linux-kernel - [PATCH v2] tpm: fix potential NULL pointer access in tpm_del_char

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20211220150635.8545-1-LinoSanfilippo@gmx.de>
Date:   Mon, 20 Dec 2021 16:06:35 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     peterhuewe@....de, jarkko@...nel.org, jgg@...pe.ca
Cc:     p.rosenberger@...bus.com, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lino Sanfilippo <LinoSanfilippo@....de>, stable@...r.kernel.org
Subject: [PATCH v2] 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")
Cc: stable@...r.kernel.org
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@....de>
---

Changes to 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..7960da490e72 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);
+	/* Check if chip->ops is still valid: In case that the controller
+	 * drivers shutdown handler unregisters the controller in its
+	 * shutdown handler we are called twice and chip->ops to NULL.
+	 */
+	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: a7904a538933c525096ca2ccde1e60d0ee62c08e
-- 
2.34.1