SU_CLEAR);
> +				tpm_chip_stop(chip);
> +			}
>   		}
> +		chip->ops = NULL;
>   	}
> -	chip->ops = NULL;
>   	up_write(&chip->ops_sem);
>   }
>   
>
> base-commit: a7904a538933c525096ca2ccde1e60d0ee62c08e


Fixes: 39d0099f9439 ("powerpc/pseries: Add shutdown() to vio_driver and 
vio_bus")

Reviewed-by: Stefan Berger <stefanb@...ux.ibm.com>

Tested-by: Stefan Berger <stefanb@...ux.ibm.com>