Creating a capabilities-based restricted execution system

[Sean R. Lynch]

I've been playing around with Zope's RestrictedPytho n, and I think I'm
on the way to making the modifications necessary to create a
capabilities-based restricted execution system. The idea is to strip out
any part of RestrictedPytho n that's not necessary for doing capabilities
and do all security using just capabilities.

The basic idea behind capabilities is that you don't give any piece of
code you don't trust a reference to something you don't want it to have
access to. You use proxies instead (E calls them "facets").

In order to be able to allow untrusted code to create proxy objects, I
needed to be able to store a reference to the proxied object in a
private attribute.

To create private attributes, I'm using "name mangling," where names
beginning with X_ within a class definition get changed to
_<uuid>_<name> , where the UUID is the same for that class. The UUIDs
don't need to be secure because it's not actually possible to create
your own name starting with an underscore in RestrictedPytho n; they just
need to be unique across all compiler invocations.

The nice thing about using this name mangling is that it's only done at
compile time and doesn't affect runtime performance. An interesting side
effect is that code defined on a class can access private attributes on
all descendants of that class, but only ones that are defined by other
code on that class, so this isn't a security issue.

I was thinking I needed read-only attributes to be able to avoid
untrusted code's being able to sabotage the revoke method on a proxy
object, but I'm thinking that just keeping around a reference to the
revoke method in the original code may be enough.

Does anyone think I'm going in completely the wrong direction here? Am I
missing anything obvious?

[Paul Rubin]

"Sean R. Lynch" <se***@chaosrin g.org> writes:

Does anyone think I'm going in completely the wrong direction here? Am
I missing anything obvious?

Well, I have a dumb question. Have you studied the security failures
of rexec/Bastion and convinced yourself that they don't happen to your
new scheme?

You might look at the PyPy architecture doc if you haven't yet.
Making a separate object space for restricted objects may fit PyPy's
design quite naturally.

[Sean R. Lynch]

Paul Rubin wrote:

Well, I have a dumb question. Have you studied the security failures
of rexec/Bastion and convinced yourself that they don't happen to your
new scheme?
If you know of a location where the known shortcomings of rexec are
documented, please let me know. So far I've only seen a couple examples
and a lot of people saying "it's not secure so let's disable it."

My current methodology is to be very careful about adding any privileges
beyond what RestrictedPytho n allows.
You might look at the PyPy architecture doc if you haven't yet.
Making a separate object space for restricted objects may fit PyPy's
design quite naturally.

I have looked at PyPy. It's very interesting, but RestrictedPytho n is
already written and in use in Zope.

I think I've figured out a way to use my name mangling scheme to make
attributes only *writable* by code defined on a class from which an
object descends: do writes through a name-mangled method, and have
RestrictedPytho n output self._mangled_s etattr(attr, val) for each
attempted attribute assignment. This will basically make it impossible
to have attributes that are writable from other classes, but I think
it's probably a prerequisite for capabilities. Most other languages
require attributes to be set via methods anyway, right?

[Martin v. Loewis]

Sean R. Lynch wrote:

If you know of a location where the known shortcomings of rexec are
documented, please let me know. So far I've only seen a couple examples
and a lot of people saying "it's not secure so let's disable it."

The biggest problem is that new-style classes are both available through
the type() builtin, and callable to create new instances.

For example, if you have managed to open a file object f, then

type(f)("/etc/passwd").read()

lets you access a different file, bypassing all machinery that may
have been designed to prevent that from happening.

Of course, for the specific case of file objects, there is additional
machinery preventing that from happening, but in the general case,
there might be more problems in that area. For example,
object.__subcla sses__() gives you access to quite a lot of stuff.

Regards,
Martin

[John Roth]

"Sean R. Lynch" <se***@chaosrin g.org> wrote in message
news:Lm******** ************@sp eakeasy.net...

[...]

Does anyone think I'm going in completely the wrong direction here? Am I
missing anything obvious?

Yes, you're missing something really obvious. Multi-level
security is a real difficult problem if you want to solve it
in a believable (that is, bullet-proof) fashion. The only way
I know of solving it is to provide separate execution
environments for the different privilege domains.
In the current Python structure, that means different
interpreters so that the object structures don't intermix.

If you have separate domains, then the only support
needed is to remove privileged modules from the
built-ins, and virtualize import so that it won't load
modules that aren't on the approved list for that
domain.

You also, of course, need some form of gate between
the untrusted and trusted domains.

Once that's done, there's no reason to layer additional
complexity on top, and there is no reason to restrict
any introspection facilities.

John Roth

[Sean R. Lynch]

John Roth wrote:

Yes, you're missing something really obvious. Multi-level
security is a real difficult problem if you want to solve it
in a believable (that is, bullet-proof) fashion. The only way
I know of solving it is to provide separate execution
environments for the different privilege domains.
In the current Python structure, that means different
interpreters so that the object structures don't intermix.

