Re: [PATCH v2] tpm: fix an invalid condition in tpm_common_poll

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2019-03-19 at 13:31 -0700, Tadeusz Struk wrote:
> The poll condition should only check response_length,
> because reads should only be issued if there is data to read.
> The response_read flag only prevents double writes.
> The problem was that the write set the response_read to false,
> enqued a tpm job, and returned. Then application called poll
> which checked the response_read flag and returned EPOLLIN.
> Then the application called read, but got nothing.
> After all that the async_work kicked in.
> Added also mutex_lock around the poll check to prevent
> other possible race conditions.
> 
> Fixes: 9488585b21bef0df12 ("tpm: add support for partial reads")
> Reported-by: Mantas Mikulėnas <grawity@gmail.com>
> Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
> ---
>  drivers/char/tpm/tpm-dev-common.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/tpm/tpm-dev-common.c
> b/drivers/char/tpm/tpm-dev-common.c
> index 5eecad233ea1..7312d3214381 100644
> --- a/drivers/char/tpm/tpm-dev-common.c
> +++ b/drivers/char/tpm/tpm-dev-common.c
> @@ -203,12 +203,14 @@ __poll_t tpm_common_poll(struct file *file,
> poll_table *wait)
>  	__poll_t mask = 0;
>  
>  	poll_wait(file, &priv->async_wait, wait);
> +	mutex_lock(&priv->buffer_mutex);
>  
> -	if (!priv->response_read || priv->response_length)
> +	if (priv->response_length)
>  		mask = EPOLLIN | EPOLLRDNORM;
>  	else
>  		mask = EPOLLOUT | EPOLLWRNORM;
>  
> +	mutex_unlock(&priv->buffer_mutex);

Just an observation on this: the mutex is now no-longer necessary
because a read on a size_t quantity is always atomic.

James