[PATCH v6] stdio-common: Fix buffer overflow in scanf %mc [BZ #34008]

[H.J. Lu]

On Sat, Apr 18, 2026 at 2:49 PM Rocket Ma <marocketbd@gmail.com> wrote:
>
> * stdio-common/vfscanf-internal.c: When enlarging allocated buffer with
> format %mc or %mC, glibc allocates one byte less, leading to
> user-controlled one byte overflow. This commit fixes BZ #34008, or
> CVE-2026-5450.
>
> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
> Signed-off-by: Rocket Ma <marocketbd@gmail.com>
> ---
>  stdio-common/Makefile              |  4 +++
>  stdio-common/tst-vfscanf-bz34008.c | 48 ++++++++++++++++++++++++++++++
>  stdio-common/vfscanf-internal.c    |  7 ++---
>  3 files changed, 55 insertions(+), 4 deletions(-)
>  create mode 100644 stdio-common/tst-vfscanf-bz34008.c
>
> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
> index 210944837e..0c0085e607 100644
> --- a/stdio-common/Makefile
> +++ b/stdio-common/Makefile
> @@ -349,6 +349,7 @@ tests := \
>    tst-vfprintf-user-type \
>    tst-vfprintf-width-i18n \
>    tst-vfprintf-width-prec-alloc \
> +  tst-vfscanf-bz34008 \
>    tst-wc-printf \
>    tstdiomisc \
>    tstgetln \
> @@ -564,6 +565,9 @@ tst-printf-bz18872-ENV = MALLOC_TRACE=$(objpfx)tst-printf-bz18872.mtrace \
>  tst-vfprintf-width-prec-ENV = \
>    MALLOC_TRACE=$(objpfx)tst-vfprintf-width-prec.mtrace \
>    LD_PRELOAD=$(common-objpfx)/malloc/libc_malloc_debug.so
> +tst-vfscanf-bz34008-ENV = \
> +  MALLOC_CHECK_=3 \
> +  LD_PRELOAD=$(common-objpfx)/malloc/libc_malloc_debug.so
>  tst-printf-bz25691-ENV = \
>    MALLOC_TRACE=$(objpfx)tst-printf-bz25691.mtrace \
>    LD_PRELOAD=$(common-objpfx)/malloc/libc_malloc_debug.so
> diff --git a/stdio-common/tst-vfscanf-bz34008.c b/stdio-common/tst-vfscanf-bz34008.c
> new file mode 100644
> index 0000000000..af746821fb
> --- /dev/null
> +++ b/stdio-common/tst-vfscanf-bz34008.c
> @@ -0,0 +1,48 @@
> +/* Regression test for vfscanf %Nmc out-of-bound write (BZ #34008)
> +   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 "malloc/mcheck.h"
> +#include <stddef.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <wchar.h>
> +#include <stdlib.h>
> +#include <malloc.h>
> +#include <support/check.h>
> +
> +#define WIDTH 0x410
> +#define SCANFSTR "%1040mc"
> +static int
> +do_test (void)
> +{
> +  mcheck_pedantic (NULL);
> +  char *input = malloc (WIDTH + 1);
> +  TEST_VERIFY (input != NULL);
> +  memset (input, 'A', WIDTH);
> +  input[WIDTH] = '\0';
> +
> +  char *buf = NULL;
> +  TEST_VERIFY (sscanf (input, SCANFSTR, &buf) != -1);
> +  TEST_VERIFY (buf != NULL);
> +
> +  free (buf);
> +  free (input);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> diff --git a/stdio-common/vfscanf-internal.c b/stdio-common/vfscanf-internal.c
> index 59fc8208aa..3d11ac261e 100644
> --- a/stdio-common/vfscanf-internal.c
> +++ b/stdio-common/vfscanf-internal.c
> @@ -855,8 +855,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
>                         {
>                           /* Enlarge the buffer.  */
>                           size_t newsize
> -                           = strsize
> -                             + (strsize >= width ? width - 1 : strsize);
> +                           = strsize + (strsize >= width ? width : strsize);
>
>                           str = (char *) realloc (*strptr, newsize);
>                           if (str == NULL)
> @@ -929,7 +928,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
>                       && wstr == (wchar_t *) *strptr + strsize)
>                     {
>                       size_t newsize
> -                       = strsize + (strsize > width ? width - 1 : strsize);
> +                       = strsize + (strsize >= width ? width : strsize);
>                       /* Enlarge the buffer.  */
>                       wstr = (wchar_t *) realloc (*strptr,
>                                                   newsize * sizeof (wchar_t));
> @@ -984,7 +983,7 @@ __vfscanf_internal (FILE *s, const char *format, va_list argptr,
>                     && wstr == (wchar_t *) *strptr + strsize)
>                   {
>                     size_t newsize
> -                     = strsize + (strsize > width ? width - 1 : strsize);
> +                     = strsize + (strsize >= width ? width : strsize);
>                     /* Enlarge the buffer.  */
>                     wstr = (wchar_t *) realloc (*strptr,
>                                                 newsize * sizeof (wchar_t));
> --
> 2.53.0

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.


-- 
H.J.