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# vim: tabstop=4 shiftwidth=4 softtabstop=4
# Copyright (c) 2012 OpenStack, LLC. # All Rights Reserved. # # Licensed under the Apache License, Version 2.0 (the "License"); you may # not use this file except in compliance with the License. You may obtain # a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, WITHOUT # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the # License for the specific language governing permissions and limitations # under the License.
""" Common Policy Engine Implementation
Policies can be expressed in one of two forms: A list of lists, or a string written in the new policy language.
In the list-of-lists representation, each check inside the innermost list is combined as with an "and" conjunction--for that check to pass, all the specified checks must pass. These innermost lists are then combined as with an "or" conjunction. This is the original way of expressing policies, but there now exists a new way: the policy language.
In the policy language, each check is specified the same way as in the list-of-lists representation: a simple "a:b" pair that is matched to the correct code to perform that check. However, conjunction operators are available, allowing for more expressiveness in crafting policies.
As an example, take the following rule, expressed in the list-of-lists representation::
[["role:admin"], ["project_id:%(project_id)s", "role:projectadmin"]]
In the policy language, this becomes::
role:admin or (project_id:%(project_id)s and role:projectadmin)
The policy language also has the "not" operator, allowing a richer policy rule::
project_id:%(project_id)s and not role:dunce
Finally, two special policy checks should be mentioned; the policy check "@" will always accept an access, and the policy check "!" will always reject an access. (Note that if a rule is either the empty list ("[]") or the empty string, this is equivalent to the "@" policy check.) Of these, the "!" policy check is probably the most useful, as it allows particular rules to be explicitly disabled. """
import abc import logging import re import urllib
import urllib2
from keystone.openstack.common.gettextutils import _ from keystone.openstack.common import jsonutils
LOG = logging.getLogger(__name__)
_rules = None _checks = {}
class Rules(dict): """ A store for rules. Handles the default_rule setting directly. """
@classmethod def load_json(cls, data, default_rule=None): """ Allow loading of JSON rule data. """
# Suck in the JSON data and parse the rules rules = dict((k, parse_rule(v)) for k, v in jsonutils.loads(data).items())
return cls(rules, default_rule)
def __init__(self, rules=None, default_rule=None): """Initialize the Rules store."""
super(Rules, self).__init__(rules or {}) self.default_rule = default_rule
def __missing__(self, key): """Implements the default rule handling."""
# If the default rule isn't actually defined, do something # reasonably intelligent if not self.default_rule or self.default_rule not in self: raise KeyError(key)
return self[self.default_rule]
def __str__(self): """Dumps a string representation of the rules."""
# Start by building the canonical strings for the rules out_rules = {} for key, value in self.items(): # Use empty string for singleton TrueCheck instances if isinstance(value, TrueCheck): out_rules[key] = '' else: out_rules[key] = str(value)
# Dump a pretty-printed JSON representation return jsonutils.dumps(out_rules, indent=4)
# Really have to figure out a way to deprecate this def set_rules(rules): """Set the rules in use for policy checks."""
global _rules
_rules = rules
# Ditto def reset(): """Clear the rules used for policy checks."""
global _rules
_rules = None
def check(rule, target, creds, exc=None, *args, **kwargs): """ Checks authorization of a rule against the target and credentials.
:param rule: The rule to evaluate. :param target: As much information about the object being operated on as possible, as a dictionary. :param creds: As much information about the user performing the action as possible, as a dictionary. :param exc: Class of the exception to raise if the check fails. Any remaining arguments passed to check() (both positional and keyword arguments) will be passed to the exception class. If exc is not provided, returns False.
