Re: [PATCH v2 4/4] tpm: Require O_EXCL for exclusive /dev/tpm access

Jarkko Sakkinen
2025-09-24T01:23:49Z

On Tue, Sep 23, 2025 at 06:10:28PM +0100, Jonathan McDowell wrote:
> From: Jonathan McDowell <noodles@meta.com>
>
> Given that /dev/tpm has not had exclusive access to the TPM since the
> existence of the kernel resource broker and other internal users, stop
> defaulted to exclusive access to the first client that opens the device.
> Continue to support exclusive access, but only with the use of the
> O_EXCL flag on device open.
>
> Signed-off-by: Jonathan McDowell <noodles@meta.com>
> ---
> drivers/char/tpm/tpm-dev.c | 25 +++++++++++++++++++------
> drivers/char/tpm/tpm-dev.h | 1 +
> 2 files changed, 20 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-dev.c b/drivers/char/tpm/tpm-dev.c
> index 80c4b3f3ad18..8921bbb541c1 100644
> --- a/drivers/char/tpm/tpm-dev.c
> +++ b/drivers/char/tpm/tpm-dev.c
> @@ -19,15 +19,21 @@ static int tpm_open(struct inode *inode, struct file *file)
> {
> struct tpm_chip *chip;
> struct file_priv *priv;
> + int rc;
>
> chip = container_of(inode->i_cdev, struct tpm_chip, cdev);
>
> /*
> - * 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 a client uses the O_EXCL flag then it expects to be the only TPM
> + * user, so we treat it as a write lock. Otherwise we do as /dev/tpmrm
> + * and use a read lock.
> */
> - if (!down_write_trylock(&chip->open_lock)) {
> + if (file->f_flags & O_EXCL)
> + rc = down_write_trylock(&chip->open_lock);
> + else
> + rc = down_read_trylock(&chip->open_lock);
> +
> + if (!rc) {
> dev_dbg(&chip->dev, "Another process owns this TPM\n");
> return -EBUSY;
> }
> @@ -35,13 +41,17 @@ static int tpm_open(struct inode *inode, struct file *file)
> priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> if (priv == NULL)
> goto out;
> + priv->exclusive = (file->f_flags & O_EXCL);
>
> tpm_common_open(file, chip, priv, NULL);
>
> return 0;
>
> out:
> - up_write(&chip->open_lock);
> + if (file->f_flags & O_EXCL)
> + up_write(&chip->open_lock);
> + else
> + up_read(&chip->open_lock);
> return -ENOMEM;
> }
>
> @@ -53,7 +63,10 @@ static int tpm_release(struct inode *inode, struct file *file)
> struct file_priv *priv = file->private_data;
>
> tpm_common_release(file, priv);
> - up_write(&priv->chip->open_lock);
> + if (priv->exclusive)
> + up_write(&priv->chip->open_lock);
> + else
> + up_read(&priv->chip->open_lock);
> kfree(priv);
>
> return 0;
> diff --git a/drivers/char/tpm/tpm-dev.h b/drivers/char/tpm/tpm-dev.h
> index f3742bcc73e3..0ad8504c73e4 100644
> --- a/drivers/char/tpm/tpm-dev.h
> +++ b/drivers/char/tpm/tpm-dev.h
> @@ -17,6 +17,7 @@ struct file_priv {
> ssize_t response_length;
> bool response_read;
> bool command_enqueued;
> + bool exclusive;
>
> u8 data_buffer[TPM_BUFSIZE];
> };
> --
> 2.51.0
>

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko