linux-kernel - [PATCH 2/4] tpm: fix reference counting for struct tpm

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Date:   Sat, 16 Jan 2021 02:22:39 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     peterhuewe@....de, jarkko@...nel.org
Cc:     jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        LinoSanfilippo@....de, p.rosenberger@...bus.com,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: [PATCH 2/4] tpm: fix reference counting for struct tpm_chip

From: Lino Sanfilippo <l.sanfilippo@...bus.com>

Commit 8979b02aaf1d ("tpm: Fix reference count to main device") tried to
fix a reference count issue which prevented the tpm_chip structure from
being freed in case that no TPM2 was used. The fix was to only get an extra
reference for chip->dev in case of TPM2 which is indicated by the
TPM_CHIP_FLAG_TPM2 flag.
Unfortunately this flag is never set, and thus the extra reference is not
taken even in the TPM2 case. This results in a refcount underflow in case
that the device file /dev/tpmrm* is used to write data after the tpm_chip
has been unregistered (e.g if the /dev/tpmrm* file has been opened before
and not yet closed at the time the chip was unregistered.)

Also the error path (label "out") in tpm_chip_alloc() results in such an
underflow, since the dev reference is put twice (one time directly and the
second time by the call of tpm_devs_release() due to the put of
chip->devs).

Fix the described issues by taking the extra reference unconditionally and
installing an additional resource management action handler which puts
chip->devs. Releasing chip->devs eventually results in the call of
tpm_devs_release() which then releases the extra reference to chip->dev.

Since we now actually take that extra reference, adjust users of
tpm_chip_alloc() like VTPM proxy and FTPM tee to release it indirectly by
putting the reference of chip->devs.

Fixes: 8979b02aaf1d ("tpm: Fix reference count to main device")
Signed-off-by: Lino Sanfilippo <l.sanfilippo@...bus.com>
---
 drivers/char/tpm/tpm-chip.c       | 11 +++++++++--
 drivers/char/tpm/tpm_ftpm_tee.c   |  2 ++
 drivers/char/tpm/tpm_vtpm_proxy.c |  1 +
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index e242d2e..596824c 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -360,8 +360,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
 	 * while cdevs is in use.  The corresponding put
 	 * is in the tpm_devs_release (TPM2 only)
 	 */
-	if (chip->flags & TPM_CHIP_FLAG_TPM2)
-		get_device(&chip->dev);
+	get_device(&chip->dev);

 	if (chip->dev_num == 0)
 		chip->dev.devt = MKDEV(MISC_MAJOR, TPM_MINOR);
@@ -425,12 +424,20 @@ struct tpm_chip *tpmm_chip_alloc(struct device *pdev,
 	if (rc)
 		goto put_dev;

+	rc = devm_add_action_or_reset(pdev,
+				      (void (*)(void *)) put_device,
+				      &chip->devs);
+	if (rc)
+		goto put_devs;
+
 	dev_set_drvdata(pdev, chip);

 	return chip;

 put_dev:
 	put_device(&chip->dev);
+put_devs:
+	put_device(&chip->devs);
 	return ERR_PTR(rc);
 }
 EXPORT_SYMBOL_GPL(tpmm_chip_alloc);
diff --git a/drivers/char/tpm/tpm_ftpm_tee.c b/drivers/char/tpm/tpm_ftpm_tee.c
index 2ccdf8a..82858c2 100644
--- a/drivers/char/tpm/tpm_ftpm_tee.c
+++ b/drivers/char/tpm/tpm_ftpm_tee.c
@@ -286,6 +286,7 @@ static int ftpm_tee_probe(struct device *dev)

 out_chip:
 	put_device(&pvt_data->chip->dev);
+	put_device(&pvt_data->chip->devs);
 out_chip_alloc:
 	tee_shm_free(pvt_data->shm);
 out_shm_alloc:
@@ -318,6 +319,7 @@ static int ftpm_tee_remove(struct device *dev)
 	tpm_chip_unregister(pvt_data->chip);

 	/* frees chip */
+	put_device(&pvt_data->chip->devs);
 	put_device(&pvt_data->chip->dev);

 	/* Free the shared memory pool */
diff --git a/drivers/char/tpm/tpm_vtpm_proxy.c b/drivers/char/tpm/tpm_vtpm_proxy.c
index 91c772e3..97b60f8 100644
--- a/drivers/char/tpm/tpm_vtpm_proxy.c
+++ b/drivers/char/tpm/tpm_vtpm_proxy.c
@@ -520,6 +520,7 @@ static struct proxy_dev *vtpm_proxy_create_proxy_dev(void)
  */
 static inline void vtpm_proxy_delete_proxy_dev(struct proxy_dev *proxy_dev)
 {
+	put_device(&proxy_dev->chip->devs);
 	put_device(&proxy_dev->chip->dev); /* frees chip */
 	kfree(proxy_dev);
 }
-- 
2.7.4