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

[Tadeusz Struk]

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 <graw...@gmail.com>
Signed-off-by: Tadeusz Struk <tadeusz.st...@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);
        return mask;
 }