[Bug stdio/17269] New: _IO_wstr_overflow integer overflow

["jsm28 at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17269

            Bug ID: 17269
           Summary: _IO_wstr_overflow integer overflow
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

libio/wstrops.c:_IO_wstr_overflow does:

          _IO_size_t new_size = 2 * old_wblen + 100;
          if (new_size < old_wblen)
            return EOF;
          new_buf
            = (wchar_t *) (*((_IO_strfile *) fp)->_s._allocate_buffer)
(new_size
                                                                        *
sizeof (wchar_t));

This allows for overflow in calculating the new size in wide characters, but
not for overflow in the multiplication to compute the size in bytes, which
could thus overflow and result in a buffer overrun copying data into the new
buffer.

(I don't know about the practical exploitability with a stream opened with
open_wmemstream.  At the doubling before the one resulting in the overflow
you'd have buffers of (nearly) 1/4 and 1/2 of the address space or more
allocated simultaneously, so it needs to be possible to allocate 3/4 of the
address space in those two contiguous regions in order to get to the point
where the overflow occurs.)

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