[2/3] realpath: Set errno to ENAMETOOLONG for result larger than PATH_MAX [BZ #28770]

realpath returns an allocated string when the result exceeds PATH_MAX,
which is unexpected when its second argument is not NULL.  This results
in the second argument (resolved) being uninitialized and also results
in a memory leak since the caller expects resolved to be the same as the
returned value.

Return NULL and set errno to ENAMETOOLONG if the result exceeds
PATH_MAX.  This fixes [BZ #28770], which is CVE-2021-3998.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
---
 NEWS                          |  4 +++
 stdlib/Makefile               |  1 +
 stdlib/canonicalize.c         | 12 +++++++--
 stdlib/tst-realpath-toolong.c | 48 +++++++++++++++++++++++++++++++++++
 4 files changed, 63 insertions(+), 2 deletions(-)
 create mode 100644 stdlib/tst-realpath-toolong.c

On 18/01/2022 06:07, Siddhesh Poyarekar wrote:
> realpath returns an allocated string when the result exceeds PATH_MAX,
> which is unexpected when its second argument is not NULL.  This results
> in the second argument (resolved) being uninitialized and also results
> in a memory leak since the caller expects resolved to be the same as the
> returned value.
> 
> Return NULL and set errno to ENAMETOOLONG if the result exceeds
> PATH_MAX.  This fixes [BZ #28770], which is CVE-2021-3998.
> 
> Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  NEWS                          |  4 +++
>  stdlib/Makefile               |  1 +
>  stdlib/canonicalize.c         | 12 +++++++--
>  stdlib/tst-realpath-toolong.c | 48 +++++++++++++++++++++++++++++++++++
>  4 files changed, 63 insertions(+), 2 deletions(-)
>  create mode 100644 stdlib/tst-realpath-toolong.c
> 
> diff --git a/NEWS b/NEWS
> index 38802f0673..5c63cef156 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -163,6 +163,10 @@ Security related changes:
>    CVE-2022-23218: Passing an overlong file name to the svcunix_create
>    legacy function could result in a stack-based buffer overflow.
>  
> +  CVE-2021-3998: Passing a path longer than PATH_MAX to the realpath
> +  function could result in a memory leak and potential access of
> +  uninitialized memory.
> +
>  The following bugs are resolved with this release:
>  
>    [The release manager will add the list generated by
> diff --git a/stdlib/Makefile b/stdlib/Makefile
> index 1e81f98fac..8236741984 100644
> --- a/stdlib/Makefile
> +++ b/stdlib/Makefile
> @@ -109,6 +109,7 @@ tests := \
>    tst-random \
>    tst-random2 \
>    tst-realpath \
> +  tst-realpath-toolong \
>    tst-secure-getenv \
>    tst-setcontext \
>    tst-setcontext2 \
> diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
> index f36bdf4c76..732dc7ea46 100644
> --- a/stdlib/canonicalize.c
> +++ b/stdlib/canonicalize.c
> @@ -400,8 +400,16 @@ realpath_stk (const char *name, char *resolved,
>  
>  error:
>    *dest++ = '\0';
> -  if (resolved != NULL && dest - rname <= get_path_max ())
> -    rname = strcpy (resolved, rname);
> +  if (resolved != NULL)
> +    {
> +      if (dest - rname <= get_path_max ())
> +	rname = strcpy (resolved, rname);
> +      else
> +	{
> +	  failed = true;
> +	  __set_errno (ENAMETOOLONG);
> +	}
> +    }
>  
>  error_nomem:
>    scratch_buffer_free (&extra_buffer);
> diff --git a/stdlib/tst-realpath-toolong.c b/stdlib/tst-realpath-toolong.c
> new file mode 100644
> index 0000000000..f10653f05b
> --- /dev/null
> +++ b/stdlib/tst-realpath-toolong.c
> @@ -0,0 +1,48 @@
> +/* Verify that realpath returns NULL with ENAMETOOLONG if the result exceeds
> +   NAME_MAX.
> +   Copyright 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 <errno.h>
> +#include <limits.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <support/check.h>
> +#include <support/temp_file.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +
> +#define BASENAME "tst-realpath-toolong."
> +
> +int
> +do_test (void)
> +{
> +  char *base = support_create_and_chdir_toolong_temp_directory (BASENAME);
> +
> +  char buf[PATH_MAX + 1];
> +  const char * const res = realpath (".", buf);
> +
> +  /* canonicalize.c states that if the real path is >= PATH_MAX, then
> +     realpath returns NULL and sets ENAMETOOLONG.  */
> +  TEST_VERIFY_EXIT (res == NULL && errno == ENAMETOOLONG);
> +
> +  free (base);
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>