CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.

The function wordexp() fails to properly handle the WRDE_NOCMD
flag when processing arithmetic inputs in the form of "$((... ``))"
where "..." can be anything valid. The backticks in the arithmetic
epxression are evaluated by in a shell even if WRDE_NOCMD forbade
command substitution. This allows an attacker to attempt to pass
dangerous commands via constructs of the above form, and bypass
the WRDE_NOCMD flag. This patch fixes this by checking for WRDE_NOCMD
in exec_comm(), the only place that can execute a shell. All other
checks for WRDE_NOCMD are superfluous and removed.

We expand the testsuite and add 3 new regression tests of roughly
the same form but with a couple of nested levels.

On top of the 3 new tests we add fork validation to the WRDE_NOCMD
testing. If any forks are detected during the execution of a wordexp()
call with WRDE_NOCMD, the test is marked as failed. This is slightly
heuristic since vfork might be used in the future, but it provides a
higher level of assurance that no shells were executed as part of
command substitution with WRDE_NOCMD in effect. In addition it doesn't
require libpthread or libdl, instead we use the public implementation
namespace function __register_atfork (already part of the public ABI
for libpthread).

Tested on x86_64 with no regressions.

1 files changed, 4 insertions, 12 deletions

diff --git a/posix/wordexp.c b/posix/wordexp.c
index b6b65dd993..26f3a2653f 100644
--- a/posix/wordexp.c
+++ b/posix/wordexp.c
@@ -893,6 +893,10 @@ exec_comm (char *comm, char **word, size_t *word_length, size_t *max_length,
   pid_t pid;
   int noexec = 0;
 
+  /* Do nothing if command substitution should not succeed.  */
+  if (flags & WRDE_NOCMD)
+    return WRDE_CMDSUB;
+
   /* Don't fork() unless necessary */
   if (!comm || !*comm)
     return 0;
@@ -2082,9 +2086,6 @@ parse_dollars (char **word, size_t *word_length, size_t *max_length,
 	    }
 	}
 
-      if (flags & WRDE_NOCMD)
-	return WRDE_CMDSUB;
-
       (*offset) += 2;
       return parse_comm (word, word_length, max_length, words, offset, flags,
 			 quoted? NULL : pwordexp, ifs, ifs_white);
@@ -2196,9 +2197,6 @@ parse_dquote (char **word, size_t *word_length, size_t *max_length,
 	  break;
 
 	case '`':
-	  if (flags & WRDE_NOCMD)
-	    return WRDE_CMDSUB;
-
 	  ++(*offset);
 	  error = parse_backtick (word, word_length, max_length, words,
 				  offset, flags, NULL, NULL, NULL);
@@ -2357,12 +2355,6 @@ wordexp (const char *words, wordexp_t *pwordexp, int flags)
 	break;
 
       case '`':
-	if (flags & WRDE_NOCMD)
-	  {
-	    error = WRDE_CMDSUB;
-	    goto do_error;
-	  }
-
 	++words_offset;
 	error = parse_backtick (&word, &word_length, &max_length, words,
 				&words_offset, flags, pwordexp, ifs,