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Subject: [PATCH v4] tpm: fix an invalid condition in tpm_common_poll
From:   Tadeusz Struk <tadeusz.struk@intel.com>
To:     jarkko.sakkinen@linux.intel.com
Cc:     grawity@gmail.com, James.Bottomley@HansenPartnership.com,
        linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, tadeusz.struk@intel.com
Date:   Wed, 27 Mar 2019 11:32:38 -0700
Message-ID: <155371155820.17863.10580533125620125669.stgit@tstruk-mobl1.jf.intel.com>
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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.

Cc: stable@vger.kernel.org
Fixes: 9488585b21bef0df12 ("tpm: add support for partial reads")
Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Tested-by: Mantas Mikulėnas <grawity@gmail.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
---
 drivers/char/tpm/tpm-dev-common.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
index 5eecad233ea1..744b0237300a 100644
--- a/drivers/char/tpm/tpm-dev-common.c
+++ b/drivers/char/tpm/tpm-dev-common.c
@@ -203,12 +203,19 @@ __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)
+	/*
+	 * The response_length indicates if there is still response
+	 * (or part of it) to be consumed. Partial reads decrease it
+	 * by the number of bytes read, and write resets it the zero.
+	 */
+	if (priv->response_length)
 		mask = EPOLLIN | EPOLLRDNORM;
 	else
 		mask = EPOLLOUT | EPOLLWRNORM;
 
+	mutex_unlock(&priv->buffer_mutex);
 	return mask;
 }