[RFC PATCH v2 5/6] tpm: add SNP SVSM vTPM driver

Stefano Garzarella
2025-02-28T17:08:07Z

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.

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@redhat.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