linux-kernel - Re: [PATCH v2 3/3] tpm: in tpm2_del_space check if ops pointer is still valid

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Date:   Wed, 3 Feb 2021 03:17:07 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     peterhuewe@....de, jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH v2 3/3] tpm: in tpm2_del_space check if ops pointer is
 still valid

On Tue, Feb 02, 2021 at 11:09:03PM +0100, Lino Sanfilippo wrote:
> From: Lino Sanfilippo <l.sanfilippo@...bus.com>
> 
> In tpm2_del_space() the sessions are flushed by means of the tpm_chip
> operations. However the concerning operations pointer my already be NULL at
                                                        ~~
                                                        may

What is "concerniog operations"? Unfamiliar term. Maybe just consistently
se chip->ops? Now you have also "tpm_chip operations" and chip->ops, in
addition to "concerning operations" in one paragraph commit message.

> this time in case that the chip has been unregistered (see
> tpm_chip_unregister() which calls tpm_del_char_device() which sets
> chip->ops to NULL).
> Before accessing chip->ops check if it is still valid. Skip flushing
> the sessions in this case.
> 
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>

Instead of cross referencing please describe the scenario (i.e.
the sequence of operations) of failure.

Fixes tag is also missing.

> ---
>  drivers/char/tpm/tpm2-space.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> index 784b8b3..9a29a40 100644
> --- a/drivers/char/tpm/tpm2-space.c
> +++ b/drivers/char/tpm/tpm2-space.c
> @@ -58,12 +58,17 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
>  
>  void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
>  {
> -	mutex_lock(&chip->tpm_mutex);
> -	if (!tpm_chip_start(chip)) {
> -		tpm2_flush_sessions(chip, space);
> -		tpm_chip_stop(chip);
> +	down_read(&chip->ops_sem);
> +	if (chip->ops) {
> +		mutex_lock(&chip->tpm_mutex);
> +		if (!tpm_chip_start(chip)) {
> +			tpm2_flush_sessions(chip, space);
> +			tpm_chip_stop(chip);
> +		}
> +		mutex_unlock(&chip->tpm_mutex);
>  	}
> -	mutex_unlock(&chip->tpm_mutex);
> +	up_read(&chip->ops_sem);
> +
>  	kfree(space->context_buf);
>  	kfree(space->session_buf);
>  }
> -- 
> 2.7.4
> 

/Jarkko