[PATCH v2] elf: Check for empty tokens before dynamic string token expansion [BZ #22625]

[Aurelien Jarno <aurelien at aurel32 dot net>]

The fillin_rpath function in elf/dl-load.c loops over each RPATH or
RUNPATH tokens and intepret 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).

To fix that, move the check for empty tokens before the dynamic string
token expansion. Change expand_dynamic_string_token to return NULL
instead of an empty string, and check for NULL pointer returned by
expand_dynamic_string_token.

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

Changelog:
	[BZ #22625]
	* elf/dl-load.c (expand_dynamic_string_token): Return NULL instead
	of an empty string.
	(fillin_rpath): Check for empty tokens before dynamic string token
	expansion. Check for NULL pointer possibly returned by
	expand_dynamic_string_token.
	(decompose_rpath): Check for empty path after dynamic string
	token expansion.
---
 ChangeLog     | 12 ++++++++++++
 NEWS          |  4 ++++
 elf/dl-load.c | 43 ++++++++++++++++++++++++++++++++++++-------
 3 files changed, 52 insertions(+), 7 deletions(-)


This new version includes the suggestions done by Dmitry. It would be
nice if it could be reviewed by at least one more person, especially 
the part modifying expand_dynamic_string_token.

diff --git a/ChangeLog b/ChangeLog
index 4a7164354f..24a5223485 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+2017-12-17  Aurelien Jarno  <aurelien@aurel32.net>
+	    Dmitry V. Levin  <ldv@altlinux.org>
+
+	[BZ #22625]
+	* elf/dl-load.c (expand_dynamic_string_token): Return NULL instead
+	of an empty string.
+	(fillin_rpath): Check for empty tokens before dynamic string token
+	expansion. Check for NULL pointer possibly returned by
+	expand_dynamic_string_token.
+	(decompose_rpath): Check for empty path after dynamic string
+	token expansion.
+
 2017-12-18  Sergei Trofimovich  <slyfox@gentoo.org>
 
 	[BZ #22624]
diff --git a/NEWS b/NEWS
index a43ff26e83..0d43e93a17 100644
--- a/NEWS
+++ b/NEWS
@@ -149,6 +149,10 @@ 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:
 
   [The release manager will add the list generated by
diff --git a/elf/dl-load.c b/elf/dl-load.c
index e7d97dcc56..827ec1c491 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -384,7 +384,17 @@ expand_dynamic_string_token (struct link_map *l, const char *s, int is_path)
   if (result == NULL)
     return NULL;
 
-  return _dl_dst_substitute (l, s, result, is_path);
+  char *retval = _dl_dst_substitute (l, s, result, is_path);
+
+  /* If substitution of dynamic string tokens resulted to an empty string,
+     return NULL as in case of insufficient memory.  */
+  if (__glibc_unlikely (*retval == '\0'))
+    {
+      free (result);
+      return NULL;
+    }
+
+  return retval;
 }
 
 
@@ -433,22 +443,33 @@ 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;
-
-      to_free = cp = expand_dynamic_string_token (l, cp, 1);
-
-      size_t len = strlen (cp);
+      char *to_free = NULL;
+      size_t len;
 
       /* `strsep' can pass an empty string.  This has to be
 	 interpreted as `use the current directory'. */
-      if (len == 0)
+      if (*cp == '\0')
 	{
 	  static const char curwd[] = "./";
 	  cp = (char *) curwd;
+	  len = 0;
+	}
+      else
+	{
+	  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;
+
+	  /* Recompute the length after dynamic string token expansion
+	     and ignore empty paths.  */
+	  len = strlen (cp);
 	}
 
       /* Remove trailing slashes (except for "/").  */
@@ -620,6 +641,14 @@ 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;
-- 
2.15.1