[COMMITTED] CVE-2014-7817: wordexp fails to honour WRDE_NOCMD.

["Carlos O'Donell" <carlos at redhat dot com>]

Committed to trunk.

CVE-2014-7817:

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.

NEWS:

* CVE-2104-7817 The wordexp function could ignore the WRDE_NOCMD flag
  under certain input conditions resulting in the execution of a shell for
  command substitution when the applicaiton did not request it. The
  implementation now checks WRDE_NOCMD immediately before executing the
  shell and returns the error WRDE_CMDSUB as expected.

2014-11-19  Carlos O'Donell  <carlos@redhat.com>
	    Florian Weimer  <fweimer@redhat.com>
	    Joseph Myers  <joseph@codesourcery.com>
	    Adam Conrad  <adconrad@0c3.net>
	    Andreas Schwab  <schwab@suse.de>
	    Brooks Moses  <bmoses@google.com>

	[BZ #17625]
	* wordexp-test.c(__dso_handle): Add prototype.
	(__register_atfork): Likewise.
	(__app_register_atfork): New function.
	(registered_forks): New global.
	(register_fork): New function.
	(test_case): Add 3 new tests for WRDE_CMDSUB.
	(main): Call __app_register_atfork.
	(testit): If WRDE_NOCMD set registered_forks to zero, run test, and
	if fork count is non-zero fail the test.
	* posix/wordexp.c (exec_comm): Return WRDE_CMDSUB if WRDE_NOCMD flag
	is set.
	(parse_dollars): Remove check for WRDE_NOCMD.
	(parse_dquote): Likewise.

diff --git a/posix/wordexp-test.c b/posix/wordexp-test.c
index 4957006..bdd65e4 100644
--- a/posix/wordexp-test.c
+++ b/posix/wordexp-test.c
@@ -27,6 +27,25 @@
 
 #define IFS " \n\t"
 
+extern void *__dso_handle __attribute__ ((__weak__, __visibility__ ("hidden")));
+extern int __register_atfork (void (*) (void), void (*) (void), void (*) (void), void *);
+
+static int __app_register_atfork (void (*prepare) (void), void (*parent) (void), void (*child) (void))
+{
+  return __register_atfork (prepare, parent, child,
+			    &__dso_handle == NULL ? NULL : __dso_handle);
+}
+
+/* Number of forks seen.  */
+static int registered_forks;
+
+/* For each fork increment the fork count.  */
+static void
+register_fork (void)
+{
+  registered_forks++;
+}
+
 struct test_case_struct
 {
   int retval;
@@ -206,6 +225,12 @@ struct test_case_struct
     { WRDE_SYNTAX, NULL, "$((2+))", 0, 0, { NULL, }, IFS },
     { WRDE_SYNTAX, NULL, "`", 0, 0, { NULL, }, IFS },
     { WRDE_SYNTAX, NULL, "$((010+4+))", 0, 0, { NULL }, IFS },
+    /* Test for CVE-2014-7817. We test 3 combinations of command
+       substitution inside an arithmetic expression to make sure that
+       no commands are executed and error is returned.  */
+    { WRDE_CMDSUB, NULL, "$((`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS },
+    { WRDE_CMDSUB, NULL, "$((1+`echo 1`))", WRDE_NOCMD, 0, { NULL, }, IFS },
+    { WRDE_CMDSUB, NULL, "$((1+$((`echo 1`))))", WRDE_NOCMD, 0, { NULL, }, IFS },
 
     { -1, NULL, NULL, 0, 0, { NULL, }, IFS },
   };
@@ -258,6 +283,15 @@ main (int argc, char *argv[])
 	  return -1;
     }
 
+  /* If we are not allowed to do command substitution, we install
+     fork handlers to verify that no forks happened.  No forks should
+     happen at all if command substitution is disabled.  */
+  if (__app_register_atfork (register_fork, NULL, NULL) != 0)
+    {
+      printf ("Failed to register fork handler.\n");
+      return -1;
+    }
+
   for (test = 0; test_case[test].retval != -1; test++)
     if (testit (&test_case[test]))
       ++fail;
@@ -367,6 +401,9 @@ testit (struct test_case_struct *tc)
 
   printf ("Test %d (%s): ", ++tests, tc->words);
 
+  if (tc->flags & WRDE_NOCMD)
+    registered_forks = 0;
+
   if (tc->flags & WRDE_APPEND)
     {
       /* initial wordexp() call, to be appended to */
@@ -378,6 +415,13 @@ testit (struct test_case_struct *tc)
     }
   retval = wordexp (tc->words, &we, tc->flags);
 
+  if ((tc->flags & WRDE_NOCMD)
+      && (registered_forks > 0))
+    {
+	  printf ("FAILED fork called for WRDE_NOCMD\n");
+	  return 1;
+    }
+
   if (tc->flags & WRDE_DOOFFS)
       start_offs = sav_we.we_offs;
 
diff --git a/posix/wordexp.c b/posix/wordexp.c
index b6b65dd..26f3a26 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,
---

Cheers,
Carlos.