v2v: -o libvirt: Prevent possible XPath injection.

Ensure the arch string is sane before using it in the following XPath
expression.  Since the arch string can be derived from untrusted guest
data [see src/filearch.c], this prevents a possible XPath injection
vulnerability.

v2v/output_libvirt.ml

@@ -30,9 +30,15 @@ module StringSet = Set.Make (String)
 let string_set_of_list =
   List.fold_left (fun set x -> StringSet.add x set) StringSet.empty
 
+let arch_sanity_re = Str.regexp "^[-_A-Za-z0-9]+$"
+
 let target_features_of_capabilities_doc doc arch =
   let xpathctx = Xml.xpath_new_context doc in
   let expr =
+    (* Check the arch is sane.  It comes from untrusted input.  This
+     * avoids XPath injection below.
+     *)
+    assert (Str.string_match arch_sanity_re arch 0);
     (* NB: Pay attention to the square brackets.  This returns the
      * <guest> nodes!
      *)