virt/coco/sev-guest: Double-buffer messages

The encryption algorithms read and write directly to shared unencrypted
memory, which may leak information as well as permit the host to tamper
with the message integrity. Instead, copy whole messages in or out as
needed before doing any computation on them.

Fixes: d5af44d ("x86/sev: Provide support for SNP guest request NAEs")
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230214164638.1189804-3-dionnaglaze@google.com

Author: deeglaze

drivers/virt/coco/sev-guest/sev-guest.c

@@ -46,7 +46,15 @@ struct snp_guest_dev {
 
 	void *certs_data;
 	struct snp_guest_crypto *crypto;
+	/* request and response are in unencrypted memory */
 	struct snp_guest_msg *request, *response;
+
+	/*
+	 * Avoid information leakage by double-buffering shared messages
+	 * in fields that are in regular encrypted memory.
+	 */
+	struct snp_guest_msg secret_request, secret_response;
+
 	struct snp_secrets_page_layout *layout;
 	struct snp_req_data input;
 	u32 *os_area_msg_seqno;
@@ -266,14 +274,17 @@ static int dec_payload(struct snp_guest_dev *snp_dev, struct snp_guest_msg *msg,
 static int verify_and_dec_payload(struct snp_guest_dev *snp_dev, void *payload, u32 sz)
 {
 	struct snp_guest_crypto *crypto = snp_dev->crypto;
-	struct snp_guest_msg *resp = snp_dev->response;
-	struct snp_guest_msg *req = snp_dev->request;
+	struct snp_guest_msg *resp = &snp_dev->secret_response;
+	struct snp_guest_msg *req = &snp_dev->secret_request;
 	struct snp_guest_msg_hdr *req_hdr = &req->hdr;
 	struct snp_guest_msg_hdr *resp_hdr = &resp->hdr;
 
 	dev_dbg(snp_dev->dev, "response [seqno %lld type %d version %d sz %d]\n",
 		resp_hdr->msg_seqno, resp_hdr->msg_type, resp_hdr->msg_version, resp_hdr->msg_sz);
 
+	/* Copy response from shared memory to encrypted memory. */
+	memcpy(resp, snp_dev->response, sizeof(*resp));
+
 	/* Verify that the sequence counter is incremented by 1 */
 	if (unlikely(resp_hdr->msg_seqno != (req_hdr->msg_seqno + 1)))
 		return -EBADMSG;
@@ -297,7 +308,7 @@ static int verify_and_dec_payload(struct snp_guest_dev *snp_dev, void *payload,
 static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8 type,
 			void *payload, size_t sz)
 {
-	struct snp_guest_msg *req = snp_dev->request;
+	struct snp_guest_msg *req = &snp_dev->secret_request;
 	struct snp_guest_msg_hdr *hdr = &req->hdr;
 
 	memset(req, 0, sizeof(*req));
@@ -417,13 +428,21 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	if (!seqno)
 		return -EIO;
 
+	/* Clear shared memory's response for the host to populate. */
 	memset(snp_dev->response, 0, sizeof(struct snp_guest_msg));
 
-	/* Encrypt the userspace provided payload */
+	/* Encrypt the userspace provided payload in snp_dev->secret_request. */
 	rc = enc_payload(snp_dev, seqno, msg_ver, type, req_buf, req_sz);
 	if (rc)
 		return rc;
 
+	/*
+	 * Write the fully encrypted request to the shared unencrypted
+	 * request page.
+	 */
+	memcpy(snp_dev->request, &snp_dev->secret_request,
+	       sizeof(snp_dev->secret_request));
+
 	rc = __handle_guest_request(snp_dev, exit_code, fw_err);
 	if (rc) {
 		if (rc == -EIO && *fw_err == SNP_GUEST_REQ_INVALID_LEN)