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Message-ID: <Z8JoMrUm9Dnoqgoi@kernel.org>
Date: Sat, 1 Mar 2025 03:51:46 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Claudio Carvalho <cclaudio@...ux.ibm.com>,
Peter Huewe <peterhuewe@....de>, x86@...nel.org,
Dov Murik <dovmurik@...ux.ibm.com>, linux-coco@...ts.linux.dev,
Dionna Glaze <dionnaglaze@...gle.com>,
James Bottomley <James.Bottomley@...senpartnership.com>,
Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
Jason Gunthorpe <jgg@...pe.ca>, linux-integrity@...r.kernel.org,
linux-kernel@...r.kernel.org,
Dave Hansen <dave.hansen@...ux.intel.com>,
Tom Lendacky <thomas.lendacky@....com>,
Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH v2 5/6] tpm: add SNP SVSM vTPM driver
On Fri, Feb 28, 2025 at 06:07:19PM +0100, Stefano Garzarella wrote:
> Add driver for the vTPM defined by the AMD SVSM spec [1].
>
> The specification defines a protocol that a SEV-SNP guest OS can use to
> discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
> in the guest context, but at a more privileged level (VMPL0).
>
> The new tpm-svsm platform driver uses two functions exposed by x86/sev
> to verify that the device is actually emulated by the platform and to
> send commands and receive responses.
>
> The vTPM is emulated through the TCG reference implementation, so this
> driver leverages tpm_tcgsim.h to fill commands and parse responses.
Why? Please don't.
>
> The device cannot be hot-plugged/unplugged as it is emulated by the
> platform, so we can use module_platform_driver_probe(). The probe
> function will only check whether in the current runtime configuration,
> SVSM is present and provides a vTPM.
>
> [1] "Secure VM Service Module for SEV-SNP Guests"
> Publication # 58019 Revision: 1.00
> https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/specifications/58019.pdf
>
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> ---
> drivers/char/tpm/tpm_svsm.c | 120 ++++++++++++++++++++++++++++++++++++
> drivers/char/tpm/Kconfig | 10 +++
> drivers/char/tpm/Makefile | 1 +
> 3 files changed, 131 insertions(+)
> create mode 100644 drivers/char/tpm/tpm_svsm.c
>
> diff --git a/drivers/char/tpm/tpm_svsm.c b/drivers/char/tpm/tpm_svsm.c
> new file mode 100644
> index 000000000000..1c34133990c5
> --- /dev/null
> +++ b/drivers/char/tpm/tpm_svsm.c
> @@ -0,0 +1,120 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
> + *
> + * Driver for the vTPM defined by the AMD SVSM spec [1].
> + *
> + * The specification defines a protocol that a SEV-SNP guest OS can use to
> + * discover and talk to a vTPM emulated by the Secure VM Service Module (SVSM)
> + * in the guest context, but at a more privileged level (usually VMPL0).
> + *
> + * The vTPM is emulated through the TCG reference implementation, so this
> + * driver leverages tpm_tcgsim.h to fill commands and parse responses.
