fix: RBAC bypass vulnerabilities in model access (backport #4270) (#4285)

Closes security gaps where RBAC checks could be bypassed:

o Inference router: Added RBAC enforcement in the fallback
  path to ensure access control is applied consistently.

o Model listing: Dynamic models fetched via provider_data were returned
  without RBAC checks. Added filtering to ensure users only see models
  they have permission to access.

Both fixes create temporary ModelWithOwner objects for RBAC validation,
maintaining security through consistent access control enforcement.

Closes: #4269
<hr>This is an automatic backport of pull request #4270 done by
[Mergify](https://mergify.com).

Signed-off-by: Derek Higgins <derekh@redhat.com>
Signed-off-by: Charlie Doern <cdoern@redhat.com>
Co-authored-by: Derek Higgins <derekh@redhat.com>

tests/unit/server/test_access_control.py

@@ -10,10 +10,12 @@ import pytest
 import yaml
 from pydantic import TypeAdapter, ValidationError
 
+from llama_stack.apis.common.errors import ModelNotFoundError
 from llama_stack.apis.datatypes import Api
 from llama_stack.apis.models import ModelType
 from llama_stack.core.access_control.access_control import AccessDeniedError, is_action_allowed
 from llama_stack.core.datatypes import AccessRule, ModelWithOwner, User
+from llama_stack.core.routers.inference import InferenceRouter
 from llama_stack.core.routing_tables.models import ModelsRoutingTable
 
 
@@ -558,3 +560,97 @@ def test_condition_reprs(condition):
     from llama_stack.core.access_control.conditions import parse_condition
 
     assert condition == str(parse_condition(condition))
+
+
+@pytest.fixture
+def restricted_user():
+    """User with limited access."""
+    return User("restricted-user", {"roles": ["user"]})
+
+
+@pytest.fixture
+def admin_user():
+    """User with admin access."""
+    return User("admin-user", {"roles": ["admin"]})
+
+
+@pytest.fixture
+def rbac_policy():
+    """RBAC policy that restricts access to certain models."""
+    from llama_stack.core.access_control.datatypes import Action, Scope
+
+    return [
+        # Admins get full access
+        AccessRule(
+            permit=Scope(actions=list(Action)),
+            when=["user with admin in roles"],
+        ),
+        # Regular users only get read access to their own resources
+        AccessRule(
+            permit=Scope(actions=[Action.READ]),
+            when=["user is owner"],
+        ),
+    ]
+
+
+class TestInferenceRouterRBACBypass:
+    """Test RBAC bypass vulnerability in inference router fallback path."""
+
+    @pytest.fixture
+    def mock_routing_table(self):
+        """Create a mock routing table for testing."""
+        routing_table = AsyncMock()
+        routing_table.impls_by_provider_id = {"test-provider": AsyncMock()}
+        routing_table.policy = []
+        return routing_table
+
+    @patch("llama_stack.core.routers.inference.get_authenticated_user")
+    async def test_registry_path_and_fallback_path_consistent(
+        self, mock_get_user, mock_routing_table, restricted_user, admin_user, rbac_policy
+    ):
+        """Test that registry path and fallback path have consistent RBAC enforcement."""
+        mock_routing_table.policy = rbac_policy
+
+        # Create a model owned by admin
+        admin_model = ModelWithOwner(
+            identifier="admin-model",
+            provider_id="test-provider",
+            provider_resource_id="admin-resource",
+            model_type=ModelType.llm,
+            type="model",
+            metadata={},
+            owner=admin_user,
+        )
+
+        # Setup router
+        router = InferenceRouter(
+            routing_table=mock_routing_table,
+            store=None,
+        )
+
+        # Test 1: Restricted user tries to access via registry (should fail)
+        mock_get_user.return_value = restricted_user
+        mock_routing_table.get_object_by_identifier.return_value = None  # RBAC blocks it
+        with pytest.raises(ModelNotFoundError):
+            await router._get_model_provider("admin-model", "llm")
+
+        # Test 2: Restricted user tries to access via fallback path (should also fail)
+        mock_routing_table.get_object_by_identifier.return_value = None
+        with pytest.raises(ModelNotFoundError):
+            await router._get_model_provider("test-provider/admin-resource", "llm")
+
+        # Test 3: Admin user can access via registry
+        mock_get_user.return_value = admin_user
+        mock_routing_table.get_object_by_identifier.return_value = admin_model
+        provider_mock = AsyncMock()
+        mock_routing_table.get_provider_impl.return_value = provider_mock
+
+        provider, resource_id = await router._get_model_provider("admin-model", "llm")
+        assert provider == provider_mock
+        assert resource_id == "admin-resource"
+
+        # Test 4: Admin user can also access via fallback path
+        mock_routing_table.get_object_by_identifier.return_value = None
+        provider, resource_id = await router._get_model_provider("test-provider/admin-resource", "llm")
+        assert provider == mock_routing_table.impls_by_provider_id["test-provider"]
+        assert resource_id == "admin-resource"