[PATCH] Add dynamic linker support for $EXEC_ORIGIN.

[Brooks Moses <bmoses at google dot com>]

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.

We have been using a slight variation of this patch at Google for about a year, with no issues.


(Note: This is my first attempt at sending a patch with git send-email; please let me know of any problems!)

Thanks,
- Brooks

---

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                 | 88 ++++++++++++++++++++++++++++++++++++++++++++++
 sysdeps/generic/ldsodefs.h |  3 ++
 5 files changed, 110 insertions(+), 3 deletions(-)

diff --git a/elf/dl-dst.h b/elf/dl-dst.h
index 3ed95d0..c656380 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..ec5952b 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -51,6 +51,12 @@ extern __typeof (__mempcpy) __mempcpy attribute_hidden;
 extern __typeof (_exit) exit_internal asm ("_exit") attribute_hidden;
 #define _exit exit_internal
 
+/* Iterate over auxv, find AT_EXECFN if any.  */
+static char * get_at_execfn(ElfW(auxv_t) *auxv);
+
+/* 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 +79,9 @@ 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 +1041,8 @@ 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 +1137,8 @@ of this helper program; chances are you did not intend to run this program.\n\
     }
   else
     {
+      set_exec_origin_path (get_at_execfn (auxv));
+
       /* 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 +2820,78 @@ 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);
+}
+
+/* Iterate over auxv, find AT_EXECFN if any.  */
+static char *
+get_at_execfn (ElfW(auxv_t) *auxv)
+{
+  assert (auxv != NULL);
+
+  for (; auxv->a_type != AT_NULL; ++auxv)
+    if (auxv->a_type == AT_EXECFN)
+      return (char *) auxv->a_un.a_val;
+
+  return NULL;
+}
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