In the utf-16 decoder after calling unicode_decode_call_errorhandler aligned_end is not updated. This may potentially cause data leaks, memory damage, and crash. The bug introduced by implementation of the issue #4868. In a similar situation in the utf-8 decoder aligned_end is updated.

There is the crasher and leaker. When Python is not crashing, there is garbage (i.e. leakage of data) at the end of the decoded string. Indeed, I see an English text in some versions of Python.

There are many other errors in utf-16 decoder (see, for example, b'\xD8\x00\xDC'.decode('utf-16be')). I'm now finishing work on a new decoder, and after that take the bug fixing in 3.2.

Here is the bugs in the utf-16 decoder:

1. `aligned_end` is not updated after calling error handler.

2. Possible silent reading of one byte over the bytes array limit when decoding of a surrogate pair. b'\xD8\x00\xDC'.decode('utf-16be')

3. Error handlers receive data without last byte.

4. After handling truncate data error it is impossible to continue decoding (unlike all the other decoders).

The proposed patch will fix only the first of these bugs. The patch in issue #14624 fixes all bugs for Python 3.3. For Python 3.2 soon I will make a patch.

[moving from Rietveld back to Roundup]

On 2012/04/20 11:15:48, storchaka wrote:
> The `aligned_end` may point outside unicode object, 
> if the unicode object was reallocated.

How so? The aligned_end *never* points into the unicode object:

q = (unsigned char *)s;
e = q + size - 1;
aligned_end = (const unsigned char *) ((size_t) e & ~LONG_PTR_MASK);

So aligned_end points into s, not into the unicode object. 
So this adjustment is necessary because the *input* may change in the callback,
not because the output may change. So the comment in decode_utf8_errors seems
just as wrong.

Why this is relevant to this issue, is unclear to me, though: the ignore handler
doesn't modify the input object.

> So this adjustment is necessary because the *input* may change in the callback,
> not because the output may change. So the comment in decode_utf8_errors seems
> just as wrong.

You're right, and my eyes in a lather. Now I saw it.

What you have to offer any comment? If someone would correct a comment
for decode_utf8_errors, I just copied it.

> Why this is relevant to this issue, is unclear to me, though: the ignore handler
> doesn't modify the input object.

I first got the crash using a custom handler, and then I saw that
"ignore" handler is enough. Even if the "ignore" handler does not have
to change the input object, other handlers can do it and this is the
reason for the crash remains.

> You're right, and my eyes in a lather. Now I saw it.
> 
> What you have to offer any comment? If someone would correct a comment
> for decode_utf8_errors, I just copied it.

"might have changed the input object"

>> Why this is relevant to this issue, is unclear to me, though: the ignore handler
>> doesn't modify the input object.
> 
> I first got the crash using a custom handler, and then I saw that
> "ignore" handler is enough. Even if the "ignore" handler does not have
> to change the input object, other handlers can do it and this is the
> reason for the crash remains.

I agree that the change is necessary. It just does not explain why it
fixes this issue.

Here is a minimal patch that corrects all bugs for 3.2. As a side effect, decoding is accelerated by 4-8%.

Now I see the problem: make_decode_exception creates a new bytes object in any case, regardless of whether the error handler will update it or not. Therefore, decoding will continue in this new bytes object.

I think the same issue also applies to the ASCII decoder in 3.3.

> I think the same issue also applies to the ASCII decoder in 3.3.

No, the ASCII decoder is not affected by this vulnerability. In a loop,
in which unicode_decode_call_errorhandler is called, do not use any
cached and not-updatable data.

Here is a patch, which took into account the Martin suggestions.

Please use CVE-2012-2135 for this issue as per http://www.openwall.com/lists/oss-security/2012/04/25/3

I have not tried the patch yet, but modifying the reproducer yields a different crash. This one seems to be a heap-based buffer overflow which is slightly more serious.

In the reproducer, you just need to replace ascii() with str().

Again works on python3 only.

I now write tests and I have a question. Should b'\xd8\x00\x41'.decode('utf-16be', 'replace') to give '\xfffd' or '\xfffd\xfffd'?

Debian bug-report: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=670389
Found in versions python3-defaults/3.2.3~rc1-2, python3-defaults/3.1.3-12+squeeze1

I tested versions 3.1.1, 3.1.2, 3.1.3, 3.1.4 and 3.1.5 and only 3.1.3 crashed with Segmentation fault:

Program received signal SIGSEGV, Segmentation fault.
0x00000000004c483a in PyObject_Call (func=0x7ffff7e4d3b0, arg=0x7ffff70fd410, kw=0x0) at Objects/abstract.c:2156
2156	    if ((call = func->ob_type->tp_call) != NULL) {

