Re: [PATCH v6 3/4] tpm: add SNP SVSM vTPM driver

Stefano Garzarella
2025-04-09T09:44:09Z

On Thu, 3 Apr 2025 at 21:05, Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Thu, Apr 03, 2025 at 12:09:41PM +0200, Stefano Garzarella wrote:
> > From: Stefano Garzarella <sgarzare@redhat.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@redhat.com>
> > ---
> > v6:
> > - removed the `locality` field (set to 0) and the FIXME comment [Jarkko]
> > 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 | 128 ++++++++++++++++++++++++++++++++++++
> > drivers/char/tpm/Kconfig | 10 +++
> > drivers/char/tpm/Makefile | 1 +
> > 3 files changed, 139 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..b9242c9eab87
> > --- /dev/null
> > +++ b/drivers/char/tpm/tpm_svsm.c
> > @@ -0,0 +1,128 @@
> > +// 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;
> > +};
> > +
> > +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, 0, 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;
> > +
> > + 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 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
> >
> >
>
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

In the next version I'm moving the snp_svsm_vtpm_probe() call from
here to sev core where the device is registered.
See https://lore.kernel.org/linux-integrity/o2u7p3wb64lcc4sziunr274hyubkgmspzdjcvihbpzkw6mkvpo@sjq3vi4y2qfl/

Do I carry your R-b for this patch (and patch 4) or would you rather
take a look at v7?

Thanks,
Stefano