[New-bugs-announce] [issue20538] Segfault in UTF-7 incremental decoder

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

UTF-7 incremental decoder can crash in debug build when decodes unfinished base-64 section. In non-debug build it just produces inconsistent unicode string. Minimal examples:

$ ./python -c "import codecs; codecs.utf_7_decode(b'a+AIA', 'strict')"
python: Objects/unicodeobject.c:403: _PyUnicode_CheckConsistency: Assertion `maxchar >= 128' failed.
Aborted (core dumped)

$ ./python -c "import codecs; codecs.utf_7_decode(b'+AIA-+AQA', 'strict')"
python: Objects/unicodeobject.c:410: _PyUnicode_CheckConsistency: Assertion `maxchar >= 0x100' failed.
Aborted (core dumped)

$ ./python -c "import codecs; codecs.utf_7_decode(b'+AQA-+2ADcAA', 'strict')"
python: Objects/unicodeobject.c:414: _PyUnicode_CheckConsistency: Assertion `maxchar >= 0x10000' failed.
Aborted (core dumped)

This happens because _PyUnicodeWriter reverts position back before unfinished base-64 section, but its buffer was already widened for characters in unfinished base-64 section.

        if (inShift) {
            writer.pos = shiftOutStart; /* back off output */
            *consumed = startinpos;
        }

And now _PyUnicodeWriter generates a string with a kind larger then needed for decoded characters.

This bug causes a lot of crashes on buildbots. E.g:
http://buildbot.python.org/all/builders/AMD64%20Snow%20Leop%203.x/builds/1197
http://buildbot.python.org/all/builders/AMD64%20Ubuntu%20LTS%203.3/builds/1446

----------
components: Interpreter Core, Unicode
messages: 210444
nosy: ezio.melotti, haypo, serhiy.storchaka
priority: high
severity: normal
stage: needs patch
status: open
title: Segfault in UTF-7 incremental decoder
type: crash
versions: Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20538>
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