CVE-2026-5928: Fix ungetwc operating on byte stream (RHEL-180340)

Resolves: RHEL-180340
2026-06-26 11:52:03 -04:00 · 2026-06-26 11:52:03 -04:00 · 2acec79102
commit 2acec79102
parent ec99e297ec
1 changed files with 102 additions and 0 deletions
--- a/glibc-RHEL-180340.patch
+++ b/glibc-RHEL-180340.patch
@ -0,0 +1,102 @@
+commit ef3bfb5f910011f3780cb06aa47e730035f53285
+Author: Rocket Ma <marocketbd@gmail.com>
+Date:   Fri May 1 20:39:07 2026 -0700
+
+    libio: Fix ungetwc operating on byte stream [BZ #33998]
+    
+    * 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>
+    Reviewed-by: Carlos O'Donell <carlos@redhat.com>
+
+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..cc4067da99
+--- /dev/null
+++ b/libio/bug-wgenops-bz33998.c
+@@ -0,0 +1,54 @@
+/* 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 "support/temp_file.h"
+#include "support/xstdio.h"
+#include "support/xunistd.h"
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <stdio.h>
+#include <wchar.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+  char *filename;
+  int fd = create_temp_file ("tst-bz33998-", &filename);
+  TEST_VERIFY (fd != -1);
+  xwrite (fd, "A", sizeof ("A")); // write "A\0" by design
+  xclose (fd);
+
+  FILE *fp = xfopen (filename, "r+");
+  TEST_COMPARE (getwc (fp), L'A');
+  /* If the bug is fixed, then ungetwc should not touch byte stream.
+     If the bug is not fixed, ungetwc firstly match last read char, L'A',
+     failed, then the pbackfail branch, matching last read char in byte
+     stream, that is, '\0' (initialized when setup wide stream). */
+  char *old_read_ptr = fp->_IO_read_ptr;
+  TEST_COMPARE (ungetwc (L'\0', fp), L'\0');
+  TEST_VERIFY (fp->_IO_read_ptr == old_read_ptr);
+
+  xfclose (fp);
+  free (filename);
+
+  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!*/