Issue #18747: Re-seed OpenSSL's pseudo-random number generator after fork.

A pthread_atfork() child handler is used to seeded the PRNG with pid, time
and some stack data.

Lib/test/test_ssl.py

@@ -143,6 +143,38 @@ class BasicSocketTests(unittest.TestCase):
         self.assertRaises(TypeError, ssl.RAND_egd, 'foo', 1)
         ssl.RAND_add("this is a random string", 75.0)
 
+    @unittest.skipUnless(os.name == 'posix', 'requires posix')
+    def test_random_fork(self):
+        status = ssl.RAND_status()
+        if not status:
+            self.fail("OpenSSL's PRNG has insufficient randomness")
+
+        rfd, wfd = os.pipe()
+        pid = os.fork()
+        if pid == 0:
+            try:
+                os.close(rfd)
+                child_random = ssl.RAND_pseudo_bytes(16)[0]
+                self.assertEqual(len(child_random), 16)
+                os.write(wfd, child_random)
+                os.close(wfd)
+            except BaseException:
+                os._exit(1)
+            else:
+                os._exit(0)
+        else:
+            os.close(wfd)
+            self.addCleanup(os.close, rfd)
+            _, status = os.waitpid(pid, 0)
+            self.assertEqual(status, 0)
+
+            child_random = os.read(rfd, 16)
+            self.assertEqual(len(child_random), 16)
+            parent_random = ssl.RAND_pseudo_bytes(16)[0]
+            