[RFC PATCH 3/4] tpm: Allow for exclusive TPM access when using /dev/tpm<n>

[Jonathan McDowell <noodles@earth.li>]

From: Jonathan McDowell <noodles@meta.com>

There are situations where userspace might reasonably desire exclusive
access to the TPM, or the kernel's internal context saving + flushing
may cause issues, for example when performing firmware upgrades. Extend
the locking already used for avoiding concurrent userspace access to
prevent internal users of the TPM when /dev/tpm<n> is in use.

The few internal users who already hold the open_lock are changed to use
tpm_internal_(try_get|put)_ops, with the old tpm_(try_get|put)_ops
functions changing to obtain read access to the open_lock.  We return
-EBUSY when another user has exclusive access, rather than adding waits.

Signed-off-by: Jonathan McDowell <noodles@meta.com>
---
 drivers/char/tpm/tpm-chip.c       | 53 +++++++++++++++++++++++++------
 drivers/char/tpm/tpm-dev-common.c |  8 ++---
 drivers/char/tpm/tpm.h            |  2 ++
 drivers/char/tpm/tpm2-space.c     |  5 ++-
 4 files changed, 52 insertions(+), 16 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index ba906966721a..3d69ccff4c2a 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -144,7 +144,7 @@ void tpm_chip_stop(struct tpm_chip *chip)
 EXPORT_SYMBOL_GPL(tpm_chip_stop);
 
 /**
- * tpm_try_get_ops() - Get a ref to the tpm_chip
+ * tpm_internal_try_get_ops() - Get a ref to the tpm_chip
  * @chip: Chip to ref
  *
  * The caller must already have some kind of locking to ensure that chip is
@@ -154,7 +154,7 @@ EXPORT_SYMBOL_GPL(tpm_chip_stop);
  *
  * Returns -ERRNO if the chip could not be got.
  */
-int tpm_try_get_ops(struct tpm_chip *chip)
+int tpm_internal_try_get_ops(struct tpm_chip *chip)
 {
 	int rc = -EIO;
 
@@ -185,22 +185,57 @@ int tpm_try_get_ops(struct tpm_chip *chip)
 	put_device(&chip->dev);
 	return rc;
 }
-EXPORT_SYMBOL_GPL(tpm_try_get_ops);
 
 /**
- * tpm_put_ops() - Release a ref to the tpm_chip
+ * tpm_internal_put_ops() - Release a ref to the tpm_chip
  * @chip: Chip to put
  *
- * This is the opposite pair to tpm_try_get_ops(). After this returns chip may
- * be kfree'd.
+ * This is the opposite pair to tpm_internal_try_get_ops(). After this returns
+ * chip may be kfree'd.
  */
-void tpm_put_ops(struct tpm_chip *chip)
+void tpm_internal_put_ops(struct tpm_chip *chip)
 {
 	tpm_chip_stop(chip);
 	mutex_unlock(&chip->tpm_mutex);
 	up_read(&chip->ops_sem);
 	put_device(&chip->dev);
 }
+
+/**
+ * tpm_try_get_ops() - Get a ref to the tpm_chip
+ * @chip: Chip to ref
+ *
+ * The caller must already have some kind of locking to ensure that chip is
+ * valid. This function will attempt to get the open_lock for the chip,
+ * ensuring no other user is expecting exclusive access, before locking the
+ * chip so that the ops member can be accessed safely. The locking prevents
+ * tpm_chip_unregister from completing, so it should not be held for long
+ * periods.
+ *
+ * Returns -ERRNO if the chip could not be got.
+ */
+int tpm_try_get_ops(struct tpm_chip *chip)
+{
+	if (!down_read_trylock(&chip->open_lock))
+		return -EBUSY;
+
+	return tpm_internal_try_get_ops(chip);
+}
+EXPORT_SYMBOL_GPL(tpm_try_get_ops);
+
+/**
+ * tpm_put_ops() - Release a ref to the tpm_chip
+ * @chip: Chip to put
+ *
+ * This is the opposite pair to tpm_try_get_ops(). After this returns
+ * chip may be kfree'd.
+ */
+void tpm_put_ops(struct tpm_chip *chip)
+{
+	tpm_internal_put_ops(chip);
+
+	up_read(&chip->open_lock);
+}
 EXPORT_SYMBOL_GPL(tpm_put_ops);
 
