Re: [PATCH v3 2/4] svsm: add header with SVSM_VTPM_CMD helpers

[Jarkko Sakkinen]

On Tue, Mar 11, 2025 at 10:42:23AM +0100, Stefano Garzarella wrote:
> 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@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Co-developed-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
> Signed-off-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
> 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/svsm_vtpm.h | 141 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 141 insertions(+)
>  create mode 100644 include/linux/svsm_vtpm.h
> 
> diff --git a/include/linux/svsm_vtpm.h b/include/linux/svsm_vtpm.h
> new file mode 100644
> index 000000000000..2ce9b1cb827e
> --- /dev/null
> +++ b/include/linux/svsm_vtpm.h
> @@ -0,0 +1,141 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2023 James.Bottomley@xxxxxxxxxxxxxxxxxxxxx
> + * 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 _SVSM_VTPM_H_
> +#define _SVSM_VTPM_H_
> +
> +#include <linux/errno.h>
> +#include <linux/string.h>
> +#include <linux/types.h>
> +
> +/*
> + * The current TCG Simulator TPM commands we support.  The complete list is
> + * in the TcpTpmProtocol header:
> + *
> + * https://github.com/TrustedComputingGroup/TPM/blob/main/TPMCmd/Simulator/include/TpmTcpProtocol.h
> + */
> +
> +#define TPM_SEND_COMMAND		8
> +#define TPM_SIGNAL_CANCEL_ON		9
> +#define TPM_SIGNAL_CANCEL_OFF		10
> +/*
> + * Any platform specific commands should be placed here and should start
> + * at 0x8000 to avoid clashes with the TCG Simulator protocol.  They should
> + * follow the same self describing buffer format below.
> + */
> +
> +#define SVSM_VTPM_MAX_BUFFER		4096 /* max req/resp buffer size */
> +

Across the board below data structures: I'd svsm_vtpm_ prefix them.
The rational is quite practical: it would easier to grep them later
on.

> +/**
> + * struct tpm_req - generic request header for single word command
> + *
> + * @cmd:	The command to send
> + */
> +struct tpm_req {
> +	u32 cmd;
> +} __packed;

__packed is useless here.

> +
> +/**
> + * struct tpm_resp - generic response header
> + *
> + * @size:	The response size (zero if nothing follows)
> + *
> + * Note: most TCG Simulator commands simply return zero here with no indication
> + * of success or failure.
> + */
> +struct tpm_resp {
> +	u32 size;
> +} __packed;

Ditto.

> +
> +/**
> + * struct tpm_send_cmd_req - Structure for a TPM_SEND_COMMAND request
> + *
> + * @hdr:	The request header whit the command (must be TPM_SEND_COMMAND)
> + * @locality:	The locality
> + * @inbuf_size:	The size of the input buffer following
> + * @inbuf:	A buffer of size inbuf_size
> + *
> + * Note that TCG Simulator expects @inbuf_size to be equal to the size of the
> + * specific TPM command, otherwise an TPM_RC_COMMAND_SIZE error is
> + * returned.
> + */
> +struct tpm_send_cmd_req {
> +	struct tpm_req hdr;

Useless nesting that makes this obfuscated: you can just as well put
that single field here, i.e.

	u32 cmd;

> +	u8 locality;
> +	u32 inbuf_size;
> +	u8 inbuf[];

Why not just buf?

> +} __packed;

Since we don't care about TCG Simulator compatibility I'd expect that
these are ordered in a way that they align nicely. E.g.,

struct svsm_vtpm_request {
	u32 command;
	u16 locality;
	u16 buffer_size;
	u8 buffer[];
};

64k should enough for any possible TPM command.

> +
> +/**
> + * struct tpm_send_cmd_req - Structure for a TPM_SEND_COMMAND response
> + *
> + * @hdr:	The response header whit the following size
> + * @outbuf:	A buffer of size hdr.size
> + */
> +struct tpm_send_cmd_resp {
> +	struct tpm_resp hdr;
> +	u8 outbuf[];
> +} __packed;

Why this does not have size? Here also __packed is useless even with the
pre-existing layout, and something like svsm_tpm_response would be a
factor more reasonable name.

> +
> +/**
> + * svsm_vtpm_fill_cmd_req() - fill a struct tpm_send_cmd_req to be sent to SVSM

> + * @req: The struct tpm_send_cmd_req 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_fill_cmd_req(struct tpm_send_cmd_req *req, u8 locality,
> +		       const u8 *buf, size_t len)

svsm_vtpm_fill_request()

> +{
> +	if (len > SVSM_VTPM_MAX_BUFFER - sizeof(*req))
> +		return -EINVAL;
> +
> +	req->hdr.cmd = TPM_SEND_COMMAND;
> +	req->locality = locality;
> +	req->inbuf_size = len;
> +
> +	memcpy(req->inbuf, buf, len);
> +
> +	return 0;
> +}
> +
> +/**
> + * svsm_vtpm_parse_cmd_resp() - Parse a struct tpm_send_cmd_resp received from
> + * SVSM
> + * @resp: The struct tpm_send_cmd_resp 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_parse_cmd_resp(const struct tpm_send_cmd_resp *resp, u8 *buf,
> +			 size_t len)

svsm_vtpm_parse_response()

> +{
> +	if (len < resp->hdr.size)
> +		return -E2BIG;
> +
> +	if (resp->hdr.size > SVSM_VTPM_MAX_BUFFER - sizeof(*resp))
> +		return -EINVAL;  // Invalid response from the platform TPM
> +
> +	memcpy(buf, resp->outbuf, resp->hdr.size);
> +
> +	return resp->hdr.size;
> +}
> +
> +#endif /* _SVSM_VTPM_H_ */
> -- 
> 2.48.1
> 

BR, Jarkko