[PATCH Part1 RFC v4 35/36] virt: sevguest: Add support to derive key

[Brijesh Singh <brijesh.singh@amd.com>]

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
sealing key or communicating with the external entities.

See SEV-SNP firmware spec for more information.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 Documentation/virt/coco/sevguest.rst  | 18 ++++++++++
 drivers/virt/coco/sevguest/sevguest.c | 48 +++++++++++++++++++++++++++
 include/uapi/linux/sev-guest.h        | 24 ++++++++++++++
 3 files changed, 90 insertions(+)

diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
index 52d5915037ef..25446670d816 100644
--- a/Documentation/virt/coco/sevguest.rst
+++ b/Documentation/virt/coco/sevguest.rst
@@ -67,3 +67,21 @@ provided by the SEV-SNP firmware to query the attestation report.
 On success, the snp_report_resp.data will contains the report. The report
 format is 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
+
+The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key.
+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 fileds passed in the key derivation request.
+
+On success, the snp_derived_key_resp.data will contains the derived key
+value.
diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
index f3f86f9b5b22..2f20db80490a 100644
--- a/drivers/virt/coco/sevguest/sevguest.c
+++ b/drivers/virt/coco/sevguest/sevguest.c
@@ -304,6 +304,50 @@ static int get_report(struct snp_guest_dev *snp_dev, struct snp_user_guest_reque
 	return rc;
 }
 
+static int get_derived_key(struct snp_guest_dev *snp_dev, struct snp_user_guest_request *arg)
+{
+	struct snp_guest_crypto *crypto = snp_dev->crypto;
+	struct snp_derived_key_resp *resp;
+	struct snp_derived_key_req req;
+	int rc, resp_len;
+
+	if (!arg->req_data || !arg->resp_data)
+		return -EINVAL;
+
+	/* Copy the request payload from the userspace */
+	if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
+		return -EFAULT;
+
+	/* Message version must be non-zero */
+	if (!req.msg_version)
+		return -EINVAL;
+
+	/*
+	 * 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;
+	resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
+	if (!resp)
+		return -ENOMEM;
+
+	/* Issue the command to get the attestation report */
+	rc = handle_guest_request(snp_dev, req.msg_version, SNP_MSG_KEY_REQ,
+				  &req.data, sizeof(req.data), resp->data, resp_len,
+				  &arg->fw_err);
+	if (rc)
+		goto e_free;
+
+	/* Copy the response payload to userspace */
+	if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
+		rc = -EFAULT;
+
+e_free:
+	kfree(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);
@@ -321,6 +365,10 @@ static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long
 		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 e8cfd15133f3..621a9167df7a 100644
--- a/include/uapi/linux/sev-guest.h
+++ b/include/uapi/linux/sev-guest.h
@@ -36,9 +36,33 @@ struct snp_user_guest_request {
 	__u64 fw_err;
 };
 
+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_req {
+	/* message version number (must be non-zero) */
+	__u8 msg_version;
+
+	struct __snp_derived_key_req data;
+};
+
+struct snp_derived_key_resp {
+	/* response data, see SEV-SNP spec for the format */
+	__u8 data[64];
+};
+
 #define SNP_GUEST_REQ_IOC_TYPE	'S'
 
 /* Get SNP attestation report */
 #define SNP_GET_REPORT _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x0, struct snp_user_guest_request)
 
+/* Get a derived key from the root */
+#define SNP_GET_DERIVED_KEY _IOWR(SNP_GUEST_REQ_IOC_TYPE, 0x1, struct snp_user_guest_request)
+
 #endif /* __UAPI_LINUX_SEV_GUEST_H_ */
-- 
2.17.1