linux-kernel - [PATCH v3] tpm: fix potential NULL pointer access in tpm_del_char

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Message-Id: <20211223040246.6575-1-LinoSanfilippo@gmx.de>
Date:   Thu, 23 Dec 2021 05:02:46 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     peterhuewe@....de, jarkko@...nel.org, jgg@...pe.ca
Cc:     p.rosenberger@...bus.com, stefanb@...ux.ibm.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lino Sanfilippo <LinoSanfilippo@....de>, stable@...r.kernel.org
Subject: [PATCH v3] tpm: fix potential NULL pointer access in tpm_del_char_device

Some SPI controller drivers unregister the controller in the shutdown
handler (e.g. BCM2835). If such a controller is used with a TPM 2 slave
chip->ops may be accessed when it is already NULL:

At system shutdown the pre-shutdown handler tpm_class_shutdown() shuts down
TPM 2 and sets chip->ops to NULL. Then at SPI controller unregistration
tpm_tis_spi_remove() is called and eventually calls tpm_del_char_device()
which tries to shut down TPM 2 again. Thereby it accesses chip->ops again:
(tpm_del_char_device calls tpm_chip_start which calls tpm_clk_enable which
calls chip->ops->clk_enable).

Avoid the NULL pointer access by testing if chip->ops is valid and skipping
the TPM 2 shutdown procedure in case it is NULL.

Fixes: dcbeab1946454 ("tpm: fix crash in tpm_tis deinitialization")
Fixes: 39d0099f9439 ("powerpc/pseries: Add shutdown() to vio_driver and vio_bus")
Cc: stable@...r.kernel.org
Tested-by: Stefan Berger <stefanb@...ux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@...ux.ibm.com>
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@....de>
---
Changes in v3:
- added tags for Stefans review and test
- corrected the source code comment

Changes in v2:
- rephrased the commit message to clarify the circumstances under which
  this bug triggers (as requested by Jarkko)

I was able to reproduce this issue with a SLB 9670 TPM chip controlled by 
a BCM2835 SPI controller. 

The approach to fix this issue in the BCM2835 driver was rejected after a
discussion on the mailing list:

https://marc.info/?l=linux-integrity&m=163285906725367&w=2

The reason for the rejection was the realization, that this issue should rather
be fixed in the TPM code:

https://marc.info/?l=linux-spi&m=163311087423271&w=2

So this is the reworked version of a patch that is supposed to do that.

 drivers/char/tpm/tpm-chip.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ddaeceb7e109..03122d624670 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -474,13 +474,19 @@ 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 (!tpm_chip_start(chip)) {
-			tpm2_shutdown(chip, TPM2_SU_CLEAR);
-			tpm_chip_stop(chip);
+	/* In case that the SPI master is unregistered in its drivers
+	 * shutdown handler, tpm_class_shutdown() has already been called
+	 * and set chip->ops to NULL. So check if it is still valid.
+	 */
+	if (chip->ops) {
+		if (chip->flags & TPM_CHIP_FLAG_TPM2) {
+			if (!tpm_chip_start(chip)) {
+				tpm2_shutdown(chip, TPM2_SU_CLEAR);
+				tpm_chip_stop(chip);
+			}
 		}
+		chip->ops = NULL;
 	}
-	chip->ops = NULL;
 	up_write(&chip->ops_sem);
 }

base-commit: bc491fb12513e79702c6f936c838f792b5389129
-- 
2.34.1