linux-kernel - Re: [PATCH] tpm: use try_get

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Date:   Tue, 8 Mar 2022 10:29:41 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Sergey Temerkhanov <s.temerkhanov@...il.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        linux-integrity@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: use try_get_ops() in tpm-space.c

On Mon, Mar 07, 2022 at 03:58:03PM -0500, James Bottomley wrote:
> As part of the series conversion to remove nested TPM operations:
> 
> https://lore.kernel.org/all/20190205224723.19671-1-jarkko.sakkinen@linux.intel.com/
> 
> exposure of the chip->tpm_mutex was removed from much of the upper
> level code.  In this conversion, tpm2_del_space() was missed.  This
> didn't matter much because it's usually called closely after a
> converted operation, so there's only a very tiny race window where the
> chip can be removed before the space flushing is done which causes a
> NULL deref on the mutex.  However, there are reports of this window
> being hit in practice, so fix this by converting tpm2_del_space() to
> use tpm_try_get_ops(), which performs all the teardown checks before
> acquring the mutex.
> 
> Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>
> Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
> ---
>  drivers/char/tpm/tpm2-space.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> index 265ec72b1d81..ffb35f0154c1 100644
> --- a/drivers/char/tpm/tpm2-space.c
> +++ b/drivers/char/tpm/tpm2-space.c
> @@ -58,12 +58,12 @@ 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)) {
> +
> +	if (tpm_try_get_ops(chip) == 0) {
>  		tpm2_flush_sessions(chip, space);
> -		tpm_chip_stop(chip);
> +		tpm_put_ops(chip);
>  	}
> -	mutex_unlock(&chip->tpm_mutex);
> +
>  	kfree(space->context_buf);
>  	kfree(space->session_buf);
>  }
> -- 
> 2.34.1
> 
> 

Thank you! 

BR, Jarkko