Code 1 :

static PyObject *
list_index_impl(PyListObject *self, PyObject *value, Py_ssize_t start,
                Py_ssize_t stop)
// ...
    for (i = start; i < stop && i < Py_SIZE(self); i++) {
        int cmp = PyObject_RichCompareBool(self->ob_item[i], value, Py_EQ);  <=  self->ob_item[i] can uaf ..

PoC :

class rewrite_list_eq(list) :
    def __eq__(self,other) :
        str(other)   #  <== that will call the object recall function tp_repr and call it ..
        return NotImplemented

class poc() :
    def __eq__(self,other) :
        list1.clear()
        return NotImplemented

list1 = [ poc() ]
list1.index(list1)   #  list_index_impl() -> PyObject_RichCompareBool()

Crash Report :

(gdb) run ../py_poc/list_poc_3.py
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /tangjitao/Python-3.8.0/python ../py_poc/list_poc_3.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
PyObject_Str (v=0x7ffff6e82d20) at Objects/object.c:573
573 if (Py_TYPE(v)->tp_str == NULL)

=====

Code 2 :

static PyObject *
list_count(PyListObject *self, PyObject *value)
{
    Py_ssize_t count = 0;
    Py_ssize_t i;

    for (i = 0; i < Py_SIZE(self); i++) {
        int cmp = PyObject_RichCompareBool(self->ob_item[i], value, Py_EQ);  //  <=

PoC :

class rewrite_list_eq(list) :
    def __eq__(self,other) :
        str(other)
        return NotImplemented

class poc() :
    def __eq__(self,other) :
        list1.clear()
        return NotImplemented

list1 = rewrite_list_eq([ poc() ])
list1.count(list1)   #  list_count() -> PyObject_RichCompareBool()

Crash Report :

(gdb) run ../py_poc/list_poc_4.py
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /tangjitao/Python-3.8.0/python ../py_poc/list_poc_4.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
PyObject_Str (v=0x7ffff6e82d20) at Objects/object.c:573
573 if (Py_TYPE(v)->tp_str == NULL)

===

Code 3 :

static PyObject *
list_remove(PyListObject *self, PyObject *value)
/*[clinic end generated code: output=f087e1951a5e30d1 input=2dc2ba5bb2fb1f82]*/
{
    Py_ssize_t i;

    for (i = 0; i < Py_SIZE(self); i++) {
		Py_INCREF(self->ob_item[i]);
        int cmp = PyObject_RichCompareBool(self->ob_item[i], value, Py_EQ);

PoC :

class rewrite_list_eq(list) :
    def __eq__(self,other) :
        str(other)
        return NotImplemented

class poc() :
    def __eq__(self,other) :
        list1.clear()
        return NotImplemented

list1 = rewrite_list_eq([ poc() ])
list1.remove(list1)   #  list_count() -> PyObject_RichCompareBool()

Crash Report :

(gdb) run ../py_poc/list_poc_5.py
Starting program: /tangjitao/Python-3.8.0/python ../py_poc/list_poc_5.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
PyObject_Str (v=0x7ffff6e82d20) at Objects/object.c:573
573 if (Py_TYPE(v)->tp_str == NULL)

I have created a patch to fix these crashes. Please consider taking a look.

New changeset d9e561d by Pablo Galindo (Zackery Spytz) in branch 'master':
bpo-38610: Fix possible crashes in several list methods (GH-17022)
d9e561d

New changeset fcaf14c by Miss Islington (bot) in branch '3.8':
bpo-38610: Fix possible crashes in several list methods (GH-17022)
fcaf14c

New changeset 296d45e by Pablo Galindo in branch '3.7':
[3.7] bpo-38610: Fix possible crashes in several list methods (GH-17022) (GH-17759)
296d45e