[PATCH v9 03/24] virt: sev-guest: Make payload a variable length array

[Nikunj A Dadhania]

Currently, guest message is PAGE_SIZE bytes and payload is hard-coded to
4000 bytes, assuming snp_guest_msg_hdr structure as 96 bytes.

Remove the structure size assumption and hard-coding of payload size and
instead use variable length array.

While at it, rename the local guest message variables for clarity.

Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
Suggested-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 drivers/virt/coco/sev-guest/sev-guest.h |  5 +-
 drivers/virt/coco/sev-guest/sev-guest.c | 74 +++++++++++++++----------
 2 files changed, 48 insertions(+), 31 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.h b/drivers/virt/coco/sev-guest/sev-guest.h
index ceb798a404d6..97796f658fd3 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.h
+++ b/drivers/virt/coco/sev-guest/sev-guest.h
@@ -60,7 +60,10 @@ struct snp_guest_msg_hdr {
 
 struct snp_guest_msg {
 	struct snp_guest_msg_hdr hdr;
-	u8 payload[4000];
+	u8 payload[];
 } __packed;
 
+#define SNP_GUEST_MSG_SIZE 4096
+#define SNP_GUEST_MSG_PAYLOAD_SIZE (SNP_GUEST_MSG_SIZE - sizeof(struct snp_guest_msg))
+
 #endif /* __VIRT_SEVGUEST_H__ */
diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 7e1bf2056b47..69bd817239d8 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -48,7 +48,7 @@ struct snp_guest_dev {
 	 * 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_guest_msg *secret_request, *secret_response;
 
 	struct snp_secrets_page *secrets;
 	struct snp_req_data input;
@@ -171,40 +171,40 @@ static struct aesgcm_ctx *snp_init_crypto(u8 *key, size_t keylen)
 
 static int verify_and_dec_payload(struct snp_guest_dev *snp_dev, void *payload, u32 sz)
 {
-	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;
+	struct snp_guest_msg *resp_msg = snp_dev->secret_response;
+	struct snp_guest_msg *req_msg = snp_dev->secret_request;
+	struct snp_guest_msg_hdr *req_msg_hdr = &req_msg->hdr;
+	struct snp_guest_msg_hdr *resp_msg_hdr = &resp_msg->hdr;
 	struct aesgcm_ctx *ctx = snp_dev->ctx;
 	u8 iv[GCM_AES_IV_SIZE] = {};
 
 	pr_debug("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);
+		 resp_msg_hdr->msg_seqno, resp_msg_hdr->msg_type, resp_msg_hdr->msg_version,
+		 resp_msg_hdr->msg_sz);
 
 	/* Copy response from shared memory to encrypted memory. */
-	memcpy(resp, snp_dev->response, sizeof(*resp));
+	memcpy(resp_msg, snp_dev->response, SNP_GUEST_MSG_SIZE);
 
 	/* Verify that the sequence counter is incremented by 1 */
-	if (unlikely(resp_hdr->msg_seqno != (req_hdr->msg_seqno + 1)))
+	if (unlikely(resp_msg_hdr->msg_seqno != (req_msg_hdr->msg_seqno + 1)))
 		return -EBADMSG;
 
 	/* Verify response message type and version number. */
-	if (resp_hdr->msg_type != (req_hdr->msg_type + 1) ||
-	    resp_hdr->msg_version != req_hdr->msg_version)
+	if (resp_msg_hdr->msg_type != (req_msg_hdr->msg_type + 1) ||
+	    resp_msg_hdr->msg_version != req_msg_hdr->msg_version)
 		return -EBADMSG;
 
 	/*
 	 * If the message size is greater than our buffer length then return
 	 * an error.
 	 */
-	if (unlikely((resp_hdr->msg_sz + ctx->authsize) > sz))
+	if (unlikely((resp_msg_hdr->msg_sz + ctx->authsize) > sz))
 		return -EBADMSG;
 
 	/* Decrypt the payload */
-	memcpy(iv, &resp_hdr->msg_seqno, min(sizeof(iv), sizeof(resp_hdr->msg_seqno)));
-	if (!aesgcm_decrypt(ctx, payload, resp->payload, resp_hdr->msg_sz,
-			    &resp_hdr->algo, AAD_LEN, iv, resp_hdr->authtag))
+	memcpy(iv, &resp_msg_hdr->msg_seqno, min(sizeof(iv), sizeof(resp_msg_hdr->msg_seqno)));
+	if (!aesgcm_decrypt(ctx, payload, resp_msg->payload, resp_msg_hdr->msg_sz,
+			    &resp_msg_hdr->algo, AAD_LEN, iv, resp_msg_hdr->authtag))
 		return -EBADMSG;
 
 	return 0;
@@ -213,12 +213,12 @@ 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->secret_request;
-	struct snp_guest_msg_hdr *hdr = &req->hdr;
+	struct snp_guest_msg *msg = snp_dev->secret_request;
+	struct snp_guest_msg_hdr *hdr = &msg->hdr;
 	struct aesgcm_ctx *ctx = snp_dev->ctx;
 	u8 iv[GCM_AES_IV_SIZE] = {};
 
