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Zope3 Security
Introduction
The Security framework provides a generic mechanism to implement
security policies on Python objects. This introduction provides a
tutorial of the framework explaining concepts, design, and going
through sample usage from the perspective of a Python programmer
using the framework outside of Zope.
Definitions
Principal
A generalization of a concept of a user. A principal may be
associated with different roles and permissions.
Permission
A kind of access, i.e. permission to READ vs. permission to
WRITE. Fundamentally the whole security framework is organized
around checking permissions on objects.
Roles
Represents a responsibility of a user in the context of an
object. Roles are associated with the permissions necessary to
fulfill the user's responsibility.
Purpose
The security framework's primary purpose is to guard and check
access to Python objects. It does this by providing mechanisms
for explicit and implicit security checks on attribute access for
objects. Attribute names are mapped onto permission names when
checking access and the implementation of the security check is
defined by the security policy, which receives the object, the
permission name, and a context.
Security contexts are containers of transient information such as
the current principal and the context stack.
To explain the concept and usage of the context stack, a little
background into the design influences of the default Zope policy
is needed, namely the Java language security model. Within the
base language, code is associated with identifiers. I.e. this code
came from "Joe Schmoe", and another code archive comes signed from
Verisign. When executing restricted code, it's important access
is checked not only for the code currently executing but for the
entire call/context stack (unless explicitly short-circuited).
I.e. if Joe Schmoe's code does haven't access to the filesystem,
but if the Verisign code does, Joe's code could circumvent the
security policy by accessing the filesystem via the Verisign code.
Its important to keep in mind that the policy provided is just a
default, and it can be substituted with one which doesn't care
about principals or context stacks at all.
Framework Components
Low Level Components
These components provide the infrastructure for guarding
attribute access and providing hooks into the higher level
security framework.
Checkers
A checker is associated with an object kind, and provides the
hooks that map attribute checks onto permissions deferring to
the security manager (which in turn defers to the policy) to
perform the check.
Additionally, checkers provide for creating proxies of objects
associated with the checker.
There are several implementation variants of checkers, such as
checkers that grant access based on attribute names.
Proxies
Wrappers around Python objects that implicitly guard access to
their wrapped contents by delegating to their associated
checker. Proxies are also viral in nature, in that values
returned by proxies are also proxied.
High Level Components
Security Management
Provides accessors for setting up security manager and global
security policy.
Security Context
Stores transient information on the current principal and the
context stack.
Security Manager
Manages security context (execution stack) and delegates
permission checks to security policy.
Security Policy
Provides a single method that accepts the object, the
permission, and the context of the access being checked and is
used to implement the application logic for the security
framework.
Narrative (agent sandbox)
As an example we take a look at constructing a multi-agent
distributed system, and then adding a security layer using the
Zope security model onto it.
Scenario
Our agent simulation consists of autonomous agents that live in
various agent homes/sandboxes and perform actions that access
services available at their current home. Agents carry around
authentication tokens which signify their level of access within
any given home. Additionally agents attempt to migrate from
home to home randomly.
The agent simulation was constructed separately from any
security aspects. now we want to define and integrate a
security model into the simulation. The full code for the
simulation and the security model is available separately; we
present only relevant code snippets here for illustration as we
go through the implementation process.
For the agent simulation we want to add a security model such
that we group agents into two authentication groups, "norse
legends", including the principals thor, odin, and loki, and
"greek men", including prometheus, archimedes, and thucydides.
We associate permissions with access to services and homes. We
differentiate the homes such that certain authentication groups
only have access to services or the home itself based on the
local settings of the home in which they reside.
We define the homes/sandboxes
- origin - all agents start here, and have access to all
services here.
- valhalla - only agents in the authentication group 'norse
legend' can reside here.
- jail - all agents can come here, but only 'norse legend's
can leave or access services.
Process
Loosely we define a process for implementing this security model
- mapping permissions onto actions
- mapping authentication tokens onto permissions
- implementing checkers and security policies that use our
authentication tokens and permissions.
- binding checkers to our simulation classes
- inserting the hooks into the original simulation code to add
proxy wrappers to automatically check security.
