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gh-125789: fix side-effects in asyncio callback scheduling methods #125833

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gh-125789: fix side-effects in asyncio callback scheduling methods #125833

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be7cac9
mitigate side-effects in futures scheduling
picnixz Oct 22, 2024
ca61908
blurb
picnixz Oct 22, 2024
640c799
fix UAF when removing a callback
picnixz Oct 22, 2024
a05cde2
Address Guido's and Yury's reviews.
picnixz Oct 23, 2024
22b21cc
Fix UAF (address Nico Posada's review)
picnixz Oct 23, 2024
f7b6730
Enforce conditions on the internal tuple
picnixz Oct 23, 2024
f72c393
Address reviews and improve coverage.
picnixz Oct 24, 2024
378fe09
fix typo
picnixz Oct 24, 2024
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3 changes: 2 additions & 1 deletion Lib/asyncio/futures.py
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Original file line number Diff line number Diff line change
Expand Up @@ -234,10 +234,11 @@ def remove_done_callback(self, fn):

Returns the number of callbacks removed.
"""
count = len(self._callbacks)
filtered_callbacks = [(f, ctx)
for (f, ctx) in self._callbacks
if f != fn]
removed_count = len(self._callbacks) - len(filtered_callbacks)
removed_count = count - len(filtered_callbacks)
if removed_count:
self._callbacks[:] = filtered_callbacks
return removed_count
Expand Down
156 changes: 156 additions & 0 deletions Lib/test/test_asyncio/test_futures.py
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Original file line number Diff line number Diff line change
Expand Up @@ -929,6 +929,136 @@ def __eq__(self, other):

fut.remove_done_callback(evil())

# Sanity checks for callback tuples corruption and Use-After-Free.
# Special thanks to Nico-Posada for the original PoCs and ideas.
# See https://github.com/python/cpython/issues/125789.

def test_schedule_callbacks_list_mutation_3(self):
raise NotImplementedError

def _test_schedule_callbacks_list_mutation_3(self, exc_type, exc_text=None):
class evil:
def __eq__(self, other):
global mem
mem = other
return False

cb_pad = lambda: ...

fut = self._new_future()
fut.add_done_callback(cb_pad) # sets fut->fut_callback0
fut.add_done_callback(evil()) # sets fut->fut_callbacks[0]
# Consume fut->fut_callback0 callback and set it to NULL before
# checking fut->fut_callbacks, thereby invoking evil.__eq__().
removed = fut.remove_done_callback(cb_pad)
self.assertEqual(removed, 1)
self.assertIs(mem, cb_pad)
# Set the malicious callback tuple and trigger a type check on it
# by re-invoking evil.__eq__() through remove_done_callback().
fut._callbacks[0] = complex(0, 0)
if exc_text is None:
self.assertRaises(exc_type, fut.remove_done_callback, evil())
else:
self.assertRaisesRegex(exc_type, exc_text,
fut.remove_done_callback, evil())
self.assertIs(mem, cb_pad)

def _evil_call_soon_event_loop(self, evil_call_soon):
fake_event_loop = lambda: ...
fake_event_loop.call_soon = evil_call_soon
fake_event_loop.get_debug = lambda: False # suppress traceback
return fake_event_loop

def test_evil_call_soon_list_mutation_1(self):
def evil_call_soon(*args, **kwargs):
fut._callbacks.clear()

loop = self._evil_call_soon_event_loop(evil_call_soon)
with mock.patch.object(self, 'loop', loop):
fut = self._new_future()
self.assertIs(fut.get_loop(), loop)

cb_pad = lambda: ...
fut.add_done_callback(cb_pad)
fut.add_done_callback(None)
fut.add_done_callback(None)

removed = fut.remove_done_callback(cb_pad)
self.assertEqual(removed, 1)
self.assertEqual(len(fut._callbacks), 2)
fut.set_result("boom")
# When there are no more callbacks, the Python implementation
# returns an empty list but the C implementation returns None.
self.assertIn(fut._callbacks, (None, []))

def test_evil_call_soon_list_mutation_2(self):
raise NotImplementedError

def _test_evil_call_soon_list_mutation_2(self, exc_type, exc_text=None):
def evil_call_soon(*args, **kwargs):
fut._callbacks[1] = complex(0, 0)

loop = self._evil_call_soon_event_loop(evil_call_soon)
with mock.patch.object(self, 'loop', loop):
fut = self._new_future()
self.assertIs(fut.get_loop(), loop)

cb_pad = lambda: ...
fut.add_done_callback(cb_pad)
fut.add_done_callback(None)
fut.add_done_callback(None)
removed = fut.remove_done_callback(cb_pad)
self.assertEqual(removed, 1)
self.assertEqual(len(fut._callbacks), 2)
# The evil 'call_soon' is executed by calling set_result().
if exc_text is None:
self.assertRaises(exc_type, fut.set_result, "boom")
else:
self.assertRaisesRegex(exc_type, exc_text, fut.set_result, "boom")

def test_use_after_free_fixed_1(self):
fut = self._new_future()

class setup:
def __eq__(self, other):
return False

# evil MUST be a subclass of setup for __eq__ to get called first
class evil(setup):
def __eq__(self, value):
fut._callbacks.clear()
return NotImplemented

cb_pad = lambda: ...
fut.add_done_callback(cb_pad) # sets fut->fut_callback0
fut.add_done_callback(setup()) # sets fut->fut_callbacks[0]
removed = fut.remove_done_callback(cb_pad)
self.assertEqual(removed, 1)

