Psyntax security hole prevents secure sandboxing in Guile

[Mark H Weaver <mhw@netris.org>]

Hello all,

Every once in a while someone asks about secure sandboxing with Guile,
and generally the response is that it should be fairly easy, by creating
a module with carefully selected bindings, but there's nothing ready
"out of the box".

I just realized that psyntax has a security hole that prevents secure
sandboxing, and wanted to post this fact before it was forgotten.

The problem is that psyntax accepts syntax-objects in the input, and
syntax-objects are simply vectors (or sexps containing vectors).
Therefore, it is always possible to _forge_ syntax-objects that refer to
arbitrary bindings in arbitrary modules, even if the usual bindings of
'@' and '@@' are not available.

In particular (although this is an internal implementation detail that
you cannot rely upon!) in Guile 2.0 the following two expressions are
treated equivalently:

  (@@ (ice-9 popen) open-pipe*)

  #(syntax-object open-pipe* ((top)) (hygiene ice-9 popen))

I don't think we can plug this hole until 2.2.

     Mark