From 8247efc90a0312928a7b7d9bbb692e666b3f7e19 Mon Sep 17 00:00:00 2001 From: Rocket Ma Date: Fri, 1 May 2026 20:39:07 -0700 Subject: [PATCH] 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 Reviewed-by: Carlos O'Donell --- libio/Makefile | 2 +- libio/bug-wgenops-bz33998.c | 54 +++++++++++++++++++++++++++++++++++++ libio/wgenops.c | 4 +-- 3 files changed, 57 insertions(+), 3 deletions(-) create mode 100644 libio/bug-wgenops-bz33998.c Index: glibc-2.31/libio/Makefile =================================================================== --- glibc-2.31.orig/libio/Makefile +++ glibc-2.31/libio/Makefile @@ -66,7 +66,7 @@ tests = tst_swprintf tst_wprintf tst_sws 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 bug-wgenops-bz33998 tests-internal = tst-vtables tst-vtables-interposed tst-readline Index: glibc-2.31/libio/bug-wgenops-bz33998.c =================================================================== --- /dev/null +++ glibc-2.31/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 + . */ + +#include "support/temp_file.h" +#include "support/xstdio.h" +#include "support/xunistd.h" +#include +#include +#include +#include +#include +#include + +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 Index: glibc-2.31/libio/wgenops.c =================================================================== --- glibc-2.31.orig/libio/wgenops.c +++ glibc-2.31/libio/wgenops.c @@ -110,8 +110,8 @@ _IO_wdefault_pbackfail (FILE *fp, wint_t { 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!*/