[PATCH v2 1/4] tpm: Ensure exclusive userspace access when using /dev/tpm

Jonathan McDowell
2025-09-23T17:10:04Z

From: Jonathan McDowell <noodles@meta.com>

There is an is_open lock on /dev/tpm<n> that dates back to at least
2013, but it only prevents multiple accesses via *this* interface. It is
perfectly possible for userspace to use /dev/tpmrm<n>, or the kernel to
use the internal interfaces, to access the TPM. For tooling expecting
exclusive access, such as firmware updates, this can cause issues.

Close the userspace loophole by changing the simple bit lock to a full
read/write mutex. Direct /dev/tpm<n> access needs an exclusive write
lock, the resource broker continues to allow concurrent access *except*
when /dev/tpm<n> is open.

Signed-off-by: Jonathan McDowell <noodles@meta.com>
---
v2: Change error path label to err instead of out. Rework commit
message.

drivers/char/tpm/tpm-chip.c | 1 +
drivers/char/tpm/tpm-dev.c | 14 ++++++++------
drivers/char/tpm/tpmrm-dev.c | 20 ++++++++++++++++++--
include/linux/tpm.h | 3 ++-
4 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index e25daf2396d3..8c8e9054762a 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -338,6 +338,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,

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

chip->ops = ops;

diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
index 97c94b5e9340..80c4b3f3ad18 100644
--- a/drivers/char/tpm/tpm-dev.c
+++ b/drivers/char/tpm/tpm-dev.c
@@ -22,10 +22,12 @@ 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)) {
+ /*
+ * Only one client is allowed to have /dev/tpm0 open at a time, so we
+ * treat it as a write lock. The shared /dev/tpmrm0 is treated as a
+ * read lock.
+ */
+ if (!down_write_trylock(&chip->open_lock)) {
dev_dbg(&chip->dev, "Another process owns this TPM\n");
return -EBUSY;
}
@@ -39,7 +41,7 @@ static int tpm_open(struct inode *inode, struct file *file)
return 0;

out:
- clear_bit(0, &chip->is_open);
+ up_write(&chip->open_lock);
return -ENOMEM;
}

@@ -51,7 +53,7 @@ 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);
+ up_write(&priv->chip->open_lock);
kfree(priv);

return 0;
diff --git a/drivers/char/tpm/tpmrm-dev.c b/drivers/char/tpm/tpmrm-dev.c
index c25df7ea064e..13322dd9ac9e 100644
--- a/drivers/char/tpm/tpmrm-dev.c
+++ b/drivers/char/tpm/tpmrm-dev.c
@@ -17,19 +17,34 @@ static int tpmrm_open(struct inode *inode, struct file *file)
int rc;

chip = container_of(inode->i_cdev, struct tpm_chip, cdevs);
+
+ /*
+ * Only one client is allowed to have /dev/tpm0 open at a time, so we
+ * treat it as a write lock. The shared /dev/tpmrm0 is treated as a
+ * read lock.
+ */
+ if (!down_read_trylock(&chip->open_lock)) {
+ dev_dbg(&chip->dev, "Another process owns this TPM\n");
+ return -EBUSY;
+ }
+
priv = kzalloc(sizeof(*priv), GFP_KERNEL);
if (priv == NULL)
- return -ENOMEM;
+ goto err;

rc = tpm2_init_space(&priv->space, TPM2_SPACE_BUFFER_SIZE);
if (rc) {
kfree(priv);
- return -ENOMEM;
+ goto err;
}

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

return 0;
+
+err:
+ up_read(&chip->open_lock);
+ return -ENOMEM;
}

static int tpmrm_release(struct inode *inode, struct file *file)
@@ -40,6 +55,7 @@ static int tpmrm_release(struct inode *inode, struct file *file)
tpm_common_release(file, fpriv);
tpm2_del_space(fpriv->chip, &priv->space);
kfree(priv);
+ up_read(&fpriv->chip->open_lock);

return 0;
}
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index b0e9eb5ef022..548362d20b32 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/rwsem.h>
#include <crypto/hash_info.h>
#include <crypto/aes.h>

@@ -168,7 +169,7 @@ struct tpm_chip {
unsigned int flags;

int dev_num; /* /dev/tpm# */
- unsigned long is_open; /* only one allowed */
+ struct rw_semaphore open_lock;

char hwrng_name[64];
struct hwrng hwrng;
--
2.51.0