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.

4 files changed, 77 insertions, 13 deletions

diff --git a/ChangeLog b/ChangeLog
index 09e308c..2fa59cf 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,25 @@
+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  <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.
+
 2014-11-19  Siddhesh Poyarekar  <siddhesh@redhat.com>
 
 	* Makeconfig (built-modules): List non-library modules to be
diff --git a/NEWS b/NEWS
index b152488..4b7eeb4 100644
--- a/NEWS
+++ b/NEWS
@@ -12,7 +12,13 @@ Version 2.21
   6652, 12926, 14132, 14138, 14171, 15215, 15884, 17266, 17344, 17363,
   17370, 17371, 17411, 17460, 17475, 17485, 17501, 17506, 17508, 17522,
   17555, 17570, 17571, 17572, 17573, 17574, 17582, 17583, 17584, 17585,
-  17589, 17594, 17616.
+  17589, 17594, 17616, 17625.
+
+* 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.
 
 * The minimum GCC version that can be used to build this version of the GNU
   C Library is GCC 4.6.  Older GCC versions, and non-GNU compilers, can
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,