Ping: [PATCH][v2] Add dynamic linker support for $EXEC_ORIGIN.

[Brooks Moses <bmoses at google dot com>]

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On Thu, Dec 12, 2013 at 2:13 PM, Brooks Moses <bmoses@google.com> wrote:
> Updated to address Paul's comments about using getsauxval().
>
> The $ORIGIN rpath token expands to the directory containing the true
> copy of the application executable -- which is to say, if the executable
> is invoked through a symlink, that symlink will be resolved in the
> $ORIGIN substitution.
>
> In some cases, such as symlink farms backed by cloud storage filesystems
> where the resolved path encodes effectively-arbitrary storage data, it
> is much more useful to have a rpath token that points to the
> non-expanded, as-called version of the executable path.  This patch adds
> the $EXEC_ORIGIN token for this purpose, resolving to the executable
> path as passed to execve().
>
> One potential security risk of such a token expansion is that, although
> copying a setuid/setgid executable (to change its $ORIGIN) loses the
> setuid/setgid bit, creating a symlink does not -- and so a user could
> inject arbitrary code into a setuid/setgid executable that references
> $EXEC_ORIGIN by creating an appropriate symlink.  In order to slam the
> door shut on this security risk, this patch simply prohibits the use of
> $EXEC_ORIGIN in setuid/setgid binaries.
>
> Ok to commit?
>
> ---
> 2013-12-12  Brooks Moses  <bmoses@google.com>
>
>         * elf/dl-support.c (_dl_exec_origin_path): New global variable.
>         * elf/dl-dst.h (DL_DST_REQUIRED): Include _dl_exec_origin_path
>         in computing dst_len.
>         * elf/dl-load.c (_dl_dst_count): Also handle "EXEC_ORIGIN"
>         token.
>         (_dl_dst_substitute): Likewise.
>         * elf/rtld.c (get_directory): New static function.
>         (set_exec_origin_path): New static function.
>         (dl_main): Call set_exec_origin_path.
>         * sysdeps/generic/ldsodefs.h (rtld_global_ro): Add
>         _dl_exec_origin_path.
>
>  elf/dl-dst.h               |  9 ++++--
>  elf/dl-load.c              | 10 ++++++-
>  elf/dl-support.c           |  3 ++
>  elf/rtld.c                 | 72 ++++++++++++++++++++++++++++++++++++++++++++++
>  sysdeps/generic/ldsodefs.h |  3 ++
>  5 files changed, 94 insertions(+), 3 deletions(-)
>
> diff --git a/elf/dl-dst.h b/elf/dl-dst.h
> index 3ed95d0..75f2893 100644
> --- a/elf/dl-dst.h
> +++ b/elf/dl-dst.h
> @@ -65,8 +65,13 @@
>         else                                                                  \
>           dst_len = (l)->l_origin == (char *) -1                              \
>             ? 0 : strlen ((l)->l_origin);                                     \
> -       dst_len = MAX (MAX (dst_len, GLRO(dl_platformlen)),                   \
> -                      strlen (DL_DST_LIB));                                  \
> +                                                                             \
> +       char *exec_origin = GLRO(dl_exec_origin_path);                        \
> +       size_t exec_origin_len =                                              \
> +         (exec_origin == NULL) ? 0 : strlen (exec_origin);                   \
> +                                                                             \
> +       dst_len = MAX (MAX (MAX (dst_len, GLRO(dl_platformlen)),              \
> +                           strlen (DL_DST_LIB)), exec_origin_len);           \
>         if (dst_len > 4)                                                      \
>           __len += __cnt * (dst_len - 4);                                     \
>        }                                                                              \
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index d3e1cf8..04956dc 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -306,7 +306,8 @@ _dl_dst_count (const char *name, int is_path)
>        if ((len = is_dst (start, name, "ORIGIN", is_path,
>                          INTUSE(__libc_enable_secure))) != 0
>           || (len = is_dst (start, name, "PLATFORM", is_path, 0)) != 0
> -         || (len = is_dst (start, name, "LIB", is_path, 0)) != 0)
> +         || (len = is_dst (start, name, "LIB", is_path, 0)) != 0
> +         || (len = is_dst (start, name, "EXEC_ORIGIN", is_path, 0)) != 0)
>         ++cnt;
>
>        name = strchr (name + len, '$');
> @@ -350,6 +351,13 @@ _dl_dst_substitute (struct link_map *l, const char *name, char *result,
>             repl = GLRO(dl_platform);
>           else if ((len = is_dst (start, name, "LIB", is_path, 0)) != 0)
>             repl = DL_DST_LIB;
> +         else if ((len = is_dst (start, name, "EXEC_ORIGIN", is_path, 0)) != 0)
> +           {
> +             if (INTUSE(__libc_enable_secure) != 0)
> +               _dl_fatal_printf ("$EXEC_ORIGIN rpath entry not allowed in setuid/setgid executables.\n");