Hmmm, can you give me an example of a Python application that works this
way? Zope seems to be doing fine using RestrictedPytho n.
RestrictedPytho n is, in fact, an attempt to provide different execution
environments within the same memory space, which is the whole point of
my exercise. Now, I know that the lack of an example of insecurity is
not proof of security, but can you think of a way to escape from
RestrictedPytho n's environment? DoS is still possible, but as I'm not
planning on using this for completely untrusted users, I'm not too
concerned about that.

[Sean R. Lynch]

Martin v. Loewis wrote:

The biggest problem is that new-style classes are both available through
the type() builtin, and callable to create new instances.

For example, if you have managed to open a file object f, then

type(f)("/etc/passwd").read()

lets you access a different file, bypassing all machinery that may
have been designed to prevent that from happening.

Of course, for the specific case of file objects, there is additional
machinery preventing that from happening, but in the general case,
there might be more problems in that area. For example,
object.__subcla sses__() gives you access to quite a lot of stuff.

RestrictedPytho n avoids this by removing the type() builtin from the
restricted __builtins__, and it doesn't allow untrusted code to create
names that start with _. Zope3 has a type() builtin, but it returns a
proxy (written in C) to the type object to prevent access.

Right now I'm providing a same_type function instead to compare types.
Later I'll probably start playing around with C proxies.

I think the main thing that's liable to introduce new security problems
(beyond what RestrictedPytho n may already have) is the fact that
RestrictedPytho n is mostly designed to protect the trusted environment
from the untrusted environment, and what I'd really like to do is give
programmers in the untrusted environment a way to create objects and
pass them around to one another; for example, in the original setup,
class statements are allowed but not very useful in the restricted
environment, because objects created from those classes would be
read-only due to the fact that you can't create any special attributes
to tell the system how to handle security from within the restricted
environment, which is why I'm adding private attributes to the system
and figuring out a way to allow methods defined on a class to assign to
attributes on instances of that class without allowing all code to do so.

[Serge Orlov]

"Sean R. Lynch" <se***@chaosrin g.org> wrote in message news:Lm******** ************@sp eakeasy.net...

I've been playing around with Zope's RestrictedPytho n, and I think I'm
on the way to making the modifications necessary to create a
capabilities-based restricted execution system. The idea is to strip out
any part of RestrictedPytho n that's not necessary for doing capabilities
and do all security using just capabilities.

The basic idea behind capabilities is that you don't give any piece of
code you don't trust a reference to something you don't want it to have
access to. You use proxies instead (E calls them "facets").
"Don't give" sounds good in theory but fails in practice. You can't prevent
leakage 100%, so any security system _must_ help programmer to keep
trusted data away from untrusted code. Do you know that rexec failed
exactly because it didn't help to prevent leakage?

In order to be able to allow untrusted code to create proxy objects, I
needed to be able to store a reference to the proxied object in a
private attribute.

To create private attributes, I'm using "name mangling," where names
beginning with X_ within a class definition get changed to
_<uuid>_<name> , where the UUID is the same for that class. The UUIDs
don't need to be secure because it's not actually possible to create
your own name starting with an underscore in RestrictedPytho n; they just
need to be unique across all compiler invocations.
This is a problem: you declare private attributes whereas you should be
declaring public attributes and consider all other attributes private. Otherwise
you don't help prevent leakage. What about doing it this way:

obj.attr means xgetattr(obj,ac c_tuple) where acc_tuple = ('attr',UUID)
and xgetattr is
def xgetattr(obj,ac c_tuple):
if not has_key(obj.__a ccdict__,acc_tu ple):
raise AccessException
return getattr(obj,acc _tuple[0])

__accdict__ is populated at the time class or its subclasses are created.
If an object without __accdict__ is passed to untrusted code it will
just fail. If new attributes are introduced but not declared in __accdict__
they are also unreachable by default.

The nice thing about using this name mangling is that it's only done at
compile time and doesn't affect runtime performance. An interesting side
effect is that code defined on a class can access private attributes on
all descendants of that class, but only ones that are defined by other
code on that class, so this isn't a security issue.

I was thinking I needed read-only attributes to be able to avoid
untrusted code's being able to sabotage the revoke method on a proxy
object, but I'm thinking that just keeping around a reference to the
revoke method in the original code may be enough.

Does anyone think I'm going in completely the wrong direction here? Am I
missing anything obvious?

It depends on what type of security do you want. Did you think about DOS
and covert channels? If you don't care about that, yeah, you don't miss
anything obvious. <wink> you should worry whether you miss something
non-obvious.

