[PATCH] tpm: avoid accessing cleared ops during shutdown

[Andrey Pronin]

This patch prevents NULL dereferencing when using chip->ops while
sending TPM2_Shutdown command if both tpm_class_shutdown handler and
tpm_del_char_device are called during system shutdown.

Both these handlers set chip->ops to NULL but don't check if it's
already NULL when they are called before using it.

This issue was revealed in Chrome OS after a recent set of changes
to the unregister order for spi controllers, such as:
  b4c6230bb0ba spi: Fix controller unregister order
  f40913d2dca1 spi: pxa2xx: Fix controller unregister order
and similar for other controllers.

Signed-off-by: Andrey Pronin <apro...@chromium.org>
---
 drivers/char/tpm/tpm-chip.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 8c77e88012e9..a410ca40a3c5 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -296,7 +296,7 @@ static int tpm_class_shutdown(struct device *dev)
        struct tpm_chip *chip = container_of(dev, struct tpm_chip, dev);
 
        down_write(&chip->ops_sem);
-       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
+       if (chip->ops && (chip->flags & TPM_CHIP_FLAG_TPM2)) {
                if (!tpm_chip_start(chip)) {
                        tpm2_shutdown(chip, TPM2_SU_CLEAR);
                        tpm_chip_stop(chip);
@@ -479,7 +479,7 @@ static void tpm_del_char_device(struct tpm_chip *chip)
 
        /* Make the driver uncallable. */
        down_write(&chip->ops_sem);
-       if (chip->flags & TPM_CHIP_FLAG_TPM2) {
+       if (chip->ops && (chip->flags & TPM_CHIP_FLAG_TPM2)) {
                if (!tpm_chip_start(chip)) {
                        tpm2_shutdown(chip, TPM2_SU_CLEAR);
                        tpm_chip_stop(chip);
-- 
2.25.1