-	memset(req, 0, sizeof(*req));
+	memset(msg, 0, SNP_GUEST_MSG_SIZE);
 
 	hdr->algo = SNP_AEAD_AES_256_GCM;
 	hdr->hdr_version = MSG_HDR_VER;
@@ -236,11 +236,11 @@ static int enc_payload(struct snp_guest_dev *snp_dev, u64 seqno, int version, u8
 	pr_debug("request [seqno %lld type %d version %d sz %d]\n",
 		 hdr->msg_seqno, hdr->msg_type, hdr->msg_version, hdr->msg_sz);
 
-	if (WARN_ON((sz + ctx->authsize) > sizeof(req->payload)))
+	if (WARN_ON((sz + ctx->authsize) > SNP_GUEST_MSG_PAYLOAD_SIZE))
 		return -EBADMSG;
 
 	memcpy(iv, &hdr->msg_seqno, min(sizeof(iv), sizeof(hdr->msg_seqno)));
-	aesgcm_encrypt(ctx, req->payload, payload, sz, &hdr->algo, AAD_LEN,
+	aesgcm_encrypt(ctx, msg->payload, payload, sz, &hdr->algo, AAD_LEN,
 		       iv, hdr->authtag);
 
 	return 0;
@@ -346,7 +346,7 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code,
 		return -EIO;
 
 	/* Clear shared memory's response for the host to populate. */
-	memset(snp_dev->response, 0, sizeof(struct snp_guest_msg));
+	memset(snp_dev->response, 0, SNP_GUEST_MSG_SIZE);
 
 	/* Encrypt the userspace provided payload in snp_dev->secret_request. */
 	rc = enc_payload(snp_dev, seqno, rio->msg_version, type, req_buf, req_sz);
@@ -357,8 +357,7 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code,
 	 * Write the fully encrypted request to the shared unencrypted
 	 * request page.
 	 */
-	memcpy(snp_dev->request, &snp_dev->secret_request,
-	       sizeof(snp_dev->secret_request));
+	memcpy(snp_dev->request, &snp_dev->secret_request, SNP_GUEST_MSG_SIZE);
 
 	rc = __handle_guest_request(snp_dev, exit_code, rio);
 	if (rc) {
@@ -842,12 +841,21 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	snp_dev->dev = dev;
 	snp_dev->secrets = secrets;
 
+	/* Allocate secret request and response message for double buffering */
+	snp_dev->secret_request = kzalloc(SNP_GUEST_MSG_SIZE, GFP_KERNEL);
+	if (!snp_dev->secret_request)
+		goto e_unmap;
+
+	snp_dev->secret_response = kzalloc(SNP_GUEST_MSG_SIZE, GFP_KERNEL);
+	if (!snp_dev->secret_response)
+		goto e_free_secret_req;
+
 	/* Allocate the shared page used for the request and response message. */
-	snp_dev->request = alloc_shared_pages(dev, sizeof(struct snp_guest_msg));
+	snp_dev->request = alloc_shared_pages(dev, SNP_GUEST_MSG_SIZE);
 	if (!snp_dev->request)
-		goto e_unmap;
+		goto e_free_secret_resp;
 
-	snp_dev->response = alloc_shared_pages(dev, sizeof(struct snp_guest_msg));
+	snp_dev->response = alloc_shared_pages(dev, SNP_GUEST_MSG_SIZE);
 	if (!snp_dev->response)
 		goto e_free_request;
 
@@ -890,9 +898,13 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 e_free_cert_data:
 	free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
 e_free_response:
-	free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
+	free_shared_pages(snp_dev->response, SNP_GUEST_MSG_SIZE);
 e_free_request:
-	free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
+	free_shared_pages(snp_dev->request, SNP_GUEST_MSG_SIZE);
+e_free_secret_resp:
+	kfree(snp_dev->secret_response);
+e_free_secret_req:
+	kfree(snp_dev->secret_request);
 e_unmap:
 	iounmap(mapping);
 	return ret;
@@ -903,8 +915,10 @@ static void __exit sev_guest_remove(struct platform_device *pdev)
 	struct snp_guest_dev *snp_dev = platform_get_drvdata(pdev);
 
 	free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
-	free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
-	free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
+	free_shared_pages(snp_dev->response, SNP_GUEST_MSG_SIZE);
+	free_shared_pages(snp_dev->request, SNP_GUEST_MSG_SIZE);
+	kfree(snp_dev->secret_response);
+	kfree(snp_dev->secret_request);
 	kfree(snp_dev->ctx);
 	misc_deregister(&snp_dev->misc);
 }
-- 
2.34.1