elf: Check for empty tokens before dynamic string token expansion [BZ #22625]

The fillin_rpath function in elf/dl-load.c loops over each RPATH or
RUNPATH tokens and interprets empty tokens as the current directory
("./"). In practice the check for empty token is done *after* the
dynamic string token expansion. The expansion process can return an
empty string for the $ORIGIN token if __libc_enable_secure is set
or if the path of the binary can not be determined (/proc not mounted).

Fix that by moving the check for empty tokens before the dynamic string
token expansion. In addition, check for NULL pointer or empty strings
return by expand_dynamic_string_token.

The above changes highlighted a bug in decompose_rpath, an empty array
is represented by the first element being NULL at the fillin_rpath
level, but by using a -1 pointer in decompose_rpath and other functions.

Changelog:
	[BZ #22625]
	* elf/dl-load.c (fillin_rpath): Check for empty tokens before dynamic
	string token expansion. Check for NULL pointer or empty string possibly
	returned by expand_dynamic_string_token.
	(decompose_rpath): Check for empty path after dynamic string
	token expansion.
(cherry picked from commit 3e3c904daef69b8bf7d5cc07f793c9f07c3553ef)

ChangeLog

@@ -1,3 +1,13 @@
+2017-12-30  Aurelien Jarno  <aurelien@aurel32.net>
+	    Dmitry V. Levin  <ldv@altlinux.org>
+
+	[BZ #22625]
+	* elf/dl-load.c (fillin_rpath): Check for empty tokens before dynamic
+	string token expansion. Check for NULL pointer or empty string possibly
+	returned by expand_dynamic_string_token.
+	(decompose_rpath): Check for empty path after dynamic string
+	token expansion.
+
 2017-12-18  Dmitry V. Levin  <ldv@altlinux.org>
 
 	[BZ #22627]

NEWS

@@ -55,6 +55,10 @@ Security related changes:
   CVE-2017-1000366 has been applied, but it is mentioned here only because
   of the CVE assignment.)  Reported by Qualys.
 
+  CVE-2017-16997: Incorrect handling of RPATH or RUNPATH containing $ORIGIN
+  for AT_SECURE or SUID binaries could be used to load libraries from the
+  current directory.
+
 The following bugs are resolved with this release:
 
   [16750] ldd: Never run file directly.

elf/dl-load.c

@@ -434,32 +434,41 @@ fillin_rpath (char *rpath, struct r_search_path_elem **result, const char *sep,
 {
   char *cp;
   size_t nelems = 0;
-  char *to_free;
 
   while ((cp = __strsep (&rpath, sep)) != NULL)
     {
       struct r_search_path_elem *dirp;
+      char *to_free = NULL;
+      size_t len = 0;
 
-      to_free = cp = expand_dynamic_string_token (l, cp, 1);
-
-      size_t len = strlen (cp);
-
-      /* `strsep' can pass an empty string.  This has to be
-	 interpreted as `use the current directory'. */
-      if (len == 0)
+      /* `strsep' can pass an empty string.  */
+      if (*cp != '\0')
 	{
-	  static const char curwd[] = "./";
-	  cp = (char *) curwd;
+	  to_free = cp = expand_dynamic_string_token (l, cp, 1);
+
+	  /* expand_dynamic_string_token can return NULL in case of empty
+	     path or memory allocation failure.  */
+	  if (cp == NULL)
+	    continue;
+
+	  /* Compute the length after dynamic string token expansion and
+	     ignore empty paths.  */
+	  len = strlen (cp);
+	  if (len == 0)
+	    {
+	      free (to_free);
+	      continue;
+	    }
+
+	  /* Remove trailing slashes (except for "/").  */
+	  while (len > 1 && cp[len - 1] == '/')
+	    --len;
+
+	  /* Now add one if there is none so far.  */
+	  if (len > 0 && cp[len - 1] != '/')
+	    cp[len++] = '/';
 	}
 
-      /* Remove trailing slashes (except for "/").  */
-      while (len > 1 && cp[len - 1] == '/')
-	--len;
-
-      /* Now add one if there is none so far.  */
-      if (len > 0 && cp[len - 1] != '/')
-	cp[len++] = '/';
-
       /* Make sure we don't use untrusted directories if we run SUID.  */
       if (__glibc_unlikely (check_trusted) && !is_trusted_path (cp, len))
 	{
@@ -622,6 +631,14 @@ decompose_rpath (struct r_search_path_struct *sps,
      necessary.  */
   free (copy);
 
+  /* There is no path after expansion.  */
+  if (result[0] == NULL)
+    {
+      free (result);
+      sps->dirs = (struct r_search_path_elem **) -1;
+      return false;
+    }
+
   sps->dirs = result;
   /* The caller will change this value if we haven't used a real malloc.  */
   sps->malloced = 1;