[PATCH v4] libio: Fix ungetwc operating on byte stream [BZ #33998]

[Rocket Ma]

* libio/wgenops.c: When _IO_wdefault_pbackfail attempts to push back one
character, it accidently compare the wchar to push back with the last
char from byte stream, instead of wide stream. Under specific coding,
attacker may exploit this to leak information. This commit fix bug
33998, or CVE-2026-5928.

Signed-off-by: Rocket Ma <marocketbd@gmail.com>
---
Removed redundant macro from previous patch.
---
 libio/Makefile              |  1 +
 libio/bug-wgenops-bz33998.c | 44 +++++++++++++++++++++++++++++++++++++
 libio/wgenops.c             |  4 ++--
 3 files changed, 47 insertions(+), 2 deletions(-)
 create mode 100644 libio/bug-wgenops-bz33998.c

diff --git a/libio/Makefile b/libio/Makefile
index 93656466df..6e0627bb88 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -84,6 +84,7 @@ tests = \
   bug-ungetwc1 \
   bug-ungetwc2 \
   bug-wfflush \
+  bug-wgenops-bz33998 \
   bug-wmemstream1 \
   bug-wsetpos \
   test-fmemopen \
diff --git a/libio/bug-wgenops-bz33998.c b/libio/bug-wgenops-bz33998.c
new file mode 100644
index 0000000000..b3f750a753
--- /dev/null
+++ b/libio/bug-wgenops-bz33998.c
@@ -0,0 +1,44 @@
+/* Regression test for ungetwc operating on byte stream (BZ #33998)
+   Copyright (C) 2026 The GNU Toolchain Authors.
+   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 <unistd.h>
+#include <sys/mman.h>
+#include <stdio.h>
+#include <wchar.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  int fd = memfd_create ("test", MFD_CLOEXEC);
+  TEST_VERIFY (fd != -1);
+  TEST_COMPARE (write (fd, (unsigned char[]){ 'A', 0, 0, 0 }, 4), 4);
+  TEST_COMPARE (lseek (fd, 0, SEEK_SET), 0);
+  FILE *fp = fdopen (fd, "r+");
+  TEST_VERIFY (fp != NULL);
+  TEST_COMPARE (getwc (fp), L'A');
+
+  /* if the bug is fixed, then ungetwc should not touch byte stream. */
+  char *old_read_ptr = fp->_IO_read_ptr;
+  TEST_COMPARE (ungetwc (0, fp), L'\0');
+  TEST_VERIFY (fp->_IO_read_ptr == old_read_ptr);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/wgenops.c b/libio/wgenops.c
index 6829477e0c..5f36bc49a1 100644
--- a/libio/wgenops.c
+++ b/libio/wgenops.c
@@ -110,8 +110,8 @@ _IO_wdefault_pbackfail (FILE *fp, wint_t c)
 {
   if (fp->_wide_data->_IO_read_ptr > fp->_wide_data->_IO_read_base
       && !_IO_in_backup (fp)
-      && (wint_t) fp->_IO_read_ptr[-1] == c)
-    --fp->_IO_read_ptr;
+      && (wint_t) fp->_wide_data->_IO_read_ptr[-1] == c)
+    --fp->_wide_data->_IO_read_ptr;
   else
     {
       /* Need to handle a filebuf in write mode (switch to read mode). FIXME!*/
-- 
2.53.0