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

Nikunj A Dadhania
2024-05-31T04:31:35Z

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@amd.com>
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
---
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