# This triggers evil.__eq__(), thereby clearing fut->fut_callbacks
# but we will still hold a reference to fut->fut_callbacks[0] until
# it is no more needed.
removed = fut.remove_done_callback(evil())
self.assertEqual(removed, 0)

def test_use_after_free_fixed_2(self):
asserter = self
fut = self._new_future()

class cb_pad:
def __eq__(self, other):
return True

class evil(cb_pad):
def __eq__(self, other):
removed = fut.remove_done_callback(None)
asserter.assertEqual(removed, 1)
return NotImplemented

fut.add_done_callback(cb_pad())
removed = fut.remove_done_callback(evil())
self.assertEqual(removed, 1)


@unittest.skipUnless(hasattr(futures, '_CFuture'),
'requires the C _asyncio module')
Expand All @@ -938,6 +1068,14 @@ class CFutureDoneCallbackTests(BaseFutureDoneCallbackTests,
def _new_future(self):
return futures._CFuture(loop=self.loop)

def test_schedule_callbacks_list_mutation_3(self):
errmsg = 'corrupted callback tuple'
super()._test_schedule_callbacks_list_mutation_3(RuntimeError, errmsg)

def test_evil_call_soon_list_mutation_2(self):
errmsg = 'corrupted callback tuple'
super()._test_evil_call_soon_list_mutation_2(RuntimeError, errmsg)


@unittest.skipUnless(hasattr(futures, '_CFuture'),
'requires the C _asyncio module')
Expand All @@ -949,13 +1087,31 @@ class CSubFuture(futures._CFuture):
pass
return CSubFuture(loop=self.loop)

def test_schedule_callbacks_list_mutation_3(self):
errmsg = 'corrupted callback tuple'
super()._test_schedule_callbacks_list_mutation_3(RuntimeError, errmsg)

def test_evil_call_soon_list_mutation_2(self):
errmsg = 'corrupted callback tuple'
super()._test_evil_call_soon_list_mutation_2(RuntimeError, errmsg)


class PyFutureDoneCallbackTests(BaseFutureDoneCallbackTests,
test_utils.TestCase):

def _new_future(self):
return futures._PyFuture(loop=self.loop)

def test_schedule_callbacks_list_mutation_3(self):
super()._test_schedule_callbacks_list_mutation_3(TypeError)

def test_evil_call_soon_list_mutation_2(self):
# For this test, the Python implementation raises an IndexError
# because the attribute fut._callbacks is set to an empty list
# *before* invoking the callbacks, while the C implementation
# does not make a temporary copy of the list of callbacks.
super()._test_evil_call_soon_list_mutation_2(IndexError)


class BaseFutureInheritanceTests:

Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2024-10-22-12-47-46.gh-issue-125789.Hk885p.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Mitigate interpreter crashes and state corruption due to side-effects in
:meth:`asyncio.Future.remove_done_callback` or other callback scheduling
methods. Patch by Bénédikt Tran.
54 changes: 41 additions & 13 deletions Modules/_asynciomodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -441,8 +441,16 @@ future_schedule_callbacks(asyncio_state *state, FutureObj *fut)
return 0;
}

for (i = 0; i < len; i++) {
// Beware: An evil 'call_soon' could change fut_callbacks or its items
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Do you have a test case for an evil 'call_soon'?

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No, you're right. I didn't test but I think we can just call fut._callbacks.clear() inside the callback that is being called and that would be it but I'd need to confirm tomorrow.

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@graingert graingert Oct 23, 2024 •
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The callback being called won't run until the loop here is finished (and whatever is running future_schedule_callbacks returns to _run_once)

You do need an evil call_soon that either calls the callback immediately or mutates fut._callbacks itself

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Oops, I mentioned it can be abused in my initial report but never actually followed up on it. Here's a simple crash POC

import asyncio

def catch(*args, **kwargs):
    fut._callbacks.clear()

loop = lambda: ...
loop.call_soon = catch
loop.get_debug = lambda: False

fut = asyncio.Future(loop=loop)

pad = lambda: ...
fut.add_done_callback(pad)
fut.add_done_callback(None)
fut.add_done_callback(None)
fut.remove_done_callback(pad)

fut.set_result("bonk")

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Thanks for that Nico; I've also added a test for when the callback tuple is changed in the evil call_soon.

