block/vmdk: fix OOB read in vmdk_read_extent()

Bounds check for marker.size doesn't account for the 12-byte marker
header, allowing zlib to read past the allocated buffer.

Move the check inside the has_marker block and subtract the marker size.

Fixes: CVE-2026-2243
Reported-by: Halil Oktay (oblivionsage) <cookieandcream560@gmail.com>
Signed-off-by: Halil Oktay (oblivionsage) <cookieandcream560@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -1951,10 +1951,10 @@ vmdk_read_extent(VmdkExtent *extent, int64_t cluster_offset,
         marker = (VmdkGrainMarker *)cluster_buf;
         compressed_data = marker->data;
         data_len = le32_to_cpu(marker->size);
-    }
-    if (!data_len || data_len > buf_bytes) {
-        ret = -EINVAL;
-        goto out;
+        if (!data_len || data_len > buf_bytes - sizeof(VmdkGrainMarker)) {
+            ret = -EINVAL;
+            goto out;
+        }
     }
     ret = uncompress(uncomp_buf, &buf_len, compressed_data, data_len);
     if (ret != Z_OK) {