Re: Google Apps Engine

If you own the interpreter codebase, shouldn't it be possible just to
hook libc's open(2) stub, and give a unique signature to calls that
originated on a trusted code path? This doesn't seem at all hard to
me.

On 4/12/08, Aidan Thornton <makosoft_at_googlemail.com> wrote:
>
> On 4/11/08, Lutz Böhne <lboehne_at_damogran.de> wrote:
> > > Even those could easily be sanitized by just some fun with function
> > > pointers.
> > >
> > > >>> open=lambda *x: "no"
> > > >>> open('/etc/passwd')
> > > 'no'
> >
> > Unless there are other ways to find these functions:
> >
> > >>> __builtins__.__dict__["open"]( '/etc/passwd')
> > <open file '/etc/passwd', mode 'r' at 0xb7dac7b8>
> >
> > or even:
> >
> > >>> open=lambda *x: "no"
> > >>> open('/etc/passwd')
> > 'no'
> > >>> del open
> > >>> open('/etc/passwd')
> > <open file '/etc/passwd', mode 'r' at 0xb7db44a0>
> >
> > Python is fun, there are so many ways to have it do what you want ;)
> >
> > It might be possible to remove these functions like this:
> >
> > >>> del __builtins__.__dict__["open"]
> > >>> open('/etc/passwd')
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > NameError: name 'open' is not defined
> > [...]
> >
> > But i don't know whether that'd get rid of all problems.
> >
> > Best regards,
> >
> > Lutz
> >
>
>
> Hi,
>
> The quick answer is no, it wouldn't be enough. For example, try
> type(sys.stdin)('/etc/passwd') or the equivalent
> sys.stdin.__class__('/etc/passwd'). Also, as
> http://mail.python.org/pipermail/python-dev/2006-July/067291.html
> points out, file can be obtained from object.__subclasses__(). (object itself can be found by working up the inheritance tree from any new-style class - say, a string - using __bases__)
>
> Python's powerful introspection support and lack of data hiding make
> doing any sort of meaningful sandboxing within the language itself very difficult. There used to be a bundled module called rexec to do this (via a combination of hooks into the interpreter and built-in support), but it was depreciated due to security issues. They might be doing something similar - it seems to strip what functions from native-code modules can be imported to some safe whitelist (and load all modules written in Python within the sandbox).
>
>
> Aidan
>
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-- 
---
Thomas H. Ptacek // matasano security
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