[New-bugs-announce] [issue24098] Multiple use after frees in obj2ast_* methods

[paul]

New submission from paul:

# 3617                for (i = 0; i < len; i++) { 
# (gdb) print *(PyListObject*)tmp
# $1 = {ob_base = {ob_base = {_ob_next = 0x4056f8f4, _ob_prev = 0x4057329c, ob_refcnt = 2, ob_type = 0x830e1c0 <PyList_Type>}, 
#     ob_size = 1337}, ob_item = 0x8491ae0, allocated = 1432}
# (gdb) n
# 3619                    res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
# (gdb) n
# 3620                    if (res != 0) goto failed;
# (gdb) print *(PyListObject*)tmp
# $2 = {ob_base = {ob_base = {_ob_next = 0x4056f8f4, _ob_prev = 0x4057329c, ob_refcnt = 2, ob_type = 0x830e1c0 <PyList_Type>}, 
#     ob_size = 1}, ob_item = 0x8491ae0, allocated = 4}
# (gdb) c
# Continuing.
# 
# Program received signal SIGSEGV, Segmentation fault.
# 0x080f2c17 in PyObject_GetAttr (v=<unknown at remote 0x405733b4>, name='lineno') at Objects/object.c:872
# 872         if (tp->tp_getattro != NULL)
# 
# Objects freed in __getattr__ are used later in the loop above. There are two
# bugs actually. One is the use-after-free and the second is using a stale size
# variable "len" to control the for(...) loop. "body" can be mutated inside
# obj2ast_stmt.


This construct:

            for (i = 0; i < len; i++) {
                stmt_ty value;
                res = obj2ast_stmt(PyList_GET_ITEM(tmp, i), &value, arena);
                if (res != 0) goto failed;
                asdl_seq_SET(body, i, value);
            }

is repeated multiple times in multiple obj2ast_ methods. It contains two bugs:
1. tmp[i] isn't protected from deletion inside python code (refcnt is not increased by GET_ITEM),
2. tmp's length can drop below "len" resulting in an OOB read, because the loop counter is static.

----------
files: poc_obj2mod.py
messages: 242315
nosy: pkt
priority: normal
severity: normal
status: open
title: Multiple use after frees in obj2ast_* methods
type: crash
versions: Python 3.4
Added file: http://bugs.python.org/file39249/poc_obj2mod.py

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue24098>
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