> + *
> + * [1] "Secure VM Service Module for SEV-SNP Guests"
> + * Publication # 58019 Revision: 1.00
> + * https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/specifications/58019.pdf
> + */
> +
> +#include <asm/sev.h>
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/platform_device.h>
> +#include <linux/tpm_tcgsim.h>
> +
> +#include "tpm.h"
> +
> +struct tpm_svsm_priv {
> + u8 buffer[TPM_TCGSIM_MAX_BUFFER];
> + u8 locality;
> +};
> +
> +static int tpm_svsm_send_recv(struct tpm_chip *chip, u8 *buf, size_t buf_len,
> + size_t to_send)
> +{
> + struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
> + int ret;
> +
> + ret = tpm_tcgsim_fill_send_cmd((struct tpm_send_cmd_req *)priv->buffer,
> + priv->locality, buf, to_send);
> + if (ret)
> + return ret;
> +
> + ret = snp_svsm_vtpm_send_command(priv->buffer);
> + if (ret)
> + return ret;
> +
> + return tpm_tcgsim_parse_send_cmd((struct tpm_send_cmd_resp *)priv->buffer,
> + buf, buf_len);
> +}
> +
> +static struct tpm_class_ops tpm_chip_ops = {
> + .flags = TPM_OPS_AUTO_STARTUP,
> + .send_recv = tpm_svsm_send_recv,
> +};
> +
> +static int __init tpm_svsm_probe(struct platform_device *pdev)
> +{
> + struct device *dev = &pdev->dev;
> + struct tpm_svsm_priv *priv;
> + struct tpm_chip *chip;
> + int err;
> +
> + if (!snp_svsm_vtpm_probe())
> + return -ENODEV;
> +
> + priv = devm_kmalloc(dev, sizeof(*priv), GFP_KERNEL);
> + if (!priv)
> + return -ENOMEM;
> +
> + /*
> + * FIXME: before implementing locality we need to agree what it means
> + * for the SNP SVSM vTPM
> + */
> + priv->locality = 0;
> +
> + chip = tpmm_chip_alloc(dev, &tpm_chip_ops);
> + if (IS_ERR(chip))
> + return PTR_ERR(chip);
> +
> + dev_set_drvdata(&chip->dev, priv);
> +
> + err = tpm2_probe(chip);
> + if (err)
> + return err;
> +
> + err = tpm_chip_register(chip);
> + if (err)
> + return err;
> +
> + dev_info(dev, "SNP SVSM vTPM %s device\n",
> + (chip->flags & TPM_CHIP_FLAG_TPM2) ? "2.0" : "1.2");
> +
> + return 0;
> +}
> +
> +static void __exit tpm_svsm_remove(struct platform_device *pdev)
> +{
> + struct tpm_chip *chip = platform_get_drvdata(pdev);
> +
> + tpm_chip_unregister(chip);
> +}
> +
> +/*
> + * tpm_svsm_remove() lives in .exit.text. For drivers registered via
> + * module_platform_driver_probe() this is ok because they cannot get unbound
> + * at runtime. So mark the driver struct with __refdata to prevent modpost
> + * triggering a section mismatch warning.
> + */
> +static struct platform_driver tpm_svsm_driver __refdata = {
> + .remove = __exit_p(tpm_svsm_remove),
> + .driver = {
> + .name = "tpm-svsm",
> + },
> +};
> +
> +module_platform_driver_probe(tpm_svsm_driver, tpm_svsm_probe);
> +
> +MODULE_DESCRIPTION("SNP SVSM vTPM Driver");
> +MODULE_LICENSE("GPL");
> +MODULE_ALIAS("platform:tpm-svsm");
> diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> index 0fc9a510e059..fc3f1d10d31d 100644
> --- a/drivers/char/tpm/Kconfig
> +++ b/drivers/char/tpm/Kconfig
> @@ -225,5 +225,15 @@ config TCG_FTPM_TEE
> help
> This driver proxies for firmware TPM running in TEE.
>
> +config TCG_SVSM
> + tristate "SNP SVSM vTPM interface"
> + depends on AMD_MEM_ENCRYPT
> + help
> + This is a driver for the AMD SVSM vTPM protocol that a SEV-SNP guest
> + OS can use to discover and talk to a vTPM emulated by the Secure VM
> + Service Module (SVSM) in the guest context, but at a more privileged
> + level (usually VMPL0). To compile this driver as a module, choose M
> + here; the module will be called tpm_svsm.
> +
> source "drivers/char/tpm/st33zp24/Kconfig"
> endif # TCG_TPM
> diff --git a/drivers/char/tpm/Makefile b/drivers/char/tpm/Makefile
> index 9bb142c75243..52d9d80a0f56 100644
> --- a/drivers/char/tpm/Makefile
> +++ b/drivers/char/tpm/Makefile
> @@ -44,3 +44,4 @@ obj-$(CONFIG_TCG_XEN) += xen-tpmfront.o
> obj-$(CONFIG_TCG_CRB) += tpm_crb.o
> obj-$(CONFIG_TCG_VTPM_PROXY) += tpm_vtpm_proxy.o
> obj-$(CONFIG_TCG_FTPM_TEE) += tpm_ftpm_tee.o
> +obj-$(CONFIG_TCG_SVSM) += tpm_svsm.o
> --
> 2.48.1
>
OK, so this is like ARM's driver architecture using SMC, and I think
tpm_ftpm_tee is probably one great reflection for this overall. Is this
correct analysis, or not?
BR, Jarkko
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