feat: fine grained access control policy (#2264)

This allows a set of rules to be defined for determining access to
resources. The rules are (loosely) based on the cedar policy format.

A rule defines a list of action either to permit or to forbid. It may
specify a principal or a resource that must match for the rule to take
effect. It may also specify a condition, either a 'when' or an 'unless',
with additional constraints as to where the rule applies.

A list of rules is held for each type to be protected and tried in order
to find a match. If a match is found, the request is permitted or
forbidden depening on the type of rule. If no match is found, the
request is denied. If no rules are specified for a given type, a rule
that allows any action as long as the resource attributes match the user
attributes is added (i.e. the previous behaviour is the default.

Some examples in yaml:

```
    model:
    - permit:
      principal: user-1
      actions: [create, read, delete]
      comment: user-1 has full access to all models
    - permit:
      principal: user-2
      actions: [read]
      resource: model-1
      comment: user-2 has read access to model-1 only
    - permit:
      actions: [read]
      when:
        user_in: resource.namespaces
      comment: any user has read access to models with matching attributes
    vector_db:
    - forbid:
      actions: [create, read, delete]
      unless:
        user_in: role::admin
      comment: only user with admin role can use vector_db resources
```

---------

Signed-off-by: Gordon Sim <gsim@redhat.com>

llama_stack/distribution/access_control/access_control.py

@@ -0,0 +1,109 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from typing import Any
+
+from llama_stack.distribution.datatypes import User
+
+from .conditions import (
+    Condition,
+    ProtectedResource,
+    parse_conditions,
+)
+from .datatypes import (
+    AccessRule,
+    Action,
+    Scope,
+)
+
+
+def matches_resource(resource_scope: str, actual_resource: str) -> bool:
+    if resource_scope == actual_resource:
+        return True
+    return resource_scope.endswith("::*") and actual_resource.startswith(resource_scope[:-1])
+
+
+def matches_scope(
+    scope: Scope,
+    action: Action,
+    resource: str,
+    user: str | None,
+) -> bool:
+    if scope.resource and not matches_resource(scope.resource, resource):
+        return False
+    if scope.principal and scope.principal != user:
+        return False
+    return action in scope.actions
+
+
+def as_list(obj: Any) -> list[Any]:
+    if isinstance(obj, list):
+        return obj
+    return [obj]
+
+
+def matches_conditions(
+    conditions: list[Condition],
+    resource: ProtectedResource,
+    user: User,
+) -> bool:
+    for condition in conditions:
+        # must match all conditions
+        if not condition.matches(resource, user):
+            return False
+    return True
+
+
+def default_policy() -> list[AccessRule]:
+    # for backwards compatibility, if no rules are provided, assume
+    # full access subject to previous attribute matching rules
+    return [
+        AccessRule(
+            permit=Scope(actions=list(Action)),
+            when=["user in owners " + name for name in ["roles", "teams", "projects", "namespaces"]],
+        ),
+    ]
+
+
+def is_action_allowed(
+    policy: list[AccessRule],
+    action: Action,
+    resource: ProtectedResource,
+    user: User | None,
+) -> bool:
+    # If user is not set, assume authentication is not enabled
+    if not user:
+        return True
+
+    if not len(policy):
+        policy = default_policy()
+
+    qualified_resource_id = resource.type + "::" + resource.identifier
+    for rule in policy:
+        if rule.forbid and matches_scope(rule.forbid, action, qualified_resource_id, user.principal):
+            if rule.when:
+                if matches_conditions(parse_conditions(as_list(rule.when)), resource, user):
+                    return False
+            elif rule.unless:
+                if not matches_conditions(parse_conditions(as_list(rule.unless)), resource, user):
+                    return False
+            else:
+                return False
+        elif rule.permit and matches_scope(rule.permit, action, qualified_resource_id, user.principal):
+            if rule.when:
+                if matches_conditions(parse_conditions(as_list(rule.when)), resource, user):
+                    return True
+            elif rule.unless:
+                if not matches_conditions(parse_conditions(as_list(rule.unless)), resource, user):
+                    return True
+            else:
+                return True
+    # assume access is denied unless we find a rule that permits access
+    return False
+
+
+class AccessDeniedError(RuntimeError):
+    pass