qcow2: Avoid overflow in alloc_clusters_noref()

alloc_clusters_noref() stores the cluster index in a uint64_t. However,
offsets are often represented as int64_t (as for example the return
value of alloc_clusters_noref() itself demonstrates). Therefore, we
should make sure all offsets in the allocated range of clusters are
representable using int64_t without overflows.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-refcount.c

@@ -653,6 +653,13 @@ retry:
             goto retry;
         }
     }
+
+    /* Make sure that all offsets in the "allocated" range are representable
+     * in an int64_t */
+    if (s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits)) {
+        return -EFBIG;
+    }
+
 #ifdef DEBUG_ALLOC2
     fprintf(stderr, "alloc_clusters: size=%" PRId64 " -> %" PRId64 "\n",
             size,