[PATCH v2] tpm: Rework open/close/shutdown to avoid races

Sergey Temerkhanov
2020-12-15T13:39:58Z

Avoid race condition at shutdown by shutting downn the TPM 2.0
devices synchronously. This eliminates the condition when the
shutdown sequence sets chip->ops to NULL leading to the following:

[ 1586.593561][ T8669] tpm2_del_space+0x28/0x73
[ 1586.598718][ T8669] tpmrm_release+0x27/0x33wq
[ 1586.603774][ T8669] __fput+0x109/0x1d
[ 1586.608380][ T8669] task_work_run+0x7c/0x90
[ 1586.613414][ T8669] prepare_exit_to_usermode+0xb8/0x128
[ 1586.619522][ T8669] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 1586.626068][ T8669] RIP: 0033:0x4cb4bb

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
---
drivers/char/tpm/tpm-chip.c | 2 ++
drivers/char/tpm/tpm-dev.c | 20 +++++++++++++-------
drivers/char/tpm/tpmrm-dev.c | 3 +++
include/linux/tpm.h | 6 ++++--
4 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ddaeceb7e109..e94148b8e180 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -295,6 +295,7 @@ static int tpm_class_shutdown(struct device *dev)
{
struct tpm_chip *chip = container_of(dev, struct tpm_chip, dev);

+ wait_event_idle(chip->waitq, !atomic_read(&chip->refcount));
down_write(&chip->ops_sem);
if (chip->flags & TPM_CHIP_FLAG_TPM2) {
if (!tpm_chip_start(chip)) {
@@ -330,6 +331,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,

mutex_init(&chip->tpm_mutex);
init_rwsem(&chip->ops_sem);
+ init_waitqueue_head(&chip->waitq);

chip->ops = ops;

diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
index e2c0baa69fef..8558f0f7382c 100644
--- a/drivers/char/tpm/tpm-dev.c
+++ b/drivers/char/tpm/tpm-dev.c
@@ -19,27 +19,32 @@ static int tpm_open(struct inode *inode, struct file *file)
{
struct tpm_chip *chip;
struct file_priv *priv;
+ int ret = 0;

chip = container_of(inode->i_cdev, struct tpm_chip, cdev);

/* It's assured that the chip will be opened just once,
- * by the check of is_open variable, which is protected
- * by driver_lock. */
- if (test_and_set_bit(0, &chip->is_open)) {
+ * by the check of the chip reference count.
+ */
+ if (atomic_fetch_inc(&chip->refcount)) {
dev_dbg(&chip->dev, "Another process owns this TPM\n");
- return -EBUSY;
+ ret = -EBUSY;
+ goto out;
}

priv = kzalloc(sizeof(*priv), GFP_KERNEL);
- if (priv == NULL)
+ if (priv == NULL) {
+ ret = -ENOMEM;
goto out;
+ }

tpm_common_open(file, chip, priv, NULL);

return 0;

out:
- clear_bit(0, &chip->is_open);
+ atomic_dec(&chip->refcount);
+ wake_up_all(&chip->waitq);
return -ENOMEM;
}

@@ -51,7 +56,8 @@ static int tpm_release(struct inode *inode, struct file *file)
struct file_priv *priv = file->private_data;

tpm_common_release(file, priv);
- clear_bit(0, &priv->chip->is_open);
+ atomic_dec(&priv->chip->refcount);
+ wake_up_all(&priv->chip->waitq);
kfree(priv);

return 0;
diff --git a/drivers/char/tpm/tpmrm-dev.c b/drivers/char/tpm/tpmrm-dev.c
index eef0fb06ea83..fb3cb7b03814 100644
--- a/drivers/char/tpm/tpmrm-dev.c
+++ b/drivers/char/tpm/tpmrm-dev.c
@@ -28,6 +28,7 @@ static int tpmrm_open(struct inode *inode, struct file *file)
}

tpm_common_open(file, chip, &priv->priv, &priv->space);
+ atomic_inc(&chip->refcount);

return 0;
}
@@ -39,6 +40,8 @@ static int tpmrm_release(struct inode *inode, struct file *file)

tpm_common_release(file, fpriv);
tpm2_del_space(fpriv->chip, &priv->space);
+ atomic_dec(&fpriv->chip->refcount);
+ wake_up_all(&fpriv->chip->waitq);
kfree(priv);

return 0;
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 8f4ff39f51e7..0c8842783823 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -22,6 +22,7 @@
#include <linux/cdev.h>
#include <linux/fs.h>
#include <linux/highmem.h>
+#include <linux/atomic.h>
#include <crypto/hash_info.h>

#define TPM_DIGEST_SIZE 20 /* Max TPM v1.2 PCR size */
@@ -128,8 +129,9 @@ struct tpm_chip {

unsigned int flags;

- int dev_num; /* /dev/tpm# */
- unsigned long is_open; /* only one allowed */
+ int dev_num; /* /dev/tpm# */
+ atomic_t refcount; /* /dev/tmp# can only be opened once */
+ wait_queue_head_t waitq; /* Wait queue for synchronous ops */

char hwrng_name[64];
struct hwrng hwrng;
--
2.25.1