#----------------------------------------------------------------------
# I, Babar K. Zafar, the author or of this code dedicate any and all
# copyright interest in this code to the public domain. I make this
# dedication for the benefit of the public at large and to the
# detriment of our heirs and successors. I intend this dedication to
# be an overt act of relinquishment in perpetuity of all present and
# future rights this code under copyright law.
#
# Version 0.1 / May 27 2006
#----------------------------------------------------------------------
import inspect, compiler.ast
import thread, time
#----------------------------------------------------------------------
# Module globals.
#----------------------------------------------------------------------
# Toggle module level debugging mode.
DEBUG = False
# List of all AST node classes in compiler/ast.py.
all_ast_nodes = \
[name for (name, obj) in inspect.getmembers(compiler.ast)
if inspect.isclass(obj) and issubclass(obj, compiler.ast.Node)]
# List of all builtin functions and types (ignoring exception classes).
all_builtins = \
[name for (name, obj) in inspect.getmembers(__builtins__)
if inspect.isbuiltin(obj) or (inspect.isclass(obj) and \
not issubclass(obj, Exception))]
#----------------------------------------------------------------------
# Utilties.
#----------------------------------------------------------------------
def classname(obj):
return obj.__class__.__name__
def is_valid_ast_node(name):
return name in all_ast_nodes
def is_valid_builtin(name):
return name in all_builtins
def get_node_lineno(node):
return (node.lineno) and node.lineno or 0
#----------------------------------------------------------------------
# Restricted AST nodes & builtins.
#----------------------------------------------------------------------
# Deny evaluation of code if the AST contain any of the following nodes:
unallowed_ast_nodes = [
# 'Add', 'And',
# 'AssAttr', 'AssList', 'AssName', 'AssTuple',
# 'Assert', 'Assign', 'AugAssign',
'Backquote',
# 'Bitand', 'Bitor', 'Bitxor', 'Break',
# 'CallFunc', 'Class', 'Compare', 'Const', 'Continue',
# 'Decorators', 'Dict', 'Discard', 'Div',
# 'Ellipsis', 'EmptyNode',
'Exec',
# 'Expression', 'FloorDiv',
# 'For',
'From',
# 'Function',
# 'GenExpr', 'GenExprFor', 'GenExprIf', 'GenExprInner',
# 'Getattr', 'Global', 'If',
'Import',
# 'Invert',
# 'Keyword', 'Lambda', 'LeftShift',
# 'List', 'ListComp', 'ListCompFor', 'ListCompIf', 'Mod',
# 'Module',
# 'Mul', 'Name', 'Node', 'Not', 'Or', 'Pass', 'Power',
# 'Print', 'Printnl',
'Raise',
# 'Return', 'RightShift', 'Slice', 'Sliceobj',
# 'Stmt', 'Sub', 'Subscript',
'TryExcept', 'TryFinally',
# 'Tuple', 'UnaryAdd', 'UnarySub',
# 'While','Yield'
]
# Deny evaluation of code if it tries to access any of the following builtins:
unallowed_builtins = [
'__import__',
# 'abs', 'apply', 'basestring', 'bool', 'buffer',
# 'callable', 'chr', 'classmethod', 'cmp', 'coerce',
'compile',
# 'complex',
'delattr',
# 'dict',
'dir',
# 'divmod', 'enumerate',
'eval', 'execfile', 'file',
# 'filter', 'float', 'frozenset',
'getattr', 'globals', 'hasattr',
# 'hash', 'hex', 'id',
'input',
# 'int', 'intern', 'isinstance', 'issubclass', 'iter',
# 'len', 'list',
'locals',
# 'long', 'map', 'max', 'min', 'object', 'oct',
'open',
# 'ord', 'pow', 'property', 'range',
'raw_input',
# 'reduce',
'reload',
# 'repr', 'reversed', 'round', 'set',
'setattr',
# 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super',
# 'tuple', 'type', 'unichr', 'unicode',
'vars',
# 'xrange', 'zip'
]
for ast_name in unallowed_ast_nodes:
assert(is_valid_ast_node(ast_name))
for name in unallowed_builtins:
assert(is_valid_builtin(name))
def is_unallowed_ast_node(kind):
return kind in unallowed_ast_nodes
def is_unallowed_builtin(name):
return name in unallowed_builtins
#----------------------------------------------------------------------
# Restricted attributes.
#----------------------------------------------------------------------
# In addition to these we deny access to all lowlevel attrs (__xxx__).
unallowed_attr = [
'im_class', 'im_func', 'im_self',
'func_code', 'func_defaults', 'func_globals', 'func_name',
'tb_frame', 'tb_next',
'f_back', 'f_builtins', 'f_code', 'f_exc_traceback',
'f_exc_type', 'f_exc_value', 'f_globals', 'f_locals']
def is_unallowed_attr(name):
return (name[:2] == '__' and name[-2:] == '__') or \
(name in unallowed_attr)
#----------------------------------------------------------------------
# SafeEvalVisitor.
#----------------------------------------------------------------------
class SafeEvalError(object):
"""
Base class for all which occur while walking the AST.
