[PATCH v11 04/20] virt: sev-guest: Ensure the SNP guest messages do not exceed a page

[Nikunj A Dadhania <nikunj@amd.com>]

Currently, snp_guest_msg includes a message header (96 bytes) and a
payload (4000 bytes). There is an implicit assumption here that the SNP
message header will always be 96 bytes, and with that assumption the
payload array size has been set to 4000 bytes magic number. If any new
member is added to the SNP message header, the SNP guest message will span
more than a page.

Instead of using magic number '4000' for the payload, declare the
snp_guest_msg in a way that payload plus the message header do not exceed a
page.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
---
 arch/x86/include/asm/sev.h              | 2 +-
 drivers/virt/coco/sev-guest/sev-guest.c | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 79bbe2be900e..ee34ab00a8d6 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -164,7 +164,7 @@ struct snp_guest_msg_hdr {
 
 struct snp_guest_msg {
 	struct snp_guest_msg_hdr hdr;
-	u8 payload[4000];
+	u8 payload[PAGE_SIZE - sizeof(struct snp_guest_msg_hdr)];
 } __packed;
 
 struct sev_guest_platform_data {
diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 3b76cbf78f41..0b950069bfcb 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -1131,6 +1131,9 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	snp_dev->dev = dev;
 	snp_dev->secrets = secrets;
 
+	/* Ensure SNP guest messages do not span more than a page */
+	BUILD_BUG_ON(sizeof(struct snp_guest_msg) > PAGE_SIZE);
+
 	/* Allocate the shared page used for the request and response message. */
 	snp_dev->request = alloc_shared_pages(dev, sizeof(struct snp_guest_msg));
 	if (!snp_dev->request)
-- 
2.34.1