[RFC PATCH v3 4/4] i386/sev: Allow measured direct kernel boot on SNP

[Dov Murik]

In SNP, the hashes page designated with a specific metadata entry
published in AmdSev OVMF.

Therefore, if the user enabled kernel hashes (for measured direct boot),
QEMU should prepare the content of hashes table, and during the
processing of the metadata entry it copy the content into the designated
page and encrypt it.

Note that in SNP (unlike SEV and SEV-ES) the measurements is done in
whole 4KB pages.  Therefore QEMU zeros the whole page that includes the
hashes table, and fills in the kernel hashes area in that page, and then
encrypts the whole page.  The rest of the page is reserved for SEV
launch secrets which are not usable anyway on SNP.

If the user disabled kernel hashes, QEMU pre-validates the kernel hashes
page as a zero page.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
---
 include/hw/i386/pc.h |  2 ++
 target/i386/sev.c    | 40 ++++++++++++++++++++++++++++++++++++----
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 2b8212b416..32b5ca45ec 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -163,6 +163,8 @@ typedef enum {
     SEV_DESC_TYPE_SNP_SECRETS,
     /* The section contains address that can be used as a CPUID page */
     SEV_DESC_TYPE_CPUID,
+    /* The section contains the region for kernel hashes for measured direct 
boot */
+    SEV_DESC_TYPE_SNP_KERNEL_HASHES = 0x10,
 
 } ovmf_sev_metadata_desc_type;
 
diff --git a/target/i386/sev.c b/target/i386/sev.c
index 126e40ec26..ef47fd748f 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -131,6 +131,9 @@ struct SevSnpGuestState {
     struct kvm_snp_init kvm_init_conf;
     struct kvm_sev_snp_launch_start kvm_start_conf;
     struct kvm_sev_snp_launch_finish kvm_finish_conf;
+
+    uint32_t kernel_hashes_offset;
+    PaddedSevHashTable *kernel_hashes_data;
 };
 
 #define DEFAULT_GUEST_POLICY    0x1 /* disable debug */
@@ -1529,6 +1532,23 @@ snp_launch_update_cpuid(SevSnpGuestState *sev_snp, 
uint32_t cpuid_addr,
     return 0;
 }
 
+static int
+snp_launch_update_kernel_hashes(SevSnpGuestState *sev_snp, uint32_t addr,
+                                void *hva, uint32_t len)
+{
+    int type = KVM_SEV_SNP_PAGE_TYPE_ZERO;
+    if (sev_snp->sev_common.kernel_hashes) {
+        assert(sev_snp->kernel_hashes_data);
+        assert((sev_snp->kernel_hashes_offset +
+                sizeof(*sev_snp->kernel_hashes_data)) <= len);
+        memset(hva, 0, len);
+        memcpy(hva + sev_snp->kernel_hashes_offset, 
sev_snp->kernel_hashes_data,
+               sizeof(*sev_snp->kernel_hashes_data));
+        type = KVM_SEV_SNP_PAGE_TYPE_NORMAL;
+    }
+    return sev_snp_launch_update(sev_snp, addr, hva, len, type);
+}
+
 static int
 snp_metadata_desc_to_page_type(int desc_type)
 {
@@ -1537,6 +1557,7 @@ snp_metadata_desc_to_page_type(int desc_type)
     case SEV_DESC_TYPE_SNP_SEC_MEM: return KVM_SEV_SNP_PAGE_TYPE_ZERO;
     case SEV_DESC_TYPE_SNP_SECRETS: return KVM_SEV_SNP_PAGE_TYPE_SECRETS;
     case SEV_DESC_TYPE_CPUID: return KVM_SEV_SNP_PAGE_TYPE_CPUID;
+    case SEV_DESC_TYPE_SNP_KERNEL_HASHES: return KVM_SEV_SNP_PAGE_TYPE_NORMAL;
     default: return -1;
     }
 }
@@ -1568,6 +1589,9 @@ snp_populate_metadata_pages(SevSnpGuestState *sev_snp,
 
         if (type == KVM_SEV_SNP_PAGE_TYPE_CPUID) {
             ret = snp_launch_update_cpuid(sev_snp, desc->base, hva, desc->len);
+        } else if (desc->type == SEV_DESC_TYPE_SNP_KERNEL_HASHES) {
+            ret = snp_launch_update_kernel_hashes(sev_snp, desc->base, hva,
+                                                  desc->len);
         } else {
             ret = sev_snp_launch_update(sev_snp, desc->base, hva, desc->len,
                                         type);
@@ -2162,10 +2186,6 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext 
*ctx, Error **errp)
         return false;
     }
 
-    if (sev_snp_enabled()) {
-        return false;
-    }
-
     area = (SevHashTableDescriptor *)data;
     if (!area->base || area->size < sizeof(PaddedSevHashTable)) {
         error_setg(errp, "SEV: guest firmware hashes table area is invalid "
@@ -2173,6 +2193,18 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext 
*ctx, Error **errp)
         return false;
     }
 
+    if (sev_snp_enabled()) {
+        /*
+         * SNP: Populate the hashes table in an area that later in
+         * snp_launch_update_kernel_hashes() will be copied to the guest memory
+         * and encrypted.
+         */
+        SevSnpGuestState *sev_snp_guest = SEV_SNP_GUEST(sev_common);
+        sev_snp_guest->kernel_hashes_offset = area->base & ~TARGET_PAGE_MASK;
+        sev_snp_guest->kernel_hashes_data = g_new0(PaddedSevHashTable, 1);
+        return build_kernel_loader_hashes(sev_snp_guest->kernel_hashes_data, 
ctx, errp);
+    }
+
     /*
      * Populate the hashes table in the guest's memory at the OVMF-designated
      * area for the SEV hashes table
-- 
2.25.1