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, 44 insertions, 0 deletions

diff --git a/posix/wordexp-test.c b/posix/wordexp-test.c
index 4957006da7..bdd65e439f 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;