379 lines
12 KiB
Python
379 lines
12 KiB
Python
"""
|
|
Transformation utilities for STIX pattern comparison expressions.
|
|
"""
|
|
import functools
|
|
import itertools
|
|
|
|
from stix2.equivalence.patterns.compare import iter_in, iter_lex_cmp
|
|
from stix2.equivalence.patterns.compare.comparison import (
|
|
comparison_expression_cmp,
|
|
)
|
|
from stix2.equivalence.patterns.transform import Transformer
|
|
from stix2.equivalence.patterns.transform.specials import (
|
|
ipv4_addr, ipv6_addr, windows_reg_key,
|
|
)
|
|
from stix2.patterns import (
|
|
AndBooleanExpression, OrBooleanExpression, ParentheticalExpression,
|
|
_BooleanExpression, _ComparisonExpression,
|
|
)
|
|
|
|
|
|
def _dupe_ast(ast):
|
|
"""
|
|
Create a duplicate of the given AST.
|
|
|
|
Note: the comparison expression "leaves", i.e. simple <path> <op> <value>
|
|
comparisons are currently not duplicated. I don't think it's necessary as
|
|
of this writing; they are never changed. But revisit this if/when
|
|
necessary.
|
|
|
|
:param ast: The AST to duplicate
|
|
:return: The duplicate AST
|
|
"""
|
|
if isinstance(ast, AndBooleanExpression):
|
|
result = AndBooleanExpression([
|
|
_dupe_ast(operand) for operand in ast.operands
|
|
])
|
|
|
|
elif isinstance(ast, OrBooleanExpression):
|
|
result = OrBooleanExpression([
|
|
_dupe_ast(operand) for operand in ast.operands
|
|
])
|
|
|
|
elif isinstance(ast, _ComparisonExpression):
|
|
# Change this to create a dupe, if we ever need to change simple
|
|
# comparison expressions as part of canonicalization.
|
|
result = ast
|
|
|
|
else:
|
|
raise TypeError("Can't duplicate " + type(ast).__name__)
|
|
|
|
return result
|
|
|
|
|
|
class ComparisonExpressionTransformer(Transformer):
|
|
"""
|
|
Transformer base class with special support for transforming comparison
|
|
expressions. The transform method implemented here performs a bottom-up
|
|
in-place transformation, with support for some comparison
|
|
expression-specific callbacks.
|
|
|
|
Specifically, subclasses can implement methods:
|
|
"transform_or" for OR nodes
|
|
"transform_and" for AND nodes
|
|
"transform_comparison" for plain comparison nodes (<prop> <op> <value>)
|
|
"transform_default" for both types of nodes
|
|
|
|
"transform_default" is a fallback, if a type-specific callback is not
|
|
found. The default implementation does nothing to the AST. The
|
|
type-specific callbacks are preferred over the default, if both exist.
|
|
|
|
In all cases, the callbacks are called with an AST for a subtree rooted at
|
|
the appropriate node type, where the subtree's children have already been
|
|
transformed. They must return the same thing as the base transform()
|
|
method: a 2-tuple with the transformed AST and a boolean for change
|
|
detection. See doc for the superclass' method.
|
|
|
|
This process currently silently drops parenthetical nodes.
|
|
"""
|
|
|
|
def transform(self, ast):
|
|
if isinstance(ast, _BooleanExpression):
|
|
changed = False
|
|
for i, operand in enumerate(ast.operands):
|
|
operand_result, this_changed = self.transform(operand)
|
|
if this_changed:
|
|
changed = True
|
|
|
|
ast.operands[i] = operand_result
|
|
|
|
result, this_changed = self.__dispatch_transform(ast)
|
|
if this_changed:
|
|
changed = True
|
|
|
|
elif isinstance(ast, _ComparisonExpression):
|
|
result, changed = self.__dispatch_transform(ast)
|
|
|
|
elif isinstance(ast, ParentheticalExpression):
|
|
# Drop these
|
|
result, changed = self.transform(ast.expression)
|
|
|
|
else:
|
|
raise TypeError("Not a comparison expression: " + str(ast))
|
|
|
|
return result, changed
|
|
|
|
def __dispatch_transform(self, ast):
|
|
"""
|
|
Invoke a transformer callback method based on the given ast root node
|
|
type.
