linux-kernel - Re: [PATCH] tpm: fix a race between poll and write in tpm-dev-common

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Date:   Thu, 21 Mar 2019 15:34:23 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Tadeusz Struk <tadeusz.struk@...el.com>, grawity@...il.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm: fix a race between poll and write in tpm-dev-common

On Mon, Mar 18, 2019 at 04:19:27PM -0700, James Bottomley wrote:
> On Mon, 2019-03-18 at 15:18 -0700, Tadeusz Struk wrote:
> > Since the poll returns EPOLLIN base on the state of two
> > variables, the response_read being false and the
> > response_length > 0 the poll needs to take the buffer_mutex
> > after it is woken up.
> > 
> > Fixes: 9488585b21bef0df12 ("tpm: add support for partial reads")
> > Reported-by: Mantas Mikulėnas <grawity@...il.com>
> > Signed-off-by: Tadeusz Struk <tadeusz.struk@...el.com>
> > ---
> >  drivers/char/tpm/tpm-dev-common.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/char/tpm/tpm-dev-common.c
> > b/drivers/char/tpm/tpm-dev-common.c
> > index 5eecad233ea1..61e458d6f652 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)
> >  		mask = EPOLLIN | EPOLLRDNORM;
> >  	else
> >  		mask = EPOLLOUT | EPOLLWRNORM;
> >  
> > +	mutex_unlock(&priv->buffer_mutex);
> 
> This doesn't do anything to address the theory that the queued work
> hasn't run before the poll wakes up, does it?  If you have an
> alternative theory, could you explain it?

I see now what you mean.

Tadeusz: before you send a new patch put this comment to that place
as a reminder:

/* Checking only response_length is correct because write() always zeros
 * it and poll() should succeed after the first partial read.
 */

/Jarkko