[Python-Dev] In defense of Capabilities [was: doc for new restricted execution design for Python]

[Talin]

Brett Cannon wrote:
> On 7/6/06, Talin <talin at acm.org> wrote:
>> And if we can call it for every operation, then we don't have to spend
>> time hunting down all of the possible loopholes and ways in which 'file'
>> or other restricted objects might be accessed.
> 
> Not true.  You have to set this object restriction flag, right?  What
> happens if you don't set it on all of the proper classes/types?  You end up
> in the exact same situation you are with crippling; making sure you cover
> your ass with what you flag as  unsafe else you risk having something get
> passed you.

But that's a much simpler problem.

With the restricted flag, it isn't just *your* code that is prevented 
from using 'file' - it's *all* code. Only approved gateways that remove 
the restriction (by setting the interpreter state) can perform 
operations on file objects without blowing up.

This means that if you call some random library function that attempts 
to open a file, it won't work, because the random library function is 
still running in restricted mode.

Similarly, if you have a reference to some externally created object 
that has a reference to a file (or the file class) somewhere in it's 
inheritance hierarchy, any attempt to access that object will fail.

Without this, you would have to chase down every bit of library code 
that opens file, or has a reference to a file.

What I am proposing shares some aspects of both the crippling and the 
capability model:

It's similar to crippling in the sense that you're protecting the object 
itself, not access to the object. So you avoid the problem of trying to 
figure out all of the possible ways an object can be accessed.

However, where it resembles capabilities is that its an 'all or nothing' 
approach - that is, you either have access to file, or you don't. Unlike 
the crippling model where fine-grained access control is implemented by 
modifying individual methods of the crippled object, in this scheme we 
cripple the object *entirely*, and then provide fine-grained access 
control via wrappers. Those wrappers, in turn, act just like 
capabilities - you can have different wrappers that have different sets 
of access permissions.

So it provides the advantage of the capability approach in that the set 
of restrictions can be extended or modified by writing new wrappers.

Thus, by providing an extremely simple but unbreakable check at the 
interpreter level, we can then write classes that build on top of that a 
richer and more sophisticated set of permissions, while still 
maintaining a strong barrier to unauthorized actions.

-- Talin