[PATCH v8 39/40] 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  | 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
+
+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 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_ */
-- 
2.25.1