virt: sev-guest: Ensure the SNP guest messages do not exceed a page

Currently, struct 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 - a 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 a magic number for the payload, declare struct
snp_guest_msg in a way that payload plus the message header do not
exceed a page.

  [ bp: Massage. ]
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240731150811.156771-5-nikunj@amd.com

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 {

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

@@ -1092,6 +1092,8 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	void __iomem *mapping;
 	int ret;
 
+	BUILD_BUG_ON(sizeof(struct snp_guest_msg) > PAGE_SIZE);
+
 	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 		return -ENODEV;