:return: Returns False if the policy does not allow the action and exc is not provided; otherwise, returns a value that evaluates to True. Note: for rules using the "case" expression, this True value will be the specified string from the expression. """
# Allow the rule to be a Check tree if isinstance(rule, BaseCheck): result = rule(target, creds) elif not _rules: # No rules to reference means we're going to fail closed result = False else: try: # Evaluate the rule result = _rules[rule](target, creds) except KeyError: # If the rule doesn't exist, fail closed result = False
# If it is False, raise the exception if requested if exc and result is False: raise exc(*args, **kwargs)
return result
class BaseCheck(object): """ Abstract base class for Check classes. """
__metaclass__ = abc.ABCMeta
@abc.abstractmethod def __str__(self): """ Retrieve a string representation of the Check tree rooted at this node. """
pass
@abc.abstractmethod def __call__(self, target, cred): """ Perform the check. Returns False to reject the access or a true value (not necessary True) to accept the access. """
pass
class FalseCheck(BaseCheck): """ A policy check that always returns False (disallow). """
def __str__(self): """Return a string representation of this check."""
return "!"
def __call__(self, target, cred): """Check the policy."""
return False
class TrueCheck(BaseCheck): """ A policy check that always returns True (allow). """
def __str__(self): """Return a string representation of this check."""
return "@"
def __call__(self, target, cred): """Check the policy."""
return True
class Check(BaseCheck): """ A base class to allow for user-defined policy checks. """
def __init__(self, kind, match): """ :param kind: The kind of the check, i.e., the field before the ':'. :param match: The match of the check, i.e., the field after the ':'. """
self.kind = kind self.match = match
def __str__(self): """Return a string representation of this check."""
return "%s:%s" % (self.kind, self.match)
class NotCheck(BaseCheck): """ A policy check that inverts the result of another policy check. Implements the "not" operator. """
def __init__(self, rule): """ Initialize the 'not' check.
:param rule: The rule to negate. Must be a Check. """
self.rule = rule
def __str__(self): """Return a string representation of this check."""
return "not %s" % self.rule
def __call__(self, target, cred): """ Check the policy. Returns the logical inverse of the wrapped check. """
return not self.rule(target, cred)
class AndCheck(BaseCheck): """ A policy check that requires that a list of other checks all return True. Implements the "and" operator. """
def __init__(self, rules): """ Initialize the 'and' check.
:param rules: A list of rules that will be tested. """
self.rules = rules
def __str__(self): """Return a string representation of this check."""
return "(%s)" % ' and '.join(str(r) for r in self.rules)
def __call__(self, target, cred): """ Check the policy. Requires that all rules accept in order to return True. """
for rule in self.rules: if not rule(target, cred): return False
return True
def add_check(self, rule): """ Allows addition of another rule to the list of rules that will be tested. Returns the AndCheck object for convenience. """
self.rules.append(rule) return self
class OrCheck(BaseCheck): """ A policy check that requires that at least one of a list of other checks returns True. Implements the "or" operator. """
def __init__(self, rules): """ Initialize the 'or' check.
:param rules: A list of rules that will be tested. """
self.rules = rules
def __str__(self): """Return a string representation of this check."""
return "(%s)" % ' or '.join(str(r) for r in self.rules)
def __call__(self, target, cred): """ Check the policy. Requires that at least one rule accept in order to return True. """
for rule in self.rules: if rule(target, cred): return True
return False
def add_check(self, rule): """ Allows addition of another rule to the list of rules that will be tested. Returns the OrCheck object for convenience. """
self.rules.append(rule) return self
def _parse_check(rule): """ Parse a single base check rule into an appropriate Check object. """
# Handle the special checks if rule == '!': return FalseCheck() elif rule == '@': return TrueCheck()
try: kind, match = rule.split(':', 1) except Exception: LOG.exception(_("Failed to understand rule %(rule)s") % locals()) # If the rule is invalid, we'll fail closed return FalseCheck()
# Find what implements the check if kind in _checks: return _checks[kind](kind, match) elif None in _checks: return _checks[None](kind, match) else: LOG.error(_("No handler for matches of kind %s") % kind) return FalseCheck()
def _parse_list_rule(rule): """ Provided for backwards compatibility. Translates the old list-of-lists syntax into a tree of Check objects. """
# Empty rule defaults to True if not rule: return TrueCheck()
# Outer list is joined by "or"; inner list by "and" or_list = [] for inner_rule in rule: # Elide empty inner lists if not inner_rule: continue
# Handle bare strings if isinstance(inner_rule, basestring): inner_rule = [inner_rule]
# Parse the inner rules into Check objects and_list = [_parse_check(r) for r in inner_rule]
# Append the appropriate check to the or_list if len(and_list) == 1: or_list.append(and_list[0]) else: or_list.append(AndCheck(and_list))
# If we have only one check, omit the "or" if len(or_list) == 0: return FalseCheck() elif len(or_list) == 1: return or_list[0]
return OrCheck(or_list)
# Used for tokenizing the policy language _tokenize_re = re.compile(r'\s+')
def _parse_tokenize(rule): """ Tokenizer for the policy language.
