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SubjectRe: [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@gmail.com>
> > Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.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

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