limited python virtual machine (WAS: Another scripting language implemented into Python itself?)

Christophe Cavalaria chris.cavalaria at free.fr
Sun Jan 30 00:05:41 CET 2005


Steven Bethard wrote:

> Fuzzyman wrote:
>  > Cameron Laird wrote:
>  > [snip..]
>  >
>  >>This is a serious issue.
>  >>
>  >>It's also one that brings Tcl, mentioned several
>  >>times in this thread, back into focus.  Tcl presents
>  >>the notion of "safe interpreter", that is, a sub-
>  >>ordinate virtual machine which can interpret only
>  >>specific commands.  It's a thrillingly powerful and
>  >>correct solution to the main problem Jeff and others
>  >>have described.
>  >
>  > A better (and of course *vastly* more powerful but unfortunately only
>  > a dream ;-) is a similarly limited python virutal machine.....
> 
> Yeah, I think there are a lot of people out there who would like
> something like this, but it's not quite clear how to go about it.  If
> you search Google Groups, there are a lot of examples of how you can use
> Python's object introspection to retrieve "unsafe" functions.
> 
> I wish there was a way to, say, exec something with no builtins and with
> import disabled, so you would have to specify all the available
> bindings, e.g.:
> 
>      exec user_code in dict(ClassA=ClassA, ClassB=ClassB)
> 
> but I suspect that even this wouldn't really solve the problem, because
> you can do things like:
> 
> py> class ClassA(object):
> ...     pass
> ...
> py> object, = ClassA.__bases__
> py> object
> <type 'object'>
> py> int = object.__subclasses__()[2]
> py> int
> <type 'int'>
> 
> so you can retrieve a lot of the builtins.  I don't know how to retrieve
>   __import__ this way, but as soon as you figure that out, you can then
> do pretty much anything you want to.
> 
> Steve

Wouldn't it be better to attach to all code objets some kind of access right
marker and to create an opcode that calls a function while reducing the
access rights ? After all, security would be easier to achieve if you
prevented the execution of all the dangerous code rather than trying to
hide all the entry points to it.



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