Attributes:
errmsg = short decription about the nature of the error
lineno = line offset to where error occured in source code
"""
def __init__(self, errmsg, lineno):
self.errmsg, self.lineno = errmsg, lineno
def __str__(self):
return "line %d : %s" % (self.lineno, self.errmsg)
class SafeEvalASTNodeError(SafeEvalError):
"Expression/statement in AST evaluates to a restricted AST node type."
pass
class SafeEvalBuiltinError(SafeEvalError):
"Expression/statement in tried to access a restricted builtin."
pass
class SafeEvalAttrError(SafeEvalError):
"Expression/statement in tried to access a restricted attribute."
pass
class SafeEvalVisitor(object):
"""
Data-driven visitor which walks the AST for some code and makes
sure it doesn't contain any expression/statements which are
declared as restricted in 'unallowed_ast_nodes'. We'll also make
sure that there aren't any attempts to access/lookup restricted
builtin declared in 'unallowed_builtins'. By default we also won't
allow access to lowlevel stuff which can be used to dynamically
access non-local envrioments.
Interface:
walk(ast) = validate AST and return True if AST is 'safe'
Attributes:
errors = list of SafeEvalError if walk() returned False
Implementation:
The visitor will automatically generate methods for all of the
available AST node types and redirect them to self.ok or self.fail
reflecting the configuration in 'unallowed_ast_nodes'. While
walking the AST we simply forward the validating step to each of
node callbacks which take care of reporting errors.
"""
def __init__(self):
"Initialize visitor by generating callbacks for all AST node types."
self.errors = []
for ast_name in all_ast_nodes:
# Don't reset any overridden callbacks.
if getattr(self, 'visit' + ast_name, None): continue
if is_unallowed_ast_node(ast_name):
setattr(self, 'visit' + ast_name, self.fail)
else:
setattr(self, 'visit' + ast_name, self.ok)
def walk(self, ast):
"Validate each node in AST and return True if AST is 'safe'."
self.visit(ast)
return self.errors == []
def visit(self, node, *args):
"Recursively validate node and all of its children."
fn = getattr(self, 'visit' + classname(node))
if DEBUG: self.trace(node)
fn(node, *args)
for child in node.getChildNodes():
self.visit(child, *args)
def visitName(self, node, *args):
"Disallow any attempts to access a restricted builtin/attr."
name = node.getChildren()[0]
lineno = get_node_lineno(node)
if is_unallowed_builtin(name):
self.errors.append(SafeEvalBuiltinError( \
"access to builtin '%s' is denied" % name, lineno))
elif is_unallowed_attr(name):
self.errors.append(SafeEvalAttrError( \
"access to attribute '%s' is denied" % name, lineno))
def visitGetattr(self, node, *args):
"Disallow any attempts to access a restricted attribute."
name = node.attrname
lineno = get_node_lineno(node)
if is_unallowed_attr(name):
self.errors.append(SafeEvalAttrError( \
"access to attribute '%s' is denied" % name, lineno))
def ok(self, node, *args):
"Default callback for 'harmless' AST nodes."
pass
def fail(self, node, *args):
"Default callback for unallowed AST nodes."
lineno = get_node_lineno(node)
self.errors.append(SafeEvalASTNodeError( \
"execution of '%s' statements is denied" % classname(node),
lineno))
def trace(self, node):
"Debugging utility for tracing the validation of AST nodes."
print classname(node)
for attr in dir(node):
if attr[:2] != '__':
print ' ' * 4, "%-15.15s" % attr, getattr(node, attr)
#----------------------------------------------------------------------
# Safe 'eval' replacement.
#----------------------------------------------------------------------
class SafeEvalException(Exception):
"Base class for all safe-eval related errors."
pass
class SafeEvalCodeException(SafeEvalException):
"""
Exception class for reporting all errors which occured while
validating AST for source code in safe_eval().
Attributes:
code = raw source code which failed to validate
errors = list of SafeEvalError
"""
def __init__(self, code, errors):
self.code, self.errors = code, errors
def __str__(self):
return '\n'.join([str(err) for err in self.errors])
class SafeEvalContextException(SafeEvalException):
"""
Exception class for reporting unallowed objects found in the dict
intended to be used as the local enviroment in safe_eval().
