tpm: Add SNP SVSM vTPM driver

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 API exposed by the x86/sev core
implementation interface to a SVSM to send commands and receive responses.

The device cannot be hot-plugged/unplugged as it is emulated by the platform,
so module_platform_driver_probe() can be used. The device will be registered
by the platform only when it's available, so the probe function just needs to
setup the tpm_chip.

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 is not implemented as recently supported by commit

  980a573621ea ("tpm: Make chip->{status,cancel,req_canceled} opt").

  [1] "Secure VM Service Module for SEV-SNP Guests"
      Publication # 58019 Revision: 1.00

  [ bp: Massage commit message. ]

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20250410135118.133240-4-sgarzare@redhat.com

diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
index fe4f3a6099346347a20b93137d8ca6077f76a87e..dddd702b2454a62f8eb02401136da4cfa9dc4453 100644 (file)
--- 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 2b004df8c04ba630f5b0e1552149514ce2e378b3..9de1b3ea34a9f2e30720eea834b822e6ca364ffa 100644 (file)
--- 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

diff --git a/drivers/char/tpm/tpm_svsm.c b/drivers/char/tpm/tpm_svsm.c
new file mode 100644 (file)
index 0000000..4280edf
--- /dev/null
+++ b/drivers/char/tpm/tpm_svsm.c
@@ -0,0 +1,125 @@
+// 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;
+
+       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");