[PATCH v3 5/6] target/i386/sev: Perform padding calculations at compile-time

[Dov Murik <dovmurik@linux.ibm.com>]

In sev_add_kernel_loader_hashes, the sizes of structs are known at
compile-time, so calculate needed padding at compile-time.

No functional change intended.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Brijesh Singh <brijesh.singh@amd.com>
---
 target/i386/sev.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/target/i386/sev.c b/target/i386/sev.c
index d11b512361..4fd258a570 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -110,9 +110,19 @@ typedef struct QEMU_PACKED SevHashTable {
     SevHashTableEntry cmdline;
     SevHashTableEntry initrd;
     SevHashTableEntry kernel;
-    uint8_t padding[];
 } SevHashTable;
 
+/*
+ * Data encrypted by sev_encrypt_flash() must be padded to a multiple of
+ * 16 bytes.
+ */
+typedef struct QEMU_PACKED PaddedSevHashTable {
+    SevHashTable ht;
+    uint8_t padding[ROUND_UP(sizeof(SevHashTable), 16) - sizeof(SevHashTable)];
+} PaddedSevHashTable;
+
+QEMU_BUILD_BUG_ON(sizeof(PaddedSevHashTable) % 16 != 0);
+
 static SevGuestState *sev_guest;
 static Error *sev_mig_blocker;
 
@@ -1216,12 +1226,12 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
     uint8_t *data;
     SevHashTableDescriptor *area;
     SevHashTable *ht;
+    PaddedSevHashTable *padded_ht;
     uint8_t cmdline_hash[HASH_SIZE];
     uint8_t initrd_hash[HASH_SIZE];
     uint8_t kernel_hash[HASH_SIZE];
     uint8_t *hashp;
     size_t hash_len = HASH_SIZE;
-    int aligned_len = ROUND_UP(sizeof(SevHashTable), 16);
 
     /*
      * Only add the kernel hashes if the sev-guest configuration explicitly
@@ -1237,7 +1247,7 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
         return false;
     }
     area = (SevHashTableDescriptor *)data;
-    if (!area->base || area->size < aligned_len) {
+    if (!area->base || area->size < sizeof(PaddedSevHashTable)) {
         error_setg(errp, "SEV: guest firmware hashes table area is invalid "
                          "(base=0x%x size=0x%x)", area->base, area->size);
         return false;
@@ -1282,7 +1292,8 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
      * Populate the hashes table in the guest's memory at the OVMF-designated
      * area for the SEV hashes table
      */
-    ht = qemu_map_ram_ptr(NULL, area->base);
+    padded_ht = qemu_map_ram_ptr(NULL, area->base);
+    ht = &padded_ht->ht;
 
     ht->guid = sev_hash_table_header_guid;
     ht->len = sizeof(*ht);
@@ -1299,13 +1310,10 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
     ht->kernel.len = sizeof(ht->kernel);
     memcpy(ht->kernel.hash, kernel_hash, sizeof(ht->kernel.hash));
 
-    /* When calling sev_encrypt_flash, the length has to be 16 byte aligned */
-    if (aligned_len != ht->len) {
-        /* zero the excess data so the measurement can be reliably calculated */
-        memset(ht->padding, 0, aligned_len - ht->len);
-    }
+    /* zero the excess data so the measurement can be reliably calculated */
+    memset(padded_ht->padding, 0, sizeof(padded_ht->padding));
 
-    if (sev_encrypt_flash((uint8_t *)ht, aligned_len, errp) < 0) {
+    if (sev_encrypt_flash((uint8_t *)padded_ht, sizeof(*padded_ht), errp) < 0) {
         return false;
     }
 
-- 
2.25.1