[PATCH v2 3/4] tpm: Allow for exclusive TPM access when using /dev/tpm

Jonathan McDowell
2025-09-23T17:10:22Z

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_(try_get|put)_ops_locked, 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>
---
v2: Switch to _locked instead of _internal_ for function names.

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..687f6d8cd601 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_try_get_ops_locked() - 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_try_get_ops_locked(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_put_ops_locked() - 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_try_get_ops_locked(). After this returns
+ * chip may be kfree'd.
*/
-void tpm_put_ops(struct tpm_chip *chip)
+void tpm_put_ops_locked(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_try_get_ops_locked(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_put_ops_locked(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_try_get_ops_locked(chip);
if (!rc) {
tpm2_end_auth_session(chip);
- tpm_put_ops(chip);
+ tpm_put_ops_locked(chip);
}
#endif

diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
index f2a5e09257dd..0f5bc63411aa 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_try_get_ops_locked(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_put_ops_locked(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_try_get_ops_locked(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_put_ops_locked(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..c2ec56e2cd24 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_try_get_ops_locked(struct tpm_chip *chip);
+void tpm_put_ops_locked(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..0ad5e18355e0 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_try_get_ops_locked(chip) == 0) {
tpm2_flush_sessions(chip, space);
- tpm_put_ops(chip);
+ tpm_put_ops_locked(chip);
}

kfree(space->context_buf);
--
2.51.0