[PATCH 2/3] sev/i386: Warn if using -kernel with invalid OVMF hashes table area

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

Commit cff03145ed3c ("sev/i386: Introduce sev_add_kernel_loader_hashes
for measured linux boot", 2021-09-30) introduced measured direct boot
with -kernel, using an OVMF-designated hashes table which QEMU fills.

However, no checks are performed on the validity of the hashes area
designated by OVMF.  Specifically, if OVMF publishes the
SEV_HASH_TABLE_RV_GUID entry but it is filled with zeroes, this will
cause QEMU to write the hashes entries over the first page of the
guest's memory (GPA 0).

Add validity checks to the published area.  If the hashes table area's
base address is zero, or its size is too small to fit the aligned hashes
table, warn and skip the hashes entries addition.  In such case, the
following warning will be displayed:

    qemu-system-x86_64: warning: SEV: OVMF's hashes table area is invalid (base=0x0 size=0x0)

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reported-by: Brijesh Singh <brijesh.singh@amd.com>
---
 target/i386/sev.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/target/i386/sev.c b/target/i386/sev.c
index 682b8ccf6c..a20ddb545e 100644
--- a/target/i386/sev.c
+++ b/target/i386/sev.c
@@ -1201,13 +1201,18 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
     uint8_t kernel_hash[HASH_SIZE];
     uint8_t *hashp;
     size_t hash_len = HASH_SIZE;
-    int aligned_len;
+    int aligned_len = ROUND_UP(sizeof(SevHashTable), 16);
 
     if (!pc_system_ovmf_table_find(SEV_HASH_TABLE_RV_GUID, &data, NULL)) {
         warn_report("SEV: kernel specified but OVMF has no hash table guid");
         return false;
     }
     area = (SevHashTableDescriptor *)data;
+    if (!area->base || area->size < aligned_len) {
+        warn_report("SEV: OVMF's hashes table area is invalid (base=0x%x size=0x%x)",
+                    area->base, area->size);
+        return false;
+    }
 
     /*
      * Calculate hash of kernel command-line with the terminating null byte. If
@@ -1266,7 +1271,6 @@ bool sev_add_kernel_loader_hashes(SevKernelLoaderContext *ctx, Error **errp)
     memcpy(ht->kernel.hash, kernel_hash, sizeof(ht->kernel.hash));
 
     /* When calling sev_encrypt_flash, the length has to be 16 byte aligned */
-    aligned_len = ROUND_UP(ht->len, 16);
     if (aligned_len != ht->len) {
         /* zero the excess data so the measurement can be reliably calculated */
         memset(ht->padding, 0, aligned_len - ht->len);
-- 
2.25.1