From: Lino Sanfilippo <redacted>

The following sequence of operations

1. open device /dev/tpmrm
2. remove the registered tpm chip driver
3. perform a write() to /dev/tpmrm

results in a refcount warning:

------------[ cut here ]------------
WARNING: CPU: 3 PID: 1161 at lib/refcount.c:25 kobject_get+0xa0/0xa4
refcount_t: addition on 0; use-after-free.
Modules linked in: tpm_tis_spi tpm_tis_core tpm mdio_bcm_unimac brcmfmac
sha256_generic libsha256 sha256_arm hci_uart btbcm bluetooth cfg80211 vc4
brcmutil ecdh_generic ecc snd_soc_core crc32_arm_ce libaes
raspberrypi_hwmon ac97_bus snd_pcm_dmaengine bcm2711_thermal snd_pcm
snd_timer genet snd phy_generic soundcore [last unloaded: spi_bcm2835]
CPU: 3 PID: 1161 Comm: hold_open Not tainted 5.10.0ls-main-dirty #2
Hardware name: BCM2711
[<c0410c3c>] (unwind_backtrace) from [<c040b580>] (show_stack+0x10/0x14)
[<c040b580>] (show_stack) from [<c1092174>] (dump_stack+0xc4/0xd8)
[<c1092174>] (dump_stack) from [<c0445a30>] (__warn+0x104/0x108)
[<c0445a30>] (__warn) from [<c0445aa8>] (warn_slowpath_fmt+0x74/0xb8)
[<c0445aa8>] (warn_slowpath_fmt) from [<c08435d0>] (kobject_get+0xa0/0xa4)
[<c08435d0>] (kobject_get) from [<bf0a715c>] (tpm_try_get_ops+0x14/0x54 [tpm])
[<bf0a715c>] (tpm_try_get_ops [tpm]) from [<bf0a7d6c>] (tpm_common_write+0x38/0x60 [tpm])
[<bf0a7d6c>] (tpm_common_write [tpm]) from [<c05a7ac0>] (vfs_write+0xc4/0x3c0)
[<c05a7ac0>] (vfs_write) from [<c05a7ee4>] (ksys_write+0x58/0xcc)
[<c05a7ee4>] (ksys_write) from [<c04001a0>] (ret_fast_syscall+0x0/0x4c)
Exception stack(0xc226bfa8 to 0xc226bff0)
bfa0:                   00000000 000105b4 00000003 beafe664 00000014 00000000
bfc0: 00000000 000105b4 000103f8 00000004 00000000 00000000 b6f9c000 beafe684
bfe0: 0000006c beafe648 0001056c b6eb6944
---[ end trace d4b8409def9b8b1f ]---

The reason is an attempt to get the chip->dev reference (see
tpm_try_get_ops()) although the last reference to this device has already
been released with the unregistration of the chip driver.

As far as I understand, the history of this bug is as follows:

Commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
introduced the support for /dev/tpmrm. Beside the new device chip->devs
this code introduced the taking of an extra reference to chip->dev to
ensure that this device is not released as long as /dev/tpmrm is still in
use:

+       chip->devs.release = tpm_devs_release;
+       /* get extra reference on main device to hold on
+        * behalf of devs.  This holds the chip structure
+        * while cdevs is in use.  The corresponding put
+        * is in the tpm_devs_release
+        */

The extra reference to chip->dev was supposed to be released as soon as the
reference to chip->devs was freed:

+static void tpm_devs_release(struct device *dev)
+{
+       struct tpm_chip *chip = container_of(dev, struct tpm_chip, devs);
+
+       /* release the master device reference */
+       put_device(&chip->dev);
+}

However the code did not work as expected, because the reference to
chip->devs was never released. Thus tpm_devs_release() was never called
which in turn also prevented the extra reference to chip->dev from being
released.

This led to behaviour which commit 8979b02aaf1d ("tpm: Fix reference count
to main device") tried to fix as an excerpt of the commit message shows:

"The main device is currently not properly released due to one additional
 reference to the 'devs' device which is only released in case of a TPM 2.
 So, also get the additional reference only in case of a TPM2."

Instead of adding the missing put for chip->devs, this patch chose another
approach by only taking the reference in case of TPM2:

+       if (chip->flags & TPM_CHIP_FLAG_TPM2)
+               get_device(&chip->dev);

This fixed the non-TPM2 case but left the TPM2 case without the required
extra reference, since there is no code path that ever sets the flag
TPM_CHIP_FLAG_TPM2, which is the situation we have today.

To fix this

1. revert commit 8979b02aaf1d ("tpm: Fix reference count to main device")
to make sure the extra reference is taken unconditionally as proposed by
commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")

2. fix commit fdc915f7f719 ("tpm: expose spaces via a device link
 /dev/tpmrm<n>") by adding an action handler to do the missing put to
chip->devs. This eventually results in the call of function
tpm_devs_release() which in turn performs the final put to the extra
reference to chip->dev.

Fixes: fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
Fixes: 8979b02aaf1d ("tpm: Fix reference count to main device")
Signed-off-by: Lino Sanfilippo <redacted>
---
 drivers/char/tpm/tpm-chip.c       | 18 +++++++++++++++---
 drivers/char/tpm/tpm_ftpm_tee.c   |  2 ++
 drivers/char/tpm/tpm_vtpm_proxy.c |  1 +
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ddaeceb..3ace199 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);

@@ -422,8 +421,21 @@ struct tpm_chip *tpmm_chip_alloc(struct device *pdev,
 	rc = devm_add_action_or_reset(pdev,
 				      (void (*)(void *)) put_device,
 				      &chip->dev);
-	if (rc)
+	if (rc) {
+		put_device(&chip->devs);
 		return ERR_PTR(rc);
+	}
+
+	rc = devm_add_action_or_reset(pdev,
+				      (void (*)(void *)) put_device,
+				      &chip->devs);
+	if (rc) {
+		devm_remove_action(pdev,
+				   (void (*)(void *)) put_device,
+				   &chip->dev);
+		put_device(&chip->dev);
+		return ERR_PTR(rc);
+	}
 
 	dev_set_drvdata(pdev, chip);
 

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