[New-bugs-announce] [issue34045] DoS due to null pointer dereference in marshal.dumps()

[Krishna Ram Prakash R]

New submission from Krishna Ram Prakash R <krp at gtux.in>:

By passing a malformed string as input to marshal.loads() an attacker can trigger a null pointer dereference resulting in DoS. 

This happens because when a Python object is unmarshalled by reference, it is assumed that the target object is fully constructed. We can construct a marshal string such that it can reference partially constructed Python objects. 

Example
-------

tuple(FrozenSet(REF(0)))

Tuple -> FrozenSet -> REF(0)

When unmarshalling of the tuple object starts, a new PyTuple_New() object is created and its address is added to p->refs array before starting to parse and load all its children elements in a loop. A FrozenSet can be added as 0th element of this tuple. And then add the 0th element of this FrozenSet as p->refs[0]. After an element is added to FrozenSet, it tries to hash it believing that it is a completely constructed Python object. 

While it tries to hash the original tuple, it does not have any valid addresses in ob_item array. This results in a null pointer dereference throwing a SIGSEGV and crashing of interpreter. 

Running the below script results in a segmentation fault.

```
#!/usr/bin/env python3

import marshal
marshal.loads(b"\xa9\x01\xbe\x01\x00\x00\x00r\x00\x00\x00\x00")
```

----------
components: Interpreter Core, Library (Lib)
messages: 321050
nosy: benjamin.peterson, rkrp
priority: normal
severity: normal
status: open
title: DoS due to null pointer dereference in marshal.dumps()
type: security
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34045>
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