Re: [PATCH v4 2/4] svsm: add header with SVSM_VTPM_CMD helpers

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Mar 24, 2025 at 11:46:47AM +0100, Stefano Garzarella wrote:
> 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>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> 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
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko