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

[Sergey Temerkhanov]

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@xxxxxxxxx>
---
 drivers/char/tpm/tpm-chip.c  | 27 ---------------------------
 drivers/char/tpm/tpm-dev.c   | 11 ++++++-----
 drivers/char/tpm/tpmrm-dev.c |  7 +++++++
 include/linux/tpm.h          |  2 +-
 4 files changed, 14 insertions(+), 33 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 1838039b0333..ede7f4790c5e 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -282,32 +282,6 @@ static void tpm_devs_release(struct device *dev)
 	put_device(&chip->dev);
 }
 
-/**
- * tpm_class_shutdown() - prepare the TPM device for loss of power.
- * @dev: device to which the chip is associated.
- *
- * Issues a TPM2_Shutdown command prior to loss of power, as required by the
- * TPM 2.0 spec. Then, calls bus- and device- specific shutdown code.
- *
- * Return: always 0 (i.e. success)
- */
-static int tpm_class_shutdown(struct device *dev)
-{
-	struct tpm_chip *chip = container_of(dev, struct tpm_chip, dev);
-
-	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);
-		}
-	}
-	chip->ops = NULL;
-	up_write(&chip->ops_sem);
-
-	return 0;
-}
-
 /**
  * tpm_chip_alloc() - allocate a new struct tpm_chip instance
  * @pdev: device to which the chip is associated
@@ -347,7 +321,6 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
 	device_initialize(&chip->devs);
 
 	chip->dev.class = tpm_class;
-	chip->dev.class->shutdown_pre = tpm_class_shutdown;
 	chip->dev.release = tpm_dev_release;
 	chip->dev.parent = pdev;
 	chip->dev.groups = chip->groups;
diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
index e2c0baa69fef..e04f3d47c64e 100644
--- a/drivers/char/tpm/tpm-dev.c
+++ b/drivers/char/tpm/tpm-dev.c
@@ -23,9 +23,9 @@ static int tpm_open(struct inode *inode, struct file *file)
 	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 is_open variable
+	 */
+	if (atomic_fetch_or(1, &chip->is_open)) {
 		dev_dbg(&chip->dev, "Another process owns this TPM\n");
 		return -EBUSY;
 	}
@@ -39,7 +39,7 @@ static int tpm_open(struct inode *inode, struct file *file)
 	return 0;
 
  out:
-	clear_bit(0, &chip->is_open);
+	atomic_set(&chip->is_open, 0);
 	return -ENOMEM;
 }
 
@@ -49,9 +49,10 @@ static int tpm_open(struct inode *inode, struct file *file)
 static int tpm_release(struct inode *inode, struct file *file)
 {
 	struct file_priv *priv = file->private_data;
+	struct tpm_chip *chip = priv->chip;
 
 	tpm_common_release(file, priv);
-	clear_bit(0, &priv->chip->is_open);
+	atomic_set(&chip->is_open, 0);
 	kfree(priv);
 
 	return 0;
diff --git a/drivers/char/tpm/tpmrm-dev.c b/drivers/char/tpm/tpmrm-dev.c
index eef0fb06ea83..ec83ca8105b8 100644
--- a/drivers/char/tpm/tpmrm-dev.c
+++ b/drivers/char/tpm/tpmrm-dev.c
@@ -27,6 +27,8 @@ static int tpmrm_open(struct inode *inode, struct file *file)
 		return -ENOMEM;
 	}
 
+	atomic_inc(&chip->is_open);
+
 	tpm_common_open(file, chip, &priv->priv, &priv->space);
 
 	return 0;
@@ -39,6 +41,11 @@ static int tpmrm_release(struct inode *inode, struct file *file)
 
 	tpm_common_release(file, fpriv);
 	tpm2_del_space(fpriv->chip, &priv->space);
+
+	if (!atomic_dec_return(&fpriv->chip->is_open)) {
+		tpm2_shutdown(fpriv->chip, TPM2_SU_CLEAR);
+		tpm_chip_stop(fpriv->chip);
+	}
 	kfree(priv);
 
 	return 0;
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index 77fdc988c610..26e070198a15 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -126,7 +126,7 @@ struct tpm_chip {
 	unsigned int flags;
 
 	int dev_num;		/* /dev/tpm# */
-	unsigned long is_open;	/* only one allowed */
+	atomic_t is_open;	/* only one allowed */
 
 	char hwrng_name[64];
 	struct hwrng hwrng;
-- 
2.25.1