 /**
@@ -644,10 +679,10 @@ void tpm_chip_unregister(struct tpm_chip *chip)
 #ifdef CONFIG_TCG_TPM2_HMAC
 	int rc;
 
-	rc = tpm_try_get_ops(chip);
+	rc = tpm_internal_try_get_ops(chip);
 	if (!rc) {
 		tpm2_end_auth_session(chip);
-		tpm_put_ops(chip);
+		tpm_internal_put_ops(chip);
 	}
 #endif
 
diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
index f2a5e09257dd..7cd0617844ed 100644
--- a/drivers/char/tpm/tpm-dev-common.c
+++ b/drivers/char/tpm/tpm-dev-common.c
@@ -65,7 +65,7 @@ static void tpm_dev_async_work(struct work_struct *work)
 
 	mutex_lock(&priv->buffer_mutex);
 	priv->command_enqueued = false;
-	ret = tpm_try_get_ops(priv->chip);
+	ret = tpm_internal_try_get_ops(priv->chip);
 	if (ret) {
 		priv->response_length = ret;
 		goto out;
@@ -73,7 +73,7 @@ static void tpm_dev_async_work(struct work_struct *work)
 
 	ret = tpm_dev_transmit(priv->chip, priv->space, priv->data_buffer,
 			       sizeof(priv->data_buffer));
-	tpm_put_ops(priv->chip);
+	tpm_internal_put_ops(priv->chip);
 
 	/*
 	 * If ret is > 0 then tpm_dev_transmit returned the size of the
@@ -220,14 +220,14 @@ ssize_t tpm_common_write(struct file *file, const char __user *buf,
 	 * lock during this period so that the tpm can be unregistered even if
 	 * the char dev is held open.
 	 */
-	if (tpm_try_get_ops(priv->chip)) {
+	if (tpm_internal_try_get_ops(priv->chip)) {
 		ret = -EPIPE;
 		goto out;
 	}
 
 	ret = tpm_dev_transmit(priv->chip, priv->space, priv->data_buffer,
 			       sizeof(priv->data_buffer));
-	tpm_put_ops(priv->chip);
+	tpm_internal_put_ops(priv->chip);
 
 	if (ret > 0) {
 		priv->response_length = ret;
diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
index 9c158c55c05f..1c34c10421c9 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -272,6 +272,8 @@ struct tpm_chip *tpm_chip_alloc(struct device *dev,
 				const struct tpm_class_ops *ops);
 struct tpm_chip *tpmm_chip_alloc(struct device *pdev,
 				 const struct tpm_class_ops *ops);
+int tpm_internal_try_get_ops(struct tpm_chip *chip);
+void tpm_internal_put_ops(struct tpm_chip *chip);
 int tpm_chip_register(struct tpm_chip *chip);
 void tpm_chip_unregister(struct tpm_chip *chip);
 
diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
index 60354cd53b5c..853acaf3d10f 100644
--- a/drivers/char/tpm/tpm2-space.c
+++ b/drivers/char/tpm/tpm2-space.c
@@ -58,10 +58,9 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
 
 void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
 {
-
-	if (tpm_try_get_ops(chip) == 0) {
+	if (tpm_internal_try_get_ops(chip) == 0) {
 		tpm2_flush_sessions(chip, space);
-		tpm_put_ops(chip);
+		tpm_internal_put_ops(chip);
 	}
 
 	kfree(space->context_buf);
-- 
2.51.0