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

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 79bbe2be900eb7a3bbea69899dc5a62dbe010927..ee34ab00a8d6d94011386d34fdf7ff4ef5e74abe 100644 (file)
--- 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 3b76cbf78f41fdccb7c29039dd05f41fb0ba2f9a..89754b019be2bab3e75da00aff8b3abe40ef7400 100644 (file)
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/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;