By the way, did you think about str.encode? Or you are not worried about
bugs in zlib too?

-- Serge.

[Sean R. Lynch]

I hate replying to myself, but I've written some more code. I hope to
have something posted soon so people can rip it apart without needing to
resort to conjecture :)

I had been considering using a name-mangled setattr for doing attribute
assignment to only allow assignment to attributes on descendants of the
class one was writing methods on, but it occurred to me that I could
probably treat "self" as a special name using only compiler
modifications, so I could eliminate RestrictedPytho n's need to turn all
Getattrs and AssAttrs (shouldn't it be GetAttr) into method calls. Now,
of course, I'm limited to static checks on names to control access, but
Python already disallows, for example, access to f.func_globals, and
RestrictedPytho n disallows names that begin with underscore.

Now I need to write a bunch of code that uses this system and attempts
to break it :)

[Sean R. Lynch]

Serge Orlov wrote:

"Sean R. Lynch" <se***@chaosrin g.org> wrote in message news:Lm******** ************@sp eakeasy.net...

I've been playing around with Zope's RestrictedPytho n, and I think I'm
on the way to making the modifications necessary to create a
capabilitie s-based restricted execution system. The idea is to strip out
any part of RestrictedPytho n that's not necessary for doing capabilities
and do all security using just capabilities.

The basic idea behind capabilities is that you don't give any piece of
code you don't trust a reference to something you don't want it to have
access to. You use proxies instead (E calls them "facets").

"Don't give" sounds good in theory but fails in practice. You can't prevent
leakage 100%, so any security system _must_ help programmer to keep
trusted data away from untrusted code. Do you know that rexec failed
exactly because it didn't help to prevent leakage?

Hmm, this is good information. I think it will probably change the way
I've been looking at this.

In order to be able to allow untrusted code to create proxy objects, I
needed to be able to store a reference to the proxied object in a
private attribute.

To create private attributes, I'm using "name mangling," where names
beginning with X_ within a class definition get changed to
_<uuid>_<name >, where the UUID is the same for that class. The UUIDs
don't need to be secure because it's not actually possible to create
your own name starting with an underscore in RestrictedPytho n; they just
need to be unique across all compiler invocations.

This is a problem: you declare private attributes whereas you should be
declaring public attributes and consider all other attributes private. Otherwise
you don't help prevent leakage. What about doing it this way:

obj.attr means xgetattr(obj,ac c_tuple) where acc_tuple = ('attr',UUID)
and xgetattr is
def xgetattr(obj,ac c_tuple):
if not has_key(obj.__a ccdict__,acc_tu ple):
raise AccessException
return getattr(obj,acc _tuple[0])

__accdict__ is populated at the time class or its subclasses are created.
If an object without __accdict__ is passed to untrusted code it will
just fail. If new attributes are introduced but not declared in __accdict__
they are also unreachable by default.

This is very interesting, and you may convince me to use something
similar, but I don't think you're quite correct in saying that the
name-mangling scheme declares private attributes; what is the difference
between saying "not having X_ in front of the attribute makes it public"
and "having X_ in front of the attribute makes it private?"

The nice thing about using this name mangling is that it's only done at
compile time and doesn't affect runtime performance. An interesting side
effect is that code defined on a class can access private attributes on
all descendants of that class, but only ones that are defined by other
code on that class, so this isn't a security issue.

I was thinking I needed read-only attributes to be able to avoid
untrusted code's being able to sabotage the revoke method on a proxy
object, but I'm thinking that just keeping around a reference to the
revoke method in the original code may be enough.

Does anyone think I'm going in completely the wrong direction here? Am I
missing anything obvious?

It depends on what type of security do you want. Did you think about DOS
and covert channels? If you don't care about that, yeah, you don't miss
anything obvious. <wink> you should worry whether you miss something
non-obvious.

I am not (particularly) concerned about DoS because I don't plan to be
running anonymous code and having to restart the server isn't that big
of a deal. I do plan to make it hard to accidentally DoS the server, but
I'm not going to sacrifice a bunch of performance for that purpose. As
for covert channels, can you give me an example of what to look for?

I am certainly worried about non-obvious things, but my intent wasn't to
put up a straw man, because if I ask if I'm missing non-obvious things,
the only possible answer is "of course."
By the way, did you think about str.encode? Or you are not worried about
bugs in zlib too?

Well, it'll only take *one* problem of that nature to force me to go
back to converting all attribute accesses to function calls. On the
other hand, as long as any problem that allows a user to access
protected data is actually a in (zlib, etc), I think I'm not going to
worry about it too much yet. If there is some method somewhere that will
allow a user access to protected data that is not considered a bug in
that particular subsystem, then I have to fix it in my scheme, which
would probably require going back to converting attribute access to
method calls.