linux-kernel - [RFC PATCH 1/4] tpm: Ensure exclusive userspace access when using /dev/tpm<n>

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Message-ID: <d2d7c8105a73e43866f23c88b188ee6e83562726.1756833527.git.noodles@meta.com>
Date: Tue, 2 Sep 2025 18:26:49 +0100
From: Jonathan McDowell <noodles@...th.li>
To: Peter Huewe <peterhuewe@....de>, Jarkko Sakkinen <jarkko@...nel.org>,
	Jason Gunthorpe <jgg@...pe.ca>
Cc: linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 1/4] tpm: Ensure exclusive userspace access when using
 /dev/tpm<n>

From: Jonathan McDowell <noodles@...a.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@...a.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