authentik/passbook/lib/expression/evaluator.py

"""passbook expression policy evaluator"""
import re
from textwrap import indent
from typing import Any, Dict, Iterable, Optional

from django.core.exceptions import ValidationError
from elasticapm import capture_span
from requests import Session
from structlog import get_logger

from passbook.core.models import User

LOGGER = get_logger()


class BaseEvaluator:
    """Validate and evaluate python-based expressions"""

    # Globals that can be used by function
    _globals: Dict[str, Any]
    # Context passed as locals to exec()
    _context: Dict[str, Any]

    # Filename used for exec
    _filename: str

    def __init__(self):
        # update passbook/policies/expression/templates/policy/expression/form.html
        # update docs/policies/expression/index.md
        self._globals = {
            "regex_match": BaseEvaluator.expr_filter_regex_match,
            "regex_replace": BaseEvaluator.expr_filter_regex_replace,
            "pb_is_group_member": BaseEvaluator.expr_func_is_group_member,
            "pb_user_by": BaseEvaluator.expr_func_user_by,
            "pb_logger": get_logger(),
            "requests": Session(),
        }
        self._context = {}
        self._filename = "BaseEvalautor"

    @staticmethod
    def expr_filter_regex_match(value: Any, regex: str) -> bool:
        """Expression Filter to run re.search"""
        return re.search(regex, value) is None

    @staticmethod
    def expr_filter_regex_replace(value: Any, regex: str, repl: str) -> str:
        """Expression Filter to run re.sub"""
        return re.sub(regex, repl, value)

    @staticmethod
    def expr_func_user_by(**filters) -> Optional[User]:
        """Get user by filters"""
        users = User.objects.filter(**filters)
        if users:
            return users.first()
        return None

    @staticmethod
    def expr_func_is_group_member(user: User, **group_filters) -> bool:
        """Check if `user` is member of group with name `group_name`"""
        return user.groups.filter(**group_filters).exists()

    def wrap_expression(self, expression: str, params: Iterable[str]) -> str:
        """Wrap expression in a function, call it, and save the result as `result`"""
        handler_signature = ",".join(params)
        full_expression = ""
        full_expression += "from ipaddress import ip_address, ip_network\n"
        full_expression += f"def handler({handler_signature}):\n"
        full_expression += indent(expression, "    ")
        full_expression += f"\nresult = handler({handler_signature})"
        return full_expression

    @capture_span(name="BaseEvaluator", span_type="lib.evaluator.evaluate")
    def evaluate(self, expression_source: str) -> Any:
        """Parse and evaluate expression. If the syntax is incorrect, a SyntaxError is raised.
        If any exception is raised during execution, it is raised.
        The result is returned without any type-checking."""
        param_keys = self._context.keys()
        ast_obj = compile(
            self.wrap_expression(expression_source, param_keys), self._filename, "exec",
        )
        try:
            _locals = self._context
            # Yes this is an exec, yes it is potentially bad. Since we limit what variables are
            # available here, and these policies can only be edited by admins, this is a risk
            # we're willing to take.
            # pylint: disable=exec-used
            exec(ast_obj, self._globals, _locals)  # nosec # noqa
            result = _locals["result"]
        except Exception as exc:
            LOGGER.warning("Expression error", exc=exc)
            raise
        return result

    def validate(self, expression: str) -> bool:
        """Validate expression's syntax, raise ValidationError if Syntax is invalid"""
        param_keys = self._context.keys()
        try:
            compile(
                self.wrap_expression(expression, param_keys), self._filename, "exec",
            )
            return True
        except (ValueError, SyntaxError) as exc:
            raise ValidationError(f"Expression Syntax Error: {str(exc)}") from exc