libio: Ensure output buffer for wchars (bug #28828)

The _IO_wfile_overflow does not check if the write pointer for wide
data is valid before access, different than _IO_file_overflow.  This
leads to crash on some cases, as described by bug 28828.

The minimal sequence to produce the crash was:

  #include <stdio.h>
  #include <wchar.h>
  int main (int ac, char **av)
  {
    setvbuf (stdout, NULL, _IOLBF, 0);
    fgetwc (stdin);
    fputwc (10, stdout); /*CRASH HERE!*/
    return 0;
  }

The "fgetwc(stdin);" is necessary since it triggers the bug by setting
the flag _IO_CURRENTLY_PUTTING on stdout indirectly (file wfileops.c,
function _IO_wfile_underflow, line 213).

Signed-off-by: Jose Bollo <jobol@nonadev.net>

libio/Makefile

@@ -66,7 +66,7 @@ tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
 	tst-ftell-append tst-fputws tst-bz22415 tst-fgetc-after-eof \
 	tst-sprintf-ub tst-sprintf-chk-ub tst-bz24051 tst-bz24153 \
-	tst-wfile-sync
+	tst-wfile-sync tst-bz28828
 
 tests-internal = tst-vtables tst-vtables-interposed
 

libio/tst-bz28828.c

@@ -0,0 +1,32 @@
+/* Unit test for BZ#28828.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/xstdio.h>
+#include <support/check.h>
+#include <wchar.h>
+
+static int
+do_test (void)
+{
+  setvbuf (stdout, NULL, _IOLBF, 0);
+  fgetwc (stdin);
+  fputwc (10, stdout); /* It should not crash here.  */
+  return 0;
+}
+
+#include <support/test-driver.c>

libio/tst-bz28828.input

@@ -0,0 +1 @@
+hello

libio/wfileops.c

@@ -412,7 +412,8 @@ _IO_wfile_overflow (FILE *f, wint_t wch)
       return WEOF;
     }
   /* If currently reading or no buffer allocated. */
-  if ((f->_flags & _IO_CURRENTLY_PUTTING) == 0)
+  if ((f->_flags & _IO_CURRENTLY_PUTTING) == 0
+      || f->_wide_data->_IO_write_base == NULL)
     {
       /* Allocate a buffer if needed. */
       if (f->_wide_data->_IO_write_base == 0)