> +
> +             repl = GLRO(dl_exec_origin_path);
> +           }
>
>           if (repl != NULL && repl != (const char *) -1)
>             {
> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index 2023bd0..d2d427a 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -67,6 +67,9 @@ void *__libc_stack_end;
>  /* Path where the binary is found.  */
>  const char *_dl_origin_path;
>
> +/* Directory where the AT_EXECFN is found.  */
> +const char *_dl_exec_origin_path;
> +
>  /* Nonzero if runtime lookup should not update the .got/.plt.  */
>  int _dl_bind_not;
>
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 3d207a3..9497b47 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -51,6 +51,9 @@ extern __typeof (__mempcpy) __mempcpy attribute_hidden;
>  extern __typeof (_exit) exit_internal asm ("_exit") attribute_hidden;
>  #define _exit exit_internal
>
> +/* Given file path, return absolute directory path.  */
> +static char * get_directory (const char *file_path);
> +
>  /* Helper function to handle errors while resolving symbols.  */
>  static void print_unresolved (int errcode, const char *objname,
>                               const char *errsting);
> @@ -73,6 +76,9 @@ enum mode { normal, list, verify, trace };
>     all the entries.  */
>  static void process_envvars (enum mode *modep);
>
> +/* Set GLRO(dl_exec_origin_path).  */
> +static void set_exec_origin_path(const char *exe_path);
> +
>  #ifdef DL_ARGV_NOT_RELRO
>  int _dl_argc attribute_hidden;
>  char **_dl_argv = NULL;
> @@ -1032,6 +1038,8 @@ of this helper program; chances are you did not intend to run this program.\n\
>                         in LIST\n\
>    --audit LIST          use objects named in LIST as auditors\n");
>
> +      set_exec_origin_path (INTUSE(_dl_argv)[1]);
> +
>        ++_dl_skip_args;
>        --_dl_argc;
>        ++INTUSE(_dl_argv);
> @@ -1126,6 +1134,8 @@ of this helper program; chances are you did not intend to run this program.\n\
>      }
>    else
>      {
> +      set_exec_origin_path ((const char *) getauxval (AT_EXECFN));
> +
>        /* Create a link_map for the executable itself.
>          This will be what dlopen on "" returns.  */
>        main_map = _dl_new_object ((char *) "", "", lt_executable, NULL,
> @@ -2807,3 +2817,65 @@ print_statistics (hp_timing_t *rtld_total_timep)
>      }
>  #endif
>  }
> +
> +/* Given file path, return an absolute directory path.
> +   Examples: in: "/foo/bar/a.out", out: "/foo/bar/";
> +   in: "./a.out", out: "/dot/resolved/to/full/path/./".  */
> +static char *
> +get_directory (const char *file_path)
> +{
> +  assert (file_path != NULL);
> +
> +  /* Find the end of the directory substring in file_path.  */
> +  size_t path_len = strlen (file_path);
> +  while (path_len > 0 && file_path[path_len - 1] != '/')
> +    --path_len;
> +
> +  /* Allocate space and set the path prefix according to whether or not
> +     this is an absolute path.  */
> +  char *dest;
> +  char *full_dir_path;
> +  if (file_path[0] == '/')
> +    {
> +      full_dir_path = malloc (path_len + 1);
> +      assert (full_dir_path != NULL);
> +      dest = full_dir_path;
> +    }
> +  else
> +    {
> +      /* For a relative path, we need to include space for the largest
> +        possible current path, a joining '/', the relevant part of
> +        file_path, and a trailing '\0'.  */
> +      full_dir_path = malloc (PATH_MAX + path_len + 2);
> +      assert (full_dir_path != NULL);
> +
> +      char *status = __getcwd (full_dir_path, PATH_MAX);
> +      assert (status != NULL);
> +
> +      char *dest = __rawmemchr (full_dir_path, '\0');
> +      if (dest[-1] != '/')
> +       *dest++ = '/';
> +    }
> +
> +  if (path_len > 0)
> +    dest = __mempcpy (dest, file_path, path_len);
> +  *dest = '\0';
> +
> +  /* Confirm that the constructed path is valid.  */
> +  struct stat64 st;
> +  assert (__xstat64 (_STAT_VER, full_dir_path, &st) == 0);
> +
> +  return full_dir_path;
> +}
> +
> +/* Set GLRO(dl_exec_origin_path).  */
> +static void
> +set_exec_origin_path (const char *exe_path)
> +{
> +  assert (GLRO(dl_exec_origin_path) == NULL);
> +
> +  if (GLRO(dl_origin_path) != NULL)
> +    GLRO(dl_exec_origin_path) = strdup (GLRO(dl_origin_path));
> +  else if (exe_path != NULL)
> +    GLRO(dl_exec_origin_path) = get_directory (exe_path);
> +}
> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 146aca4..561a763 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -518,6 +518,9 @@ struct rtld_global_ro
>    /* Location of the binary.  */
>    EXTERN const char *_dl_origin_path;
>
> +  /* Directory where the AT_EXECFN is found.  */
> +  EXTERN const char *_dl_exec_origin_path;
> +
>    /* -1 if the dynamic linker should honor library load bias,
>       0 if not, -2 use the default (honor biases for normal
>       binaries, don't honor for PIEs).  */
> --
> 1.8.5.1
>