[PATCH v6 2/4] svsm: add header with SVSM_VTPM_CMD helpers

Stefano Garzarella
2025-04-03T10:09:59Z

From: Stefano Garzarella <sgarzare@redhat.com>

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>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
v5:
- added Jarkko's R-b
v4:
- used svsm_vtpm_ prefix consistently [Jarkko]
- removed __packed where not needed [Jarkko]
- expanded headers to avoid obfuscation [Jarkko]
- used `buf` instead of `inbuf`/`outbuf` [Jarkko]
- added more documentation quoting the specification
- removed TPM_* macros since we only use TPM_SEND_COMMAND in one place
and don't want dependencies on external headers, but put the value
directly as specified in the AMD SVSM specification
- header renamed in tpm_svsm.h so it will fall under TPM DEVICE DRIVER
section [Borislav, Jarkko]
v3:
- renamed header and prefix to make clear it's related to the SVSM vTPM
protocol
- renamed fill/parse functions [Tom]
- removed link to the spec because those URLs are unstable [Borislav]
---
include/linux/tpm_svsm.h | 149 +++++++++++++++++++++++++++++++++++++++
1 file changed, 149 insertions(+)
create mode 100644 include/linux/tpm_svsm.h

diff --git a/include/linux/tpm_svsm.h b/include/linux/tpm_svsm.h
new file mode 100644
index 000000000000..38e341f9761a
--- /dev/null
+++ b/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_ */
--
2.49.0