// (see https://github.com/python/cpython/issues/125789 for details).
for (i = 0;
fut->fut_callbacks != NULL && i < PyList_GET_SIZE(fut->fut_callbacks);
i++) {
PyObject *cb_tup = PyList_GET_ITEM(fut->fut_callbacks, i);
if (!PyTuple_CheckExact(cb_tup) || PyTuple_GET_SIZE(cb_tup) != 2) {
PyErr_SetString(PyExc_RuntimeError, "corrupted callback tuple");
return -1;
}
PyObject *cb = PyTuple_GET_ITEM(cb_tup, 0);
PyObject *ctx = PyTuple_GET_ITEM(cb_tup, 1);

Expand Down Expand Up @@ -1017,7 +1025,13 @@ _asyncio_Future_remove_done_callback_impl(FutureObj *self, PyTypeObject *cls,
ENSURE_FUTURE_ALIVE(state, self)

if (self->fut_callback0 != NULL) {
int cmp = PyObject_RichCompareBool(self->fut_callback0, fn, Py_EQ);
// Beware: An evil PyObject_RichCompareBool could change fut_callback0
// (see https://github.com/python/cpython/issues/125789 for details)
// In addition, the reference to self->fut_callback0 may be cleared,
// so we need to temporarily hold it explicitly.
PyObject *fut_callback0 = Py_NewRef(self->fut_callback0);
int cmp = PyObject_RichCompareBool(fut_callback0, fn, Py_EQ);
Py_DECREF(fut_callback0);
if (cmp == -1) {
return NULL;
}
Expand All @@ -1041,8 +1055,17 @@ _asyncio_Future_remove_done_callback_impl(FutureObj *self, PyTypeObject *cls,

if (len == 1) {
PyObject *cb_tup = PyList_GET_ITEM(self->fut_callbacks, 0);
int cmp = PyObject_RichCompareBool(
PyTuple_GET_ITEM(cb_tup, 0), fn, Py_EQ);
// Beware: An evil PyObject_RichCompareBool could change fut_callbacks
// or its items (see https://github.com/python/cpython/issues/97592 or
// https://github.com/python/cpython/issues/125789 for details).
if (!PyTuple_CheckExact(cb_tup) || PyTuple_GET_SIZE(cb_tup) != 2) {
PyErr_SetString(PyExc_RuntimeError, "corrupted callback tuple");
return NULL;
}
Py_INCREF(cb_tup);
PyObject *cb = PyTuple_GET_ITEM(cb_tup, 0);
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A little nitpicky, but the incref should be on cb not cb_tup. We don't necessarily care if the tuple gets deleted in PyObject_RichCompareBool, we care if cb gets deleted. This technically works because I don't think there's a way to delete cb at this point if the tuple can't be deleted, but it's still worth noting.

int cmp = PyObject_RichCompareBool(cb, fn, Py_EQ);
Py_DECREF(cb_tup);
if (cmp == -1) {
return NULL;
}
Expand All @@ -1060,24 +1083,29 @@ _asyncio_Future_remove_done_callback_impl(FutureObj *self, PyTypeObject *cls,
return NULL;
}

// Beware: PyObject_RichCompareBool below may change fut_callbacks.
// See GH-97592.
// Beware: An evil PyObject_RichCompareBool could change fut_callbacks
// or its items (see https://github.com/python/cpython/issues/97592 or
// https://github.com/python/cpython/issues/125789 for details).
for (i = 0;
self->fut_callbacks != NULL && i < PyList_GET_SIZE(self->fut_callbacks);
i++) {
int ret;
PyObject *item = PyList_GET_ITEM(self->fut_callbacks, i);
Py_INCREF(item);
ret = PyObject_RichCompareBool(PyTuple_GET_ITEM(item, 0), fn, Py_EQ);
PyObject *cb_tup = PyList_GET_ITEM(self->fut_callbacks, i);
if (!PyTuple_CheckExact(cb_tup) || PyTuple_GET_SIZE(cb_tup) != 2) {
PyErr_SetString(PyExc_RuntimeError, "corrupted callback tuple");
goto fail;
}
Py_INCREF(cb_tup);
PyObject *cb = PyTuple_GET_ITEM(cb_tup, 0);
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Same thing here, I don't think it's possible to delete cb once the tuple has been incref'd, but if it's somehow possible to replace the first item in the tuple then cb will be freed possibly causing a UAF.

int ret = PyObject_RichCompareBool(cb, fn, Py_EQ);
if (ret == 0) {
if (j < len) {
PyList_SET_ITEM(newlist, j, item);
PyList_SET_ITEM(newlist, j, cb_tup);
j++;
continue;
}
ret = PyList_Append(newlist, item);
ret = PyList_Append(newlist, cb_tup);
}
Py_DECREF(item);
Py_DECREF(cb_tup);
if (ret < 0) {
goto fail;
}
Expand Down
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