Re: [PATCH v5 3/4] tpm: add SNP SVSM vTPM driver

On Mon, Mar 31, 2025 at 10:34 AM Jarkko Sakkinen [off-list ref] wrote:

On Mon, Mar 31, 2025 at 12:38:56PM +0200, Stefano Garzarella wrote:

quoted

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>
---
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?

quoted

+
+     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?

quoted

+
+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)