(gdb) bt
#0  0x00000000004c483a in PyObject_Call (func=0x7ffff7e4d3b0, arg=0x7ffff70fd410, kw=0x0) at Objects/abstract.c:2156
#1  0x000000000045c437 in do_call (f=0x8929b0, throwflag=<value optimized out>) at Python/ceval.c:3982
#2  call_function (f=0x8929b0, throwflag=<value optimized out>) at Python/ceval.c:3785
#3  PyEval_EvalFrameEx (f=0x8929b0, throwflag=<value optimized out>) at Python/ceval.c:2548
#4  0x000000000045e675 in PyEval_EvalCodeEx (co=0x7ffff7159e30, globals=<value optimized out>, locals=<value optimized out>, args=0x0, argcount=1, kws=<value optimized out>, 
    kwcount=0, defs=0x0, defcount=0, kwdefs=0x0, closure=0x0) at Python/ceval.c:3198
#5  0x000000000045e77b in PyEval_EvalCode (co=0x7ffff7e4d3b0, globals=0x7ffff70fd410, locals=0x0) at Python/ceval.c:668
#6  0x00000000004800b2 in run_mod (fp=<value optimized out>, filename=<value optimized out>, flags=0x7fffffffe390) at Python/pythonrun.c:1711
#7  PyRun_InteractiveOneFlags (fp=<value optimized out>, filename=<value optimized out>, flags=0x7fffffffe390) at Python/pythonrun.c:1104
#8  0x00000000004803ce in PyRun_InteractiveLoopFlags (fp=0x7ffff75346a0, filename=0x5312a1 "<stdin>", flags=0x7fffffffe390) at Python/pythonrun.c:1006
#9  0x0000000000480bab in PyRun_AnyFileExFlags (fp=0x7ffff75346a0, filename=0x5312a1 "<stdin>", closeit=0, flags=0x7fffffffe390) at Python/pythonrun.c:975
#10 0x0000000000496422 in Py_Main (argc=<value optimized out>, argv=<value optimized out>) at Modules/main.c:607
#11 0x0000000000416e6e in main (argc=<value optimized out>, argv=<value optimized out>) at ./Modules/python.c:152

I thought it was one error, and not two.

The updated patch adds tests and fixes minor mistake. 2.7 is not
affected by main security issue, but it contains one of mentioned bugs
(read 1 byte outside of the input array). A patch for 2.7 fixes this bug
and also includes tests.

I ran tests of utf16_error_handling-3.2_4.patch on Python 3.1. Two tests are failing:
 - b'\x00\xd8'.decode('utf-16le', 'replace')='\ufffd\ufffd' != '\ufffd'
 - b'\xd8\x00'.decode('utf-16be', 'replace')='\ufffd\ufffd' != '\ufffd'

I don't think that the test is correct: UTF-16 should resynchronize as early as possible (ignore the first invalid byte and restart at the following byte), so '\ufffd\ufffd' is the correct answer.

Another examples:
 - b'\xd8\x00\x41'.decode('utf-16be', 'replace') should return '�A' (\ufffdA')
 - with UTF-8 decoder: (b'\xC3' + '\xe9'.encode('utf-8')).decode('utf-8', 'replace') returns '\ufffd\xe9'

> I ran tests of utf16_error_handling-3.2_4.patch on Python 3.1. Two tests are failing:
>  - b'\x00\xd8'.decode('utf-16le', 'replace')='\ufffd\ufffd' != '\ufffd'
>  - b'\xd8\x00'.decode('utf-16be', 'replace')='\ufffd\ufffd' != '\ufffd'
> 
> I don't think that the test is correct: UTF-16 should resynchronize as
> early as possible (ignore the first invalid byte and restart at the
> following byte), so '\ufffd\ufffd' is the correct answer.

UTF-16 units are 16-bit words, not bytes, so '\uffffd' sounds correct to
me. You resynchronize on the word boundary: the invalid word is skipped.

>  - with UTF-8 decoder: (b'\xC3' +
> '\xe9'.encode('utf-8')).decode('utf-8', 'replace') returns '\ufffd
> \xe9'

That's because UTF-8 operates on bytes: the invalid byte is skipped.

> UTF-16 units are 16-bit words, not bytes, so '\uffffd' sounds correct to
> me. You resynchronize on the word boundary: the invalid word is skipped.

I agree. The only odd case is when the number of bytes is not even
(pun intended). In that case, anybody can guess which of the bytes is
extra. The most natural (IMO) assumption is that the data is truncated,
so it would be the last byte which is extra.

Please, can anyone do a final review and commit?

Here are three patches for three Python versions:

2.7: utf16_error_handling-2.7.patch. Fix for one minor bug (overreading) and tests.

3.2: utf16_error_handling-3.2_4.patch. Fix for one critical security bug (CVE-2012-2135) and several minor bugs, tests.

3.3: utf16_error_handling-3.3.patch. Only tests.

There are spurious print() calls in the 2.7 patch.

New changeset 034ff986019d by Antoine Pitrou in branch '3.2':
Issue #14579: Fix CVE-2012-2135: vulnerability in the utf-16 decoder after error handling.
http://hg.python.org/cpython/rev/034ff986019d

New changeset 118fe0ee6921 by Antoine Pitrou in branch 'default':
Port additional tests from #14579 (the issue is already fixed).
http://hg.python.org/cpython/rev/118fe0ee6921

New changeset 4cadf91aaddd by Antoine Pitrou in branch '2.7':
Issue #14579: Fix error handling bug in the utf-16 decoder.
http://hg.python.org/cpython/rev/4cadf91aaddd

Thanks for the patches, Serhiy! They're now pushed.

> There are spurious print() calls in the 2.7 patch.

Oh, my inattentiveness. Thank you for pushing, Antoine. And thank Martin for 
review.