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Using python for writing models: How to run models in restricted python mode?

 
 
vinjvinj
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      11-07-2005
I have an application which allows multiple users to write models.
These models get distributed on a grid of compute engines. users submit
their models through a web interface. I want to

1. restrict the user from doing any file io, exec, import, eval, etc. I
was thinking of writing a plugin for pylint to do all the checks? Is
this is a good way given that there is no restricted python. What are
the things I should serach for in python code

2. restrict the amount of memory a module uses as well. For instance
how can I restrict a user from doing a = range(10000000000) or similar
tasks so that my whole compute farm does not come down.

Thanks for your help

 
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Mike Meyer
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      11-07-2005
"vinjvinj" <(E-Mail Removed)> writes:
> 1. restrict the user from doing any file io, exec, import, eval, etc. I
> was thinking of writing a plugin for pylint to do all the checks? Is
> this is a good way given that there is no restricted python. What are
> the things I should serach for in python code


Um - I've got a restricted python module: rexec.py. Of course, it
doesn't work correctly, in that it isn't really secure. Python is very
powerful, and creating a secure sandbox is difficult - so much so that
the task has never been accomplished. If you want something that will
keep the obvious things from working, rexec.py might be for you - but
don't kid yourself that it's secure. If you need real security, I'd
consider switching to Jython, which at least has a VM which was
designed with building such sandboxes as a possibility.

> 2. restrict the amount of memory a module uses as well. For instance
> how can I restrict a user from doing a = range(10000000000) or similar
> tasks so that my whole compute farm does not come down.


This is equivalent to trying to limit the amount of CPU time the
module uses, which is better known as the halting problem. There's no
algorithmic solution to that. If you want verify that some module will
only use so much memory before executing it, the best you can do is
verify that they don't do anything obvious. If you want to restrict
them while they are running, you can probably get the OS to
help. Exactly how will depend on your requirements, and the OS
involved.

<Mike
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Mike Meyer <(E-Mail Removed)> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
 
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vinjvinj
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      11-07-2005
While I understand 2 is very hard (if not impossible) to do in single
unix process. I'm not sure why 1 would be hard to do. Since I have
complete control to what code I can allow or not allow on my grid. Can
i not just search for certain strings and disallow the model if it
fails certain conditions. It might not be 100% secure but will it not
get me at 90%...

 
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Mike Meyer
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      11-07-2005
"vinjvinj" <(E-Mail Removed)> writes:

> While I understand 2 is very hard (if not impossible) to do in single
> unix process. I'm not sure why 1 would be hard to do. Since I have
> complete control to what code I can allow or not allow on my grid. Can
> i not just search for certain strings and disallow the model if it
> fails certain conditions. It might not be 100% secure but will it not
> get me at 90%...


Sure you can search for certain strings. Python lets you build strings
dynamically, so you'd have to search for every possible way to create
those strings. Further, Python provides lots of tools for
introspection, meaning there are lots of ways to find these
"forbidden" objects other than mentioning their name.

You can get to *every* builtin function through any python module. For
instance, are you going to prevent them from using regular
rexpressions? If not, consider:

>>> getattr(re, ''.join([chr(x + 1) for x in [94, 94, 97, 116, 104, 107, 115, 104, 109, 114, 94, 94]]))['fi' + 'le'] is open

True
>>>


String searches only prevent the most obvious abuses, and may well
miss things that are merely not quite so obvious. If you think of your
"security" as a notice to the end user that they are doing something
wrong, as opposed to a tool that will prevent them from doing it, then
you'll have the right idea. In which case, I'd still recommend looking
into the rexec module.

<mike
--
Mike Meyer <(E-Mail Removed)> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
 
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Steven D'Aprano
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      11-08-2005
vinjvinj wrote:

> While I understand 2 is very hard (if not impossible) to do in single
> unix process. I'm not sure why 1 would be hard to do. Since I have
> complete control to what code I can allow or not allow on my grid. Can
> i not just search for certain strings and disallow the model if it
> fails certain conditions. It might not be 100% secure but will it not
> get me at 90%...


You might be able to think of and disallow the most
obvious security holes, but how confident are you that
you will think of the bad code that your users will
think of?

Are you concerned about malicious users, or just
incompetent users?

I suspect your best bet might be to write a
mini-language using Python, and get your users to use
that. You will take a small performance hit, but
security will be very much improved.

What do others think?


--
Steven.

 
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Paul Rubin
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      11-08-2005
Steven D'Aprano <(E-Mail Removed)> writes:
> I suspect your best bet might be to write a mini-language using
> Python, and get your users to use that. You will take a small
> performance hit, but security will be very much improved.
>
> What do others think?


That is the only approach that makes any sense. Even with restricted
execution there's no way to stop memory exhaustion with restricted
Python statements. Consider

xxx = 'x'*10000000000
 
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vinjvinj
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      11-08-2005
I'm more worried about incompetent users then malicious users. I'm
going to take the following steps:

1. My users will be paying a decent amount of money to run models on
the compute grid. If they are intentionaly writing malicious code then
their account will be disabled.

2. Since their models will be fairly basic.
- No imports in the code.
- No special charters allowed.
- No access to special builtins.

The users write functions which get called man many times with
different variables. I'm not sure how this would work with the rexec
module especially since I'll be passing values to th functions and the
functions will be returning either None, yes, or False.

3. Pylint has a pretty cool way to write your onw custom plugins. You
can write custom handlers for each sort of available node at:
http://www.python.org/doc/current/li...piler.ast.html
this will allow me to compile a module and give users feedback on what
is wrong and what is not allowed.

4. I'll set up a test sandbox where the models will be run with a
smaller dataaset before then can be pushed into production. if the
models pass the sandbox test then they will be run in production.

I'm going to have write some custom performance monitoring functions to
get notified when some models are running for ever and be able to
terminate them.

vinjvinj

 
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vinjvinj
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      11-08-2005
I have so many things to do to get this to production and writing a
mini language would be a full project in itself. :-<.

Is there an easy way to do this? If not, I'll go with the steps
outlined in my other post.

vinjvinj

 
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Jeffrey Schwab
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      11-08-2005
vinjvinj wrote:
> I have so many things to do to get this to production and writing a
> mini language would be a full project in itself. :-<.
>
> Is there an easy way to do this? If not, I'll go with the steps
> outlined in my other post.


Do you really think it will be faster to start parsing Python code,
looking for potentially dangerous constructs?
 
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vinjvinj
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      11-08-2005
No. I was hoping to leverage the work done for restricted pythonscript
by zope at:

http://www.zope.org/Control_Panel/Pr...ythonScript.py

which is similar to what I want to do as well.

vinjvinj

 
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