Re: [PATCH v8 39/40] virt: sevguest: Add support to derive key

[Liam Merwick]

On 10/12/2021 15:43, Brijesh Singh wrote:
> The SNP_GET_DERIVED_KEY ioctl interface can be used by the SNP guest to
> ask the firmware to provide a key derived from a root key. The derived
> key may be used by the guest for any purposes it choose, such as a

nit: choose -> chooses

> sealing key or communicating with the external entities.
>
> See SEV-SNP firmware spec for more information.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>   Documentation/virt/coco/sevguest.rst  | 17 ++++++++++
>   drivers/virt/coco/sevguest/sevguest.c | 45 +++++++++++++++++++++++++++
>   include/uapi/linux/sev-guest.h        | 17 ++++++++++
>   3 files changed, 79 insertions(+)
>
> diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
> index 47ef3b0821d5..8c22d514d44f 100644
> --- a/Documentation/virt/coco/sevguest.rst
> +++ b/Documentation/virt/coco/sevguest.rst
> @@ -72,6 +72,23 @@ On success, the snp_report_resp.data will contains the report. The report
>   contain the format described in the SEV-SNP specification. See the SEV-SNP
>   specification for further details.
>   
> +2.2 SNP_GET_DERIVED_KEY
> +-----------------------
> +:Technology: sev-snp
> +:Type: guest ioctl
> +:Parameters (in): struct snp_derived_key_req
> +:Returns (out): struct snp_derived_key_req on success, -negative on error
> +

Does it return 'struct snp_derived_key_resp' on success?


> +The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key.

nit: derive -> derived ?

Otherwise

Reviewed-by: Liam Merwick <liam.merwick@xxxxxxxxxx>

> +The derived key can be used by the guest for any purpose, such as sealing keys
> +or communicating with external entities.
> +
> +The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the
> +SEV-SNP firmware to derive the key. See SEV-SNP specification for further details
> +on the various fields passed in the key derivation request.
> +
> +On success, the snp_derived_key_resp.data contains the derived key value. See
> +the SEV-SNP specification for further details.
>   
>   Reference
>   ---------
> diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
> index b3b080c9b2d6..d8dcafc32e11 100644
> --- a/drivers/virt/coco/sevguest/sevguest.c
> +++ b/drivers/virt/coco/sevguest/sevguest.c
> @@ -391,6 +391,48 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_io
>   	return rc;
>   }
>   
> +static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> +{
> +	struct snp_guest_crypto *crypto = snp_dev->crypto;
> +	struct snp_derived_key_resp resp = {0};
> +	struct snp_derived_key_req req;
> +	int rc, resp_len;
> +	u8 buf[64+16]; /* Response data is 64 bytes and max authsize for GCM is 16 bytes */
> +
> +	if (!arg->req_data || !arg->resp_data)
> +		return -EINVAL;
> +
> +	/* Copy the request payload from userspace */
> +	if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
> +		return -EFAULT;
> +
> +	/*
> +	 * The intermediate response buffer is used while decrypting the
> +	 * response payload. Make sure that it has enough space to cover the
> +	 * authtag.
> +	 */
> +	resp_len = sizeof(resp.data) + crypto->a_len;
> +	if (sizeof(buf) < resp_len)
> +		return -ENOMEM;
> +
> +	/* Issue the command to get the attestation report */
> +	rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg->msg_version,
> +				  SNP_MSG_KEY_REQ, &req, sizeof(req), buf, resp_len,
> +				  &arg->fw_err);
> +	if (rc)
> +		goto e_free;
> +
> +	/* Copy the response payload to userspace */
> +	memcpy(resp.data, buf, sizeof(resp.data));
> +	if (copy_to_user((void __user *)arg->resp_data, &resp, sizeof(resp)))
> +		rc = -EFAULT;
> +
> +e_free:
> +	memzero_explicit(buf, sizeof(buf));
> +	memzero_explicit(&resp, sizeof(resp));
> +	return rc;
> +}
> +
>   static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
>   {
>   	struct snp_guest_dev *snp_dev = to_snp_dev(file);
> @@ -420,6 +462,9 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
>   	case SNP_GET_REPORT:
>   		ret = get_report(snp_dev, &input);
>   		break;
> +	case SNP_GET_DERIVED_KEY:
> +		ret = get_derived_key(snp_dev, &input);
> +		break;
>   	default:
>   		break;
>   	}
> diff --git a/include/uapi/linux/sev-guest.h b/include/uapi/linux/sev-guest.h
> index 0bfc162da465..ce595539e00c 100644
> --- a/include/uapi/linux/sev-guest.h
> +++ b/include/uapi/linux/sev-guest.h
> @@ -27,6 +27,20 @@ struct snp_report_resp {
>   	__u8 data[4000];
>   };
>   
> +struct snp_derived_key_req {
> +	__u32 root_key_select;
> +	__u32 rsvd;
> +	__u64 guest_field_select;
> +	__u32 vmpl;
> +	__u32 guest_svn;
> +	__u64 tcb_version;
> +};
> +
> +struct snp_derived_key_resp {
> +	/* response data, see SEV-SNP spec for the format */
> +	__u8 data[64];
> +};
> +
>   struct snp_guest_request_ioctl {
>   	/* message version number (must be non-zero) */
>   	__u8 msg_version;
> @@ -44,4 +58,7 @@ struct snp_guest_request_ioctl {
>   /* Get SNP attestation report */
>   #define SNP_GET_REPORT _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x0, struct snp_guest_request_ioctl)
>   
> +/* Get a derived key from the root */
> +#define SNP_GET_DERIVED_KEY _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x1, struct snp_guest_request_ioctl)
> +
>   #endif /* __UAPI_LINUX_SEV_GUEST_H_ */