|
|
|
|
:param ast: The AST
|
|
:return: The callback's result
|
|
"""
|
|
|
|
if isinstance(ast, AndBooleanExpression):
|
|
meth = getattr(self, "transform_and", self.transform_default)
|
|
|
|
elif isinstance(ast, OrBooleanExpression):
|
|
meth = getattr(self, "transform_or", self.transform_default)
|
|
|
|
elif isinstance(ast, _ComparisonExpression):
|
|
meth = getattr(
|
|
self, "transform_comparison", self.transform_default,
|
|
)
|
|
|
|
else:
|
|
meth = self.transform_default
|
|
|
|
return meth(ast)
|
|
|
|
def transform_default(self, ast):
|
|
"""
|
|
Override to handle transforming AST nodes which don't have a more
|
|
specific method implemented.
|
|
"""
|
|
return ast, False
|
|
|
|
|
|
class OrderDedupeTransformer(
|
|
ComparisonExpressionTransformer
|
|
):
|
|
"""
|
|
Canonically order the children of all nodes in the AST. Because the
|
|
deduping algorithm is based on sorted data, this transformation also does
|
|
deduping.
|
|
|
|
E.g.:
|
|
A and A => A
|
|
A or A => A
|
|
"""
|
|
|
|
def __transform(self, ast):
|
|
"""
|
|
Sort/dedupe children. AND and OR can be treated identically.
|
|
|
|
:param ast: The comparison expression AST
|
|
:return: The same AST node, but with sorted children
|
|
"""
|
|
sorted_children = sorted(
|
|
ast.operands, key=functools.cmp_to_key(comparison_expression_cmp),
|
|
)
|
|
|
|
deduped_children = [
|
|
# Apparently when using a key function, groupby()'s "keys" are the
|
|
# key wrappers, not actual sequence values. Obviously we don't
|
|
# need key wrappers in our ASTs!
|
|
k.obj for k, _ in itertools.groupby(
|
|
sorted_children, key=functools.cmp_to_key(
|
|
comparison_expression_cmp,
|
|
),
|
|
)
|
|
]
|
|
|
|
changed = iter_lex_cmp(
|
|
ast.operands, deduped_children, comparison_expression_cmp,
|
|
) != 0
|
|
|
|
ast.operands = deduped_children
|
|
|
|
return ast, changed
|
|
|
|
def transform_or(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
def transform_and(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
|
|
class FlattenTransformer(ComparisonExpressionTransformer):
|
|
"""
|
|
Flatten all nodes of the AST. E.g.:
|
|
|
|
A and (B and C) => A and B and C
|
|
A or (B or C) => A or B or C
|
|
(A) => A
|
|
"""
|
|
|
|
def __transform(self, ast):
|
|
"""
|
|
Flatten children. AND and OR can be treated mostly identically. The
|
|
little difference is that we can absorb AND children if we're an AND
|
|
ourselves; and OR for OR.
|
|
|
|
:param ast: The comparison expression AST
|
|
:return: The same AST node, but with flattened children
|
|
"""
|
|
|
|
changed = False
|
|
if len(ast.operands) == 1:
|
|
# Replace an AND/OR with one child, with the child itself.
|
|
ast = ast.operands[0]
|
|
changed = True
|
|
|
|
else:
|
|
flat_operands = []
|
|
for operand in ast.operands:
|
|
if isinstance(operand, _BooleanExpression) \
|
|
and ast.operator == operand.operator:
|
|
flat_operands.extend(operand.operands)
|
|
changed = True
|
|
|
|
else:
|
|
flat_operands.append(operand)
|
|
|
|
ast.operands = flat_operands
|
|
|
|
return ast, changed
|
|
|
|
def transform_or(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
def transform_and(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
|
|
class AbsorptionTransformer(
|
|
ComparisonExpressionTransformer
|
|
):
|
|
"""
|
|
Applies boolean "absorption" rules for AST simplification. E.g.:
|
|
|
|
A and (A or B) = A
|
|
A or (A and B) = A
|
|
"""
|
|
|
|
def __transform(self, ast):
|
|
|
|
changed = False
|
|
secondary_op = "AND" if ast.operator == "OR" else "OR"
|
|
|
|
to_delete = set()
