Author thatch
Recipients thatch
Date 2014-11-11.22:55:25
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Message-id <1415746526.05.0.824048300267.issue22849@psf.upfronthosting.co.za>
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There's a reproducible bug in textio.c that causes a double DECREF on codecs.  The conditions to trigger are probably rare in real life, so not remotely exploitable (sandbox escape is the worst I can think of on its own, and I'm not aware of any on 3.x):

* You need to create a TextIOWrapper wrapping a file-like object that only partially supports the protocol.  For example, supporting readable(), writable(), and seekable() but not tell().

The crash I experience most of the time appears to be that the memory being reused, such that the PyObject ob_type field is no longer a valid pointer.

Affected:
  Source 3.5.0a0 (latest default branch yesterday, 524a004e93dd)
  Archlinux: 3.3.5 and 3.4.2
  Ubuntu: 3.4.0
Unaffected:
  Centos: 3.3.2
  All 2.7 branch (doesn't contain the faulty commit)

Here's where it's introduced -- https://hg.python.org/cpython/rev/f3ec00d2b75e/#l5.76

/* Modules/_io/textio.c line 1064 */

Py_DECREF(codec_info); 
/* does not set codec_info = NULL; */
...
if(...) goto error;
...
error:
  Py_XDECREF(codec_info);

The attached script is close to minimal -- I think at most you can reduce by one TextIOWrapper instantiation.  Sample stacktrace follows (which is after the corruption occurs, on subsequent access to v->ob_type (which is invalid).

#0  0x00000000004c8829 in PyObject_GetAttr (v=<unknown at remote 0x7ffff7eb9688>, 
    name='_is_text_encoding') at Objects/object.c:872
#1  0x00000000004c871d in _PyObject_GetAttrId (v=<unknown at remote 0x7ffff7eb9688>, 
    name=0x945d50 <PyId__is_text_encoding.10143>) at Objects/object.c:835
#2  0x00000000005c6674 in _PyCodec_LookupTextEncoding (
    encoding=0x7ffff6f40220 "utf-8", alternate_command=0x6c2fcd "codecs.open()")
    at Python/codecs.c:541
#3  0x000000000064286e in textiowrapper_init (self=0x7ffff7f9ecb8, 
    args=(<F at remote 0x7ffff6f40a18>,), kwds={'encoding': 'utf-8'})
    at ./Modules/_io/textio.c:965
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Date User Action Args
2014-11-11 22:55:26thatchsetrecipients: + thatch
2014-11-11 22:55:26thatchsetmessageid: <1415746526.05.0.824048300267.issue22849@psf.upfronthosting.co.za>
2014-11-11 22:55:26thatchlinkissue22849 messages
2014-11-11 22:55:25thatchcreate