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

Jonathan McDowell
2025-09-02T17:26:52Z

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.

This can cause problems with userspace expecting exclusive access and
something changing state underneath it, for example while performing a
TPM firmware upgrade.

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>
---
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..40c139a080b6 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 out;

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

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

return 0;
+
+out:
+ 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