[Python-Dev] Challenge: Please break this! (a.k.a restricted mode revisited)

[Jon Ribbens]

On Tue, Apr 12, 2016 at 03:02:54AM +1000, Chris Angelico wrote:
> On Tue, Apr 12, 2016 at 2:53 AM, Jon Ribbens
> <jon+python-dev at unequivocal.co.uk> wrote:
> > On Mon, Apr 11, 2016 at 04:04:21PM +0100, Paul Moore wrote:
> >> However, it's not at all clear (to me at least) what you *are* trying
> >> to do.
> >
> > I'm trying to see to what extent we can use ast node inspection to
> > remedy the failures of prior attempts at Python sandboxing. Is there
> > *any* extent to which Python can be sandboxed, or is even trying to
> > use it as a calculator function unfixably insecure?
> 
> It all depends on how much functionality you want. If all you need is
> a numeric expression evaluator, that's not too hard - disallow all
> forms of attribute access, etc, and just have simple numbers and
> operators. That's pretty useful, and safe.

By "calculator" I didn't necessarily mean to imply numeric-only,
sorry if I was unclear. Also perhaps I should have said "non-trivial",
inasmuch as if we restrict it that far then it would quite possibly be
simpler and quicker just to write the expression evaluator from scratch
and not use the Python interpreter at all.

> Alternatively, go completely the other way. Let people run whatever
> code they like... in an environment where it can't hurt anyone else.
> That's what PyPyJS does - don't bother looking for security holes in
> it, because all you're doing is attacking your own computer.

That's a very specific use case though: running client-side in the
user's browser.

> So before you can ask whether Python is unfixably insecure, you first
> have to decide what the minimum level of functionality is that you'll
> accept. Do you need basic arithmetic plus trignometric functions? Easy
> enough - disallow all attribute access and imports, and populate
> builtins with "from math import *". Need them to be able to assign
> variables and define functions? That's gonna be harder.

I think calling functions and accessing variables and attributes is
likely a minimum. Defining functions would be useful, and of course
defining classes would be another useful step further.