Re: [PATCH v3 2/2] tpm: in tpm2_del_space check if ops pointer is still valid

[James Bottomley]

On Fri, 2021-02-05 at 00:50 +0100, Lino Sanfilippo wrote:
> From: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
> 
> In tpm2_del_space() chip->ops is used for flushing the sessions.
> However
> this function may be called after tpm_chip_unregister() which sets
> the chip->ops pointer to NULL.
> Avoid a possible NULL pointer dereference by checking if chip->ops is
> still
> valid before accessing it.
> 
> Fixes: a3fbfae82b4c ("tpm: take TPM chip power gating out of
> tpm_transmit()")
> Signed-off-by: Lino Sanfilippo <l.sanfilippo@xxxxxxxxxx>
> ---
>  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);
>  }


Actually, this still isn't right.  As I said to the last person who
reported this, we should be doing a get/put on the ops, not rolling our
own here:

https://lore.kernel.org/linux-integrity/e7566e1e48f5be9dca034b4bfb67683b5d3cb88f.camel@xxxxxxxxxxxxxxxxxxxxx/

The reporter went silent before we could get this tested, but could you
try, please, because your patch is still hand rolling the ops get/put,
just slightly better than it had been done previously.

James