Most of the single-character tokens are specified in the _tokenize_re; however, parentheses need to be handled specially, because they can appear inside a check string. Thankfully, those parentheses that appear inside a check string can never occur at the very beginning or end ("%(variable)s" is the correct syntax). """
for tok in _tokenize_re.split(rule): # Skip empty tokens if not tok or tok.isspace(): continue
# Handle leading parens on the token clean = tok.lstrip('(') for i in range(len(tok) - len(clean)): yield '(', '('
# If it was only parentheses, continue if not clean: continue else: tok = clean
# Handle trailing parens on the token clean = tok.rstrip(')') trail = len(tok) - len(clean)
# Yield the cleaned token lowered = clean.lower() if lowered in ('and', 'or', 'not'): # Special tokens yield lowered, clean elif clean: # Not a special token, but not composed solely of ')' if len(tok) >= 2 and ((tok[0], tok[-1]) in [('"', '"'), ("'", "'")]): # It's a quoted string yield 'string', tok[1:-1] else: yield 'check', _parse_check(clean)
# Yield the trailing parens for i in range(trail): yield ')', ')'
class ParseStateMeta(type): """ Metaclass for the ParseState class. Facilitates identifying reduction methods. """
def __new__(mcs, name, bases, cls_dict): """ Create the class. Injects the 'reducers' list, a list of tuples matching token sequences to the names of the corresponding reduction methods. """
reducers = []
for key, value in cls_dict.items(): if not hasattr(value, 'reducers'): continue for reduction in value.reducers: reducers.append((reduction, key))
cls_dict['reducers'] = reducers
return super(ParseStateMeta, mcs).__new__(mcs, name, bases, cls_dict)
def reducer(*tokens): """ Decorator for reduction methods. Arguments are a sequence of tokens, in order, which should trigger running this reduction method. """
def decorator(func): # Make sure we have a list of reducer sequences if not hasattr(func, 'reducers'): func.reducers = []
# Add the tokens to the list of reducer sequences func.reducers.append(list(tokens))
return func
return decorator
class ParseState(object): """ Implement the core of parsing the policy language. Uses a greedy reduction algorithm to reduce a sequence of tokens into a single terminal, the value of which will be the root of the Check tree.
Note: error reporting is rather lacking. The best we can get with this parser formulation is an overall "parse failed" error. Fortunately, the policy language is simple enough that this shouldn't be that big a problem. """
__metaclass__ = ParseStateMeta
def __init__(self): """Initialize the ParseState."""
self.tokens = [] self.values = []
def reduce(self): """ Perform a greedy reduction of the token stream. If a reducer method matches, it will be executed, then the reduce() method will be called recursively to search for any more possible reductions. """
for reduction, methname in self.reducers: if (len(self.tokens) >= len(reduction) and self.tokens[-len(reduction):] == reduction): # Get the reduction method meth = getattr(self, methname)
# Reduce the token stream results = meth(*self.values[-len(reduction):])
# Update the tokens and values self.tokens[-len(reduction):] = [r[0] for r in results] self.values[-len(reduction):] = [r[1] for r in results]
# Check for any more reductions return self.reduce()
def shift(self, tok, value): """Adds one more token to the state. Calls reduce()."""
