svsm: Add header with SVSM_VTPM_CMD helpers

Add helpers for the SVSM_VTPM_CMD calls used by the vTPM protocol defined by
the AMD SVSM spec [1].

The vTPM protocol follows the Official TPM 2.0 Reference Implementation
(originally by Microsoft, now part of the TCG) simulator protocol.

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

Co-developed-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Co-developed-by: Claudio Carvalho <cclaudio@linux.ibm.com>
Signed-off-by: Claudio Carvalho <cclaudio@linux.ibm.com>
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/20250403100943.120738-3-sgarzare@redhat.com

Author: stefano-garzarella

include/linux/tpm_svsm.h

@@ -0,0 +1,149 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2023 James.Bottomley@HansenPartnership.com
+ * Copyright (C) 2025 Red Hat, Inc. All Rights Reserved.
+ *
+ * Helpers for the SVSM_VTPM_CMD calls used by the vTPM protocol defined by the
+ * AMD SVSM spec [1].
+ *
+ * The vTPM protocol follows the Official TPM 2.0 Reference Implementation
+ * (originally by Microsoft, now part of the TCG) simulator protocol.
+ *
+ * [1] "Secure VM Service Module for SEV-SNP Guests"
+ *     Publication # 58019 Revision: 1.00
+ */
+#ifndef _TPM_SVSM_H_
+#define _TPM_SVSM_H_
+
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/types.h>
+
+#define SVSM_VTPM_MAX_BUFFER		4096 /* max req/resp buffer size */
+
+/**
+ * struct svsm_vtpm_request - Generic request for single word command
+ * @cmd:	The command to send
+ *
+ * Defined by AMD SVSM spec [1] in section "8.2 SVSM_VTPM_CMD Call" -
+ * Table 15: vTPM Common Request/Response Structure
+ *     Byte      Size       In/Out    Description
+ *     Offset    (Bytes)
+ *     0x000     4          In        Platform command
+ *                          Out       Platform command response size
+ */
+struct svsm_vtpm_request {
+	u32 cmd;
+};
+
+/**
+ * struct svsm_vtpm_response - Generic response
+ * @size:	The response size (zero if nothing follows)
+ *
+ * Defined by AMD SVSM spec [1] in section "8.2 SVSM_VTPM_CMD Call" -
+ * Table 15: vTPM Common Request/Response Structure
+ *     Byte      Size       In/Out    Description
+ *     Offset    (Bytes)
+ *     0x000     4          In        Platform command
+ *                          Out       Platform command response size
+ *
+ * Note: most TCG Simulator commands simply return zero here with no indication
+ * of success or failure.
+ */
+struct svsm_vtpm_response {
+	u32 size;
+};
+
+/**
+ * struct svsm_vtpm_cmd_request - Structure for a TPM_SEND_COMMAND request
+ * @cmd:	The command to send (must be TPM_SEND_COMMAND)
+ * @locality:	The locality
+ * @buf_size:	The size of the input buffer following
+ * @buf:	A buffer of size buf_size
+ *
+ * Defined by AMD SVSM spec [1] in section "8.2 SVSM_VTPM_CMD Call" -
+ * Table 16: TPM_SEND_COMMAND Request Structure
+ *     Byte      Size       Meaning
+ *     Offset    (Bytes)
+ *     0x000     4          Platform command (8)
+ *     0x004     1          Locality (must-be-0)
+ *     0x005     4          TPM Command size (in bytes)
+ *     0x009     Variable   TPM Command
+ *
+ * Note: the TCG Simulator expects @buf_size to be equal to the size of the
+ * specific TPM command, otherwise an TPM_RC_COMMAND_SIZE error is returned.
+ */
+struct svsm_vtpm_cmd_request {
+	u32 cmd;
+	u8 locality;
+	u32 buf_size;
+	u8 buf[];
+} __packed;
+
+/**
+ * struct svsm_vtpm_cmd_response - Structure for a TPM_SEND_COMMAND response
+ * @buf_size:	The size of the output buffer following
+ * @buf:	A buffer of size buf_size
+ *
+ * Defined by AMD SVSM spec [1] in section "8.2 SVSM_VTPM_CMD Call" -
+ * Table 17: TPM_SEND_COMMAND Response Structure
+ *     Byte      Size       Meaning
+ *     Offset    (Bytes)
+ *     0x000     4          Response size (in bytes)
+ *     0x004     Variable   Response
+ */
+struct svsm_vtpm_cmd_response {
+	u32 buf_size;
+	u8 buf[];
+};
+
+/**
+ * svsm_vtpm_cmd_request_fill() - Fill a TPM_SEND_COMMAND request to be sent to SVSM
+ * @req: The struct svsm_vtpm_cmd_request to fill
+ * @locality: The locality
+ * @buf: The buffer from where to copy the payload of the command
+ * @len: The size of the buffer
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+static inline int
+svsm_vtpm_cmd_request_fill(struct svsm_vtpm_cmd_request *req, u8 locality,
+			   const u8 *buf, size_t len)
+{
+	if (len > SVSM_VTPM_MAX_BUFFER - sizeof(*req))
+		return -EINVAL;
+
+	req->cmd = 8; /* TPM_SEND_COMMAND */
+	req->locality = locality;
+	req->buf_size = len;
+
+	memcpy(req->buf, buf, len);
+
+	return 0;
+}
+
+/**
+ * svsm_vtpm_cmd_response_parse() - Parse a TPM_SEND_COMMAND response received from SVSM
+ * @resp: The struct svsm_vtpm_cmd_response to parse
+ * @buf: The buffer where to copy the response
+ * @len: The size of the buffer
+ *
+ * Return: buffer size filled with the response on success, negative error
+ * code on failure.
+ */
+static inline int
+svsm_vtpm_cmd_response_parse(const struct svsm_vtpm_cmd_response *resp, u8 *buf,
+			     size_t len)
+{
+	if (len < resp->buf_size)
+		return -E2BIG;
+
+	if (resp->buf_size > SVSM_VTPM_MAX_BUFFER - sizeof(*resp))
+		return -EINVAL;  // Invalid response from the platform TPM
+
+	memcpy(buf, resp->buf, resp->buf_size);
+
+	return resp->buf_size;
+}
+
+#endif /* _TPM_SVSM_H_ */