[PATCH v2] realpath: Set errno to ENAMETOOLONG for result larger than PATH_MAX [BZ #28770]

[Siddhesh Poyarekar]

On 13/01/2022 19:30, Adhemerval Zanella wrote:
> 
> 
> On 13/01/2022 02:59, Siddhesh Poyarekar via Libc-alpha 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>
> 
> CCing Paul because I believe he might want to fix it on gnulib as well.
> 
> Looks good, it just need to fix the Hurd build below.
> 
>> ---
>>   stdlib/Makefile               |  1 +
>>   stdlib/canonicalize.c         | 12 ++++-
>>   stdlib/tst-realpath-toolong.c | 84 +++++++++++++++++++++++++++++++++++
>>   3 files changed, 95 insertions(+), 2 deletions(-)
>>   create mode 100644 stdlib/tst-realpath-toolong.c
>>
>> diff --git a/stdlib/Makefile b/stdlib/Makefile
>> index 52e4d8cf19..897d4ea79d 100644
>> --- a/stdlib/Makefile
>> +++ b/stdlib/Makefile
>> @@ -108,6 +108,7 @@ tests		:=  bug-fmtmsg1 \
>>   		    tst-random \
>>   		    tst-random2 \
>>   		    tst-realpath \
>> +		    tst-realpath-toolong \
>>   		    tst-secure-getenv \
>>   		    tst-setcontext \
>>   		    tst-setcontext2 \
> 
> Ok.
> 
>> 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);
> 
> Ok.
> 
>> diff --git a/stdlib/tst-realpath-toolong.c b/stdlib/tst-realpath-toolong.c
>> new file mode 100644
>> index 0000000000..71c0829a80
>> --- /dev/null
>> +++ b/stdlib/tst-realpath-toolong.c
>> @@ -0,0 +1,84 @@
>> +/* 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."
>> +size_t nest_count;
>> +char dir[NAME_MAX + 1];
>> +char *base;
>> +
>> +static void
>> +delete_subdirectories (const char *name)
>> +{
>> +  TEST_VERIFY (chdir (name) == 0);
>> +  if (--nest_count > 0)
>> +    delete_subdirectories (dir);
>> +  TEST_VERIFY (chdir ("..") == 0);
>> +  TEST_VERIFY (rmdir (name) == 0);
>> +}
>> +
>> +static void
>> +do_cleanup (void)
>> +{
>> +  TEST_VERIFY (chdir (base) == 0);
>> +  if (nest_count > 0)
>> +    delete_subdirectories (dir);
>> +}
>> +
>> +int
>> +do_test (void)
>> +{
>> +  base = support_create_temp_directory (BASENAME);
>> +
>> +  TEST_VERIFY_EXIT (chdir (base) == 0);
>> +
>> +  memset (dir, 'X', sizeof (dir) - 1);
>> +  dir[NAME_MAX] = '\0';
>> +
>> +  /* Create directories and descend into them so that the final path is larger
>> +     than PATH_MAX.  */
>> +  for (nest_count = 0; nest_count <= PATH_MAX / sizeof (dir); nest_count++)
> 
> This fails to build on Hurd, so I am not sure if it would be better to just
> set it as UNSUPPORTED in this case or use the assumption on realpath:
> 
>    #ifdef PATH_MAX
>    size_t path_max = PATH_MAX;
>    #else
>    size_t path_max = pathconf ("/", _PC_PATH_MAX);
>    path_max = path_max < 0 ? 1024 : path_max <  ? path_max : PTRDIFF_MAX;
>    #endif

Ahh yes, I'll fix up the test for hurd using this snippet.

Thanks,
Siddhesh