From https://bugzilla.redhat.com/show_bug.cgi?id=1157689
Modified for eglibc.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>

WARNING !!! WARNING !!! WARNING !!! WARNING !!! WARNING !!! WARNING !!!
EMBARGOED !!! EMBARGOED !!! EMARGOED !!! EMBARGOED !!! EMBARGOED !!!
SECURITY !!! SECURITY !!! SECURITY !!! SECURITY !!! SECURITY !!!

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 parse_arith(). The patch also hardens parse_backticks() and 
parse_comm() to check for WRDE_NOCMD flag and return an error instead
of ever running a shell.

We expand the testsuite and add 3 new regression tests of roughtly
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, 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.

2014-10-27  Carlos O'Donell  <carlos@redhat.com>

	* 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 (parse_arith): Return WRDE_NOCMD if WRDE_NOCMD flag
	is set and parsing '`'. 
	(parse_comm): Return WRDE_NOCMD if WRDE_NOCMD flag is set.
	(parse_backtick): Return WRDE_NOCMD if WRDE_NOCMD flag is set and
	parsing '`'.

diff --git a/posix/wordexp-test.c b/posix/wordexp-test.c
index 4957006..5ce2a1b 100644
--- a/libc/posix/wordexp-test.c
+++ b/libc/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..d6a158f 100644
--- a/libc/posix/wordexp.c
+++ b/libc/posix/wordexp.c
@@ -693,6 +693,12 @@ parse_arith (char **word, size_t *word_length, size_t *max_length,
 	  break;
 
 	case '`':
+	   if (flags & WRDE_NOCMD)
+	     {
+	       free (expr);
+	       return WRDE_NOCMD;
+	     }
+
 	  (*offset)++;
 	  error = parse_backtick (&expr, &expr_length, &expr_maxlen,
 				  words, offset, flags, NULL, NULL, NULL);
@@ -1144,6 +1150,10 @@ parse_comm (char **word, size_t *word_length, size_t *max_length,
   size_t comm_maxlen;
   char *comm = w_newword (&comm_length, &comm_maxlen);
 
+  /* Do nothing if command substitution should not succeed.  */
+  if (flags & WRDE_NOCMD)
+    return WRDE_CMDSUB;
+
   for (; words[*offset]; ++(*offset))
     {
       switch (words[*offset])
@@ -2121,6 +2131,9 @@ parse_backtick (char **word, size_t *word_length, size_t *max_length,
       switch (words[*offset])
 	{
 	case '`':
+	  if (flags & WRDE_NOCMD)
+	    return WRDE_NOCMD;
+
 	  /* Go -- give the script to the shell */
 	  error = exec_comm (comm, word, word_length, max_length, flags,
 			     pwordexp, ifs, ifs_white);