self.tokens.append(tok) self.values.append(value)
# Do a greedy reduce... self.reduce()
@property def result(self): """ Obtain the final result of the parse. Raises ValueError if the parse failed to reduce to a single result. """
if len(self.values) != 1: raise ValueError("Could not parse rule") return self.values[0]
@reducer('(', 'check', ')') @reducer('(', 'and_expr', ')') @reducer('(', 'or_expr', ')') def _wrap_check(self, _p1, check, _p2): """Turn parenthesized expressions into a 'check' token."""
return [('check', check)]
@reducer('check', 'and', 'check') def _make_and_expr(self, check1, _and, check2): """ Create an 'and_expr' from two checks joined by the 'and' operator. """
return [('and_expr', AndCheck([check1, check2]))]
@reducer('and_expr', 'and', 'check') def _extend_and_expr(self, and_expr, _and, check): """ Extend an 'and_expr' by adding one more check. """
return [('and_expr', and_expr.add_check(check))]
@reducer('check', 'or', 'check') def _make_or_expr(self, check1, _or, check2): """ Create an 'or_expr' from two checks joined by the 'or' operator. """
return [('or_expr', OrCheck([check1, check2]))]
@reducer('or_expr', 'or', 'check') def _extend_or_expr(self, or_expr, _or, check): """ Extend an 'or_expr' by adding one more check. """
return [('or_expr', or_expr.add_check(check))]
@reducer('not', 'check') def _make_not_expr(self, _not, check): """Invert the result of another check."""
return [('check', NotCheck(check))]
def _parse_text_rule(rule): """ Translates a policy written in the policy language into a tree of Check objects. """
# Empty rule means always accept if not rule: return TrueCheck()
# Parse the token stream state = ParseState() for tok, value in _parse_tokenize(rule): state.shift(tok, value)
try: return state.result except ValueError: # Couldn't parse the rule LOG.exception(_("Failed to understand rule %(rule)r") % locals())
# Fail closed return FalseCheck()
def parse_rule(rule): """ Parses a policy rule into a tree of Check objects. """
# If the rule is a string, it's in the policy language if isinstance(rule, basestring): return _parse_text_rule(rule) return _parse_list_rule(rule)
def register(name, func=None): """ Register a function or Check class as a policy check.
:param name: Gives the name of the check type, e.g., 'rule', 'role', etc. If name is None, a default check type will be registered. :param func: If given, provides the function or class to register. If not given, returns a function taking one argument to specify the function or class to register, allowing use as a decorator. """
# Perform the actual decoration by registering the function or # class. Returns the function or class for compliance with the # decorator interface. def decorator(func): _checks[name] = func return func
# If the function or class is given, do the registration if func: return decorator(func)
return decorator
@register("rule") class RuleCheck(Check): def __call__(self, target, creds): """ Recursively checks credentials based on the defined rules. """
try: return _rules[self.match](target, creds) except KeyError: # We don't have any matching rule; fail closed return False
@register("role") class RoleCheck(Check): def __call__(self, target, creds): """Check that there is a matching role in the cred dict."""
return self.match.lower() in [x.lower() for x in creds['roles']]
@register('http') class HttpCheck(Check): def __call__(self, target, creds): """ Check http: rules by calling to a remote server.
This example implementation simply verifies that the response is exactly 'True'. """
url = ('http:' + self.match) % target data = {'target': jsonutils.dumps(target), 'credentials': jsonutils.dumps(creds)} post_data = urllib.urlencode(data) f = urllib2.urlopen(url, post_data) return f.read() == "True"
@register(None) class GenericCheck(Check): def __call__(self, target, creds): """ Check an individual match.
Matches look like:
tenant:%(tenant_id)s role:compute:admin """
# TODO(termie): do dict inspection via dot syntax match = self.match % target if self.kind in creds: return match == unicode(creds[self.kind]) return False |