Message-ID: <CAAH4kHY-Orjr_+rcYNemuWrAOOtyrCMD5s6KsOrkX740AARXiA@mail.gmail.com>
Date: Mon, 31 Mar 2025 13:56:20 -0700
From: Dionna Amalie Glaze <dionnaglaze@...gle.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Stefano Garzarella <sgarzare@...hat.com>, linux-kernel@...r.kernel.org, 
	Jason Gunthorpe <jgg@...pe.ca>, James Bottomley <James.Bottomley@...senpartnership.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, 
	linux-integrity@...r.kernel.org, Peter Huewe <peterhuewe@....de>, 
	Ingo Molnar <mingo@...hat.com>, linux-coco@...ts.linux.dev, 
	Dov Murik <dovmurik@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Joerg Roedel <jroedel@...e.de>, x86@...nel.org, Tom Lendacky <thomas.lendacky@....com>, 
	Claudio Carvalho <cclaudio@...ux.ibm.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v5 3/4] tpm: add SNP SVSM vTPM driver

On Mon, Mar 31, 2025 at 10:34 AM Jarkko Sakkinen <jarkko@...nel.org> wrote:
>
> On Mon, Mar 31, 2025 at 12:38:56PM +0200, Stefano Garzarella wrote:
> > From: Stefano Garzarella <sgarzare@...hat.com>
> >
> > 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 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.
> >
> > This device does not support interrupts and sends responses to commands
> > synchronously. In order to have .recv() called just after .send() in
> > tpm_try_transmit(), the .status() callback returns 0, and both
> > .req_complete_mask and .req_complete_val are set to 0.
> >
> > [1] "Secure VM Service Module for SEV-SNP Guests"
> >     Publication # 58019 Revision: 1.00
> >
> > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> > ---
> > v5:
> > - removed cancel/status/req_* ops after rebase on master that cotains
> >   commit 980a573621ea ("tpm: Make chip->{status,cancel,req_canceled} opt")
> > v4:
> > - moved "asm" includes after the "linux" includes [Tom]
> > - allocated buffer separately [Tom/Jarkko/Jason]
> > v3:
> > - removed send_recv() ops and followed the ftpm driver implementing .status,
> >   .req_complete_mask, .req_complete_val, etc. [Jarkko]
> > - removed link to the spec because those URLs are unstable [Borislav]
> > ---
> >  drivers/char/tpm/tpm_svsm.c | 135 ++++++++++++++++++++++++++++++++++++
> >  drivers/char/tpm/Kconfig    |  10 +++
> >  drivers/char/tpm/Makefile   |   1 +
> >  3 files changed, 146 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..04c532421ff2
> > --- /dev/null
> > +++ b/drivers/char/tpm/tpm_svsm.c
> > @@ -0,0 +1,135 @@
> > +// 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).
> > + *
> > + * [1] "Secure VM Service Module for SEV-SNP Guests"
> > + *     Publication # 58019 Revision: 1.00
> > + */
> > +
> > +#include <linux/module.h>
> > +#include <linux/kernel.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/tpm_svsm.h>
> > +
> > +#include <asm/sev.h>
> > +
> > +#include "tpm.h"
> > +
> > +struct tpm_svsm_priv {
> > +     void *buffer;
> > +     u8 locality;
> > +};
> > +
> > +static int tpm_svsm_send(struct tpm_chip *chip, u8 *buf, size_t len)
> > +{
> > +     struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
> > +     int ret;
> > +
> > +     ret = svsm_vtpm_cmd_request_fill(priv->buffer, priv->locality, buf, len);
> > +     if (ret)
> > +             return ret;
> > +
> > +     /*
> > +      * The SVSM call uses the same buffer for the command and for the
> > +      * response, so after this call, the buffer will contain the response
> > +      * that can be used by .recv() op.
> > +      */
> > +     return snp_svsm_vtpm_send_command(priv->buffer);
> > +}
> > +
> > +static int tpm_svsm_recv(struct tpm_chip *chip, u8 *buf, size_t len)
> > +{
> > +     struct tpm_svsm_priv *priv = dev_get_drvdata(&chip->dev);
> > +
> > +     /*
> > +      * The internal buffer contains the response after we send the command
> > +      * to SVSM.
> > +      */
> > +     return svsm_vtpm_cmd_response_parse(priv->buffer, buf, len);
> > +}
> > +
> > +static struct tpm_class_ops tpm_chip_ops = {
> > +     .flags = TPM_OPS_AUTO_STARTUP,
> > +     .recv = tpm_svsm_recv,
> > +     .send = tpm_svsm_send,
> > +};
> > +
> > +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;
> > +
> > +     /*
> > +      * The maximum buffer supported is one page (see SVSM_VTPM_MAX_BUFFER
> > +      * in tpm_svsm.h).
> > +      */
> > +     priv->buffer = (void *)devm_get_free_pages(dev, GFP_KERNEL, 0);
> > +     if (!priv->buffer)
> > +             return -ENOMEM;
> > +
> > +     /*
> > +      * FIXME: before implementing locality we need to agree what it means
> > +      * for the SNP SVSM vTPM
> > +      */
> > +     priv->locality = 0;
>
> I don't think we want FIXME's to mainline. Instead, don't declare the
> field at all if you don't use it. Just pass zero to *_request_fill().
>
> Maybe "not have the field" is even a better reminder than a random fixme
> comment?
>
> > +
> > +     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);

I might be unclear on how I should be testing this, but I do see
/dev/tpm0 and /dev/tpmrm0 when I build with CONFIG_TCG_SVSM=y, but I
don't see the event log in securityfs. What am I missing?

> > +
> > +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 fe4f3a609934..dddd702b2454 100644
> > --- a/drivers/char/tpm/Kconfig
> > +++ b/drivers/char/tpm/Kconfig
> > @@ -234,5 +234,15 @@ 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 2b004df8c04b..9de1b3ea34a9 100644
> > --- a/drivers/char/tpm/Makefile
> > +++ b/drivers/char/tpm/Makefile
> > @@ -45,3 +45,4 @@ obj-$(CONFIG_TCG_CRB) += tpm_crb.o
> >  obj-$(CONFIG_TCG_ARM_CRB_FFA) += tpm_crb_ffa.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
> > --
> > 2.49.0
> >
>
> BR, Jarkko



-- 
-Dionna Glaze, PhD, CISSP, CCSP (she/her)

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