[PATCH v4 3/4] tpm: add SNP SVSM vTPM driver

[Stefano Garzarella <sgarzare@redhat.com>]

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>
---
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 | 155 ++++++++++++++++++++++++++++++++++++
 drivers/char/tpm/Kconfig    |  10 +++
 drivers/char/tpm/Makefile   |   1 +
 3 files changed, 166 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..1281ff265927
--- /dev/null
+++ b/drivers/char/tpm/tpm_svsm.c
@@ -0,0 +1,155 @@
+// 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 void tpm_svsm_cancel(struct tpm_chip *chip)
+{
+	/* not supported */
+}
+
+static u8 tpm_svsm_status(struct tpm_chip *chip)
+{
+	return 0;
+}
+
+static bool tpm_svsm_req_canceled(struct tpm_chip *chip, u8 status)
+{
+	return false;
+}
+
+static struct tpm_class_ops tpm_chip_ops = {
+	.flags = TPM_OPS_AUTO_STARTUP,
+	.recv = tpm_svsm_recv,
+	.send = tpm_svsm_send,
+	.cancel = tpm_svsm_cancel,
+	.status = tpm_svsm_status,
+	.req_complete_mask = 0,
+	.req_complete_val = 0,
+	.req_canceled = tpm_svsm_req_canceled,
+};
+
+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;
+
+	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
-- 
2.49.0