- inserting hooks into the original simulation to register the
agents as the active principal within a security manager's
context....
Defining Permission Model
We define the following permissions::
NotAllowed = 'Not Allowed'
Public = Checker.CheckerPublic
TransportAgent = 'Transport Agent'
AccessServices = 'Access Services'
AccessAgents = 'Access Agents'
AccessTimeService = 'Access Time Services'
AccessAgentService = 'Access Agent Service'
AccessHomeService = 'Access Home Service'
and create a dictionary database mapping homes to authentication
groups which are linked to associated permissions.
Defining and Binding Checkers
Checkers are the foundational unit for the security framework.
They define what attributes can be accessed or set on a given
instance. They can be used implicitly via Proxy objects, to
guard all attribute access automatically or explicitly to check a
given access for an operation.
Checker construction expects two functions or dictionaries, one
is used to map attribute names to permissions for attribute
access and another to do the same for setting attributes.
We use the following checker factory function::
def PermissionMapChecker(permissions_map={},
setattr_permission_func=NoSetAttr):
res = {}
for k,v in permissions_map.items():
for iv in v:
res[iv]=k
return Checker.Checker(res.get, setattr_permission_func)
time_service_checker = PermissionMapChecker(
# permission : [methods]
{'AccessTimeService':['getTime']}
)
with the NoSetAttr function defined as a lambda which always
return the permission NotAllowed
To bind the checkers to the simulation classes we register our
checkers with the security model's global checker registry::
import sandbox_simulation
from zope.security.checker import defineChecker
defineChecker(sandbox_simulation.TimeService, time_service_checker)
Defining a Security Policy
We implement our security policy such that it checks the current
agent's authentication token against the given permission in the
home of the object being accessed::
class SimulationSecurityPolicy:
__implements__ = ISecurityPolicy
def checkPermission(self, permission, object, context):
token = context.user.getAuthenticationToken()
home = object.getHome()
db = getattr(SimulationSecurityDatabase, home.getId(), None)
if db is None:
return False
allowed = db.get('any', ())
if permission in allowed or ALL in allowed:
return True
allowed = db.get(token, ())
if permission in allowed:
return True
return False
There is some additional code present to allow for shortcuts in
defining the permission database when defining permissions for
all auth groups and all permissions.
Integration
At this point we have implemented our security model, and we
need to integrate it with our simulation model. We do so in
three separate steps.
First we make it such that agents only access homes that are
wrapped in a security proxy. By doing this all access to homes
and services (proxies have proxied return values for their
methods) is implicitly guarded by our security policy.
The second step is that we want to associate the active agent
with the security context so the security policy will know which
agent's authentication token to validate against.
The third step is to set our security policy as the default
policy for the Zope security framework. It is possible to
create custom security policies at a finer grained than global,
but such is left as an exercise for the reader.
Security Manager Access
The *default* implementation of the security management
interfaces defines security managers on a per thread basis with
a function for an accessor. This model is not appropriate for
all systems, as it restricts one to a single active user per
thread at any given moment. Reimplementing the manager access
methods though is easily doable and is noted here for
completeness.
Perspectives
It's important to keep in mind that there is a lot more that is
possible using the security framework than what's been presented
here. All of the interactions are interface based, such that if
you need to re-implement the semantics to suite your application
a new implementation of the interface will be sufficient.
Additional possibilities range from restricted interpreters and
dynamic loading of untrusted code to non Zope web application
security systems. Insert imagination here ;-).
Zope Perspective
A Zope3 programmer will never commonly need to interact with the
low level security framework. Zope3 defines a second security
package over top the low level framework that implements concepts
of roles, and authentication sources and checkers are handled
via zcml registration. Still those developing Zope3 will
hopefully find this useful as an introduction into the
underpinnings of the security framework.
Code
The complete code for this example is available.
sandbox.py - the agent framework
sandbox_security.py - the security implementation and binding to
the agent framework.
Author
Kapil Thangavelu <hazmat at objectrealms.net>
Guido Wesdorp <guido at infrae.com>
