[PATCH] vTPM: Fix missing NULL check

["Hon Ching(Vicky) Lo" <honclo@linux.vnet.ibm.com>]

The current code passes the address of tpm_chip as the argument to
dev_get_drvdata() without prior NULL check in
tpm_ibmvtpm_get_desired_dma.  This resulted an oops during kernel
boot when vTPM is enabled in Power partition configured in active
memory sharing mode.

The vio_driver's get_desired_dma() is called before the probe(), which
for vtpm is tpm_ibmvtpm_probe, and it's this latter function that
initializes the driver and set data.  Attempting to get data before
the probe() caused the problem.

This patch adds a NULL check to the tpm_ibmvtpm_get_desired_dma.

fixes: 9e0d39d8a6a0 ("tpm: Remove useless priv field in struct tpm_vendor_specific")
Cc: <stable@vger.kernel.org>
Signed-off-by: Hon Ching(Vicky) Lo <honclo@linux.vnet.ibm.com>
---
 drivers/char/tpm/tpm_ibmvtpm.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/drivers/char/tpm/tpm_ibmvtpm.c b/drivers/char/tpm/tpm_ibmvtpm.c
index 1b9d61f..a88ee25 100644
--- a/drivers/char/tpm/tpm_ibmvtpm.c
+++ b/drivers/char/tpm/tpm_ibmvtpm.c
@@ -313,7 +313,10 @@ static int tpm_ibmvtpm_remove(struct vio_dev *vdev)
 static unsigned long tpm_ibmvtpm_get_desired_dma(struct vio_dev *vdev)
 {
 	struct tpm_chip *chip = dev_get_drvdata(&vdev->dev);
-	struct ibmvtpm_dev *ibmvtpm = dev_get_drvdata(&chip->dev);
+	struct ibmvtpm_dev *ibmvtpm = NULL;
+
+	if (chip)
+		ibmvtpm = dev_get_drvdata(&chip->dev);
 
 	/*
 	 * ibmvtpm initializes at probe time, so the data we are
-- 
1.7.1