linux-kernel - Re: [RFC PATCH] tpm, tpm_vtpm

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Message-ID: <9193c59f-3982-42f4-9b05-c6cdefe3b05c@molgen.mpg.de>
Date: Wed, 16 Jul 2025 14:23:04 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: linux-kernel@...r.kernel.org, keyrings@...r.kernel.org,
 linux-integrity@...r.kernel.org, Stefan Berger <stefanb@...ux.ibm.com>,
 Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>,
 Jonathan Corbet <corbet@....net>, Peter Huewe <peterhuewe@....de>,
 Jason Gunthorpe <jgg@...pe.ca>, Andrew Morton <akpm@...ux-foundation.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Steven Rostedt
 <rostedt@...dmis.org>, Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, Arnd Bergmann <arnd@...db.de>,
 Frank van der Linden <fvdl@...gle.com>, linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH] tpm, tpm_vtpm_proxy: boot-time TPM

Dear Jarkko,


Am 16.07.25 um 14:18 schrieb Jarkko Sakkinen:
> From: Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>

Congratulations on the new(?) job! Big thanks to that company to do 
upstream Linux kernel work.

> Provide a kernel command-line parameter named as `supplicant`, which
> contains a path to an TPM emulator binary. When defind, the kernel will

defin*e*d

> launch the program during boot-time.
> 
> This feature is most useful in feature testing e.g., in environments
> where other means are not possible, such as CI runners. Its original use
> case highlights also quite well of its applicability for pre-production
> hardware: it was used to provide a TPM implemnentation for a RISC-V SoC

implementation

> running on FPGA with no TPM HW implementation at the time.
> 
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>
> ---
> Bumped into this in my archives so thought to make it available just in
> case anyone is interested.

Do you have such a TPM emulator binary?

Thank you for upstreaming this.

> ---
>   .../admin-guide/kernel-parameters.txt         | 14 +++++
>   drivers/char/tpm/tpm_vtpm_proxy.c             | 51 +++++++++++++++++++
>   2 files changed, 65 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f1f2c0874da9..e062de99480e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -7230,6 +7230,20 @@
>   			defined by Trusted Computing Group (TCG) see
>   			https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/
>   
> +	tpm_vtpm_proxy.supplicant= [TPM]
> +			When defined, this field must contain a legit path to a
> +			program emulating a TPM chip, which will be started
> +			during the driver initialization, thus providing a
> +			mechanism for the user space have an emulated TPM from
> +			the get go. Kernel prepares the process with a file
> +			pre-opened file descriptor in the index 3 for
> +			/dev/vtpmx.
> +
> +			An emulator can optionally provide support for
> +			localities by reacting to the vendor command defined
> +			by the driver: 0x20001000. Its payload is a single
> +			byte containing the new locality.
> +
>   	tp_printk	[FTRACE]
>   			Have the tracepoints sent to printk as well as the
>   			tracing ring buffer. This is useful for early boot up
> diff --git a/drivers/char/tpm/tpm_vtpm_proxy.c b/drivers/char/tpm/tpm_vtpm_proxy.c
> index 0818bb517805..612f5251fdc0 100644
> --- a/drivers/char/tpm/tpm_vtpm_proxy.c
> +++ b/drivers/char/tpm/tpm_vtpm_proxy.c
> @@ -51,6 +51,8 @@ struct proxy_dev {
>   #define VTPM_PROXY_FLAGS_ALL  (VTPM_PROXY_FLAG_TPM2)
>   
>   static struct workqueue_struct *workqueue;
> +static char *supplicant;
> +module_param(supplicant, charp, 0);
>   
>   static void vtpm_proxy_delete_device(struct proxy_dev *proxy_dev);
>   
> @@ -678,6 +680,55 @@ static const struct file_operations vtpmx_fops = {
>   	.llseek = noop_llseek,
>   };
>   
> +static int vtpmx_supplicant_setup(struct subprocess_info *info, struct cred *new)
> +{
> +	struct vtpm_proxy_new_dev dev = { .flags = VTPM_PROXY_FLAG_TPM2 };
> +	struct file *file = vtpm_proxy_create_device(&dev);
> +
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +
> +	fd_install(dev.fd, file);
> +	return 0;
> +}
> +
> +static void vtpmx_supplicant_cleanup(struct subprocess_info *info)
> +{
> +}
> +
> +static int vtpmx_supplicant_init(void)
> +{
> +	static const char * const argv[] = { supplicant, NULL };
> +	struct subprocess_info *info;
> +	int ret;
> +
> +	if (!supplicant)
> +		return 0;
> +
> +	info = call_usermodehelper_setup(argv[0], (char **)argv, NULL,
> +					 GFP_KERNEL, vtpmx_supplicant_setup,
> +					 vtpmx_supplicant_cleanup, NULL);
> +	if (!info)
> +		return -ENOMEM;
> +
> +	ret = call_usermodehelper_exec(info, UMH_KILLABLE | UMH_NO_WAIT);
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> +
> +static int vtpmx_init(void)
> +{
> +	int ret;
> +
> +	ret = vtpmx_supplicant_init();
> +	if (ret)
> +		return ret;
> +
> +	return misc_register(&vtpmx_miscdev);
> +}
> +
>   static struct miscdevice vtpmx_miscdev = {
>   	.minor = MISC_DYNAMIC_MINOR,
>   	.name = "vtpmx",


Kind regards,

Paul