|
|
|
|
# Check i (child1) against j to see if we can delete j.
|
|
for i, child1 in enumerate(ast.operands):
|
|
if i in to_delete:
|
|
continue
|
|
|
|
for j, child2 in enumerate(ast.operands):
|
|
if i == j or j in to_delete:
|
|
continue
|
|
|
|
# We're checking if child1 is contained in child2, so
|
|
# child2 has to be a compound object, not just a simple
|
|
# comparison expression. We also require the right operator
|
|
# for child2: "AND" if ast is "OR" and vice versa.
|
|
if not isinstance(child2, _BooleanExpression) \
|
|
or child2.operator != secondary_op:
|
|
continue
|
|
|
|
# The simple check: is child1 contained in child2?
|
|
if iter_in(
|
|
child1, child2.operands, comparison_expression_cmp,
|
|
):
|
|
to_delete.add(j)
|
|
|
|
# A more complicated check: does child1 occur in child2
|
|
# in a "flattened" form?
|
|
elif child1.operator == child2.operator:
|
|
if all(
|
|
iter_in(
|
|
child1_operand, child2.operands,
|
|
comparison_expression_cmp,
|
|
)
|
|
for child1_operand in child1.operands
|
|
):
|
|
to_delete.add(j)
|
|
|
|
if to_delete:
|
|
changed = True
|
|
|
|
for i in reversed(sorted(to_delete)):
|
|
del ast.operands[i]
|
|
|
|
return ast, changed
|
|
|
|
def transform_or(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
def transform_and(self, ast):
|
|
return self.__transform(ast)
|
|
|
|
|
|
class DNFTransformer(ComparisonExpressionTransformer):
|
|
"""
|
|
Convert a comparison expression AST to DNF. E.g.:
|
|
|
|
A and (B or C) => (A and B) or (A and C)
|
|
"""
|
|
def transform_and(self, ast):
|
|
or_children = []
|
|
other_children = []
|
|
changed = False
|
|
|
|
# Sort AND children into two piles: the ORs and everything else
|
|
for child in ast.operands:
|
|
if isinstance(child, _BooleanExpression) and child.operator == "OR":
|
|
# Need a list of operand lists, so we can compute the
|
|
# product below.
|
|
or_children.append(child.operands)
|
|
else:
|
|
other_children.append(child)
|
|
|
|
if or_children:
|
|
distributed_children = [
|
|
AndBooleanExpression([
|
|
# Make dupes: distribution implies adding repetition, and
|
|
# we should ensure each repetition is independent of the
|
|
# others.
|
|
_dupe_ast(sub_ast) for sub_ast in itertools.chain(
|
|
other_children, prod_seq,
|
|
)
|
|
])
|
|
for prod_seq in itertools.product(*or_children)
|
|
]
|
|
|
|
# Need to recursively continue to distribute AND over OR in
|
|
# any of our new sub-expressions which need it. This causes
|
|
# more downward recursion in the midst of this bottom-up transform.
|
|
# It's not good for performance. I wonder if a top-down
|
|
# transformation algorithm would make more sense in this phase?
|
|
# But then we'd be using two different algorithms for the same
|
|
# thing... Maybe this transform should be completely top-down
|
|
# (no bottom-up component at all)?
|
|
distributed_children = [
|
|
self.transform(child)[0] for child in distributed_children
|
|
]
|
|
|
|
result = OrBooleanExpression(distributed_children)
|
|
changed = True
|
|
|
|
else:
|
|
# No AND-over-OR; nothing to do
|
|
result = ast
|
|
|
|
return result, changed
|
|
|
|
|
|
class SpecialValueCanonicalization(ComparisonExpressionTransformer):
|
|
"""
|
|
Try to find particular leaf-node comparison expressions whose rhs (i.e. the
|
|
constant) can be canonicalized. This is an idiosyncratic transformation
|
|
based on some ideas people had for context-sensitive semantic equivalence
|
|
in constant values.
|
|
"""
|
|
def transform_comparison(self, ast):
|
|
if ast.lhs.object_type_name == "windows-registry-key":
|
|
windows_reg_key(ast)
|
|
|
|
elif ast.lhs.object_type_name == "ipv4-addr":
|
|
ipv4_addr(ast)
|
|
|
|
elif ast.lhs.object_type_name == "ipv6-addr":
|
|
ipv6_addr(ast)
|
|
|
|
# Hard-code False here since this particular canonicalization is never
|
|
# worth doing more than once. I think it's okay to pretend nothing has
|
|
# changed.
|
|
return ast, False
|