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Ping^5? Re: [PATCH][v3] Add dynamic linker support for $EXEC_ORIGIN.


Ping^5?  It's now been 79 days since I posted the updated patch.

Original here: https://sourceware.org/ml/libc-alpha/2013-12/msg00816.html

On Sat, Dec 28, 2013 at 10:50 AM, Brooks Moses <bmoses@google.com> wrote:
> Andreas, Carlos, Roland -
>
> As best I can tell from git history, you three are the people most
> familiar with the ld.so code.  Any chance one of you could review this
> before the code freeze?  If so, I'd appreciate it, and if not, I'll
> hold off on re-pinging until after the release.
>
> Thanks,
> - Brooks
>
> On Mon, Dec 23, 2013 at 11:46 AM, Brooks Moses <bmoses@google.com> wrote:
>> Updated to include Mike Frysinger's comments, as below.  Mike suggests
>> that this still needs signoff from someone more familiar with the ld.so
>> code; any volunteers?
>>
>> On Sun, Dec 22, 2013 at 11:51 PM, Mike Frysinger <vapier@gentoo.org> wrote:
>>> On Thursday 12 December 2013 17:13:39 Brooks Moses wrote:
>>>> --- a/elf/dl-dst.h
>>>> +++ b/elf/dl-dst.h
>>>>
>>>> +     char *exec_origin = GLRO(dl_exec_origin_path);                        \
>>>
>>> shouldn't that be const ?
>>
>> Yup, it should.  Fixed.
>>
>>>> --- a/elf/rtld.c
>>>> +++ b/elf/rtld.c
>>>>
>>>> +static void set_exec_origin_path(const char *exe_path);
>>>
>>> space before the (
>>
>> Also fixed.
>>
>>> otherwise, looks OK, but probably should get sign off from someone more
>>> familiar with the ldso code
>>
>> Thanks!
>> - Brooks
>>
>> ---
>> 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.
>>
>> ---
>> 2013-12-08  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_at_execfn): New static function.
>>         (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..13cd0e7 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));                                  \
>> +                                                                             \
>> +       const 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..0b3dd1f 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
>>


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