Attributes:
keys = list of keys of the unallowed objects
errors = list of strings describing the nature of the error
for each key in 'keys'
"""
def __init__(self, keys, errors):
self.keys, self.errors = keys, errors
def __str__(self):
return '\n'.join([str(err) for err in self.errors])
class SafeEvalTimeoutException(SafeEvalException):
"""
Exception class for reporting that code evaluation execeeded
the given timelimit.
Attributes:
timeout = time limit in seconds
"""
def __init__(self, timeout):
self.timeout = timeout
def __str__(self):
return "Timeout limit execeeded (%s secs) during exec" % self.timeout
def exec_timed(code, context, timeout_secs):
"""
Dynamically execute 'code' using 'context' as the global enviroment.
SafeEvalTimeoutException is raised if execution does not finish within
the given timelimit.
"""
assert(timeout_secs > 0)
signal_finished = False
def alarm(secs):
def wait(secs):
for n in xrange(timeout_secs):
time.sleep(1)
if signal_finished: break
else:
thread.interrupt_main()
thread.start_new_thread(wait, (secs,))
try:
alarm(timeout_secs)
exec code in context
signal_finished = True
except KeyboardInterrupt:
raise SafeEvalTimeoutException(timeout_secs)
def safe_eval(code, context = {}, timeout_secs = 5):
"""
Validate source code and make sure it contains no unauthorized
expression/statements as configured via 'unallowed_ast_nodes' and
'unallowed_builtins'. By default this means that code is not
allowed import modules or access dangerous builtins like 'open' or
'eval'. If code is considered 'safe' it will be executed via
'exec' using 'context' as the global environment. More details on
how code is executed can be found in the Python Reference Manual
section 6.14 (ignore the remark on '__builtins__'). The 'context'
enviroment is also validated and is not allowed to contain modules
or builtins. The following exception will be raised on errors:
if 'context' contains unallowed objects =
SafeEvalContextException
if code is didn't validate and is considered 'unsafe' =
SafeEvalCodeException
if code did not execute within the given timelimit =
SafeEvalTimeoutException
"""
ctx_errkeys, ctx_errors = [], []
for (key, obj) in context.items():
if inspect.isbuiltin(obj):
ctx_errkeys.append(key)
ctx_errors.append("key '%s' : unallowed builtin %s" % (key, obj))
if inspect.ismodule(obj):
ctx_errkeys.append(key)
ctx_errors.append("key '%s' : unallowed module %s" % (key, obj))
if ctx_errors:
raise SafeEvalContextException(ctx_errkeys, ctx_errors)
ast = compiler.parse(code)
checker = SafeEvalVisitor()
if checker.walk(ast):
exec_timed(code, context, timeout_secs)
else:
raise SafeEvalCodeException(code, checker.errors)
#----------------------------------------------------------------------
# Basic tests.
#----------------------------------------------------------------------
import unittest
class TestSafeEval(unittest.TestCase):
def test_builtin(self):
# attempt to access a unsafe builtin
self.assertRaises(SafeEvalException,
safe_eval, "open('test.txt', 'w')")
def test_getattr(self):
# attempt to get arround direct attr access
self.assertRaises(SafeEvalException, \
safe_eval, "getattr(int, '__abs__')")
def test_func_globals(self):
# attempt to access global enviroment where fun was defined
self.assertRaises(SafeEvalException, \
safe_eval, "def x(): pass; print x.func_globals")
def test_lowlevel(self):
# lowlevel tricks to access 'object'
self.assertRaises(SafeEvalException, \
safe_eval, "().__class__.mro()[1].__subclasses__()")
def test_timeout_ok(self):
# attempt to exectute 'slow' code which finishes within timelimit
def test(): time.sleep(2)
env = {'test':test}
safe_eval("test()", env, timeout_secs = 5)
def test_timeout_exceed(self):
# attempt to exectute code which never teminates
self.assertRaises(SafeEvalException, \
safe_eval, "while 1: pass")
def test_invalid_context(self):
# can't pass an enviroment with modules or builtins
env = {'f' : __builtins__.open, 'g' : time}
self.assertRaises(SafeEvalException, \
safe_eval, "print 1", env)
def test_callback(self):
# modify local variable via callback
self.value = 0
def test(): self.value = 1
env = {'test':test}
safe_eval("test()", env)
self.assertEqual(self.value, 1)
if __name__ == "__main__":
unittest.main()
#----------------------------------------------------------------------
# The End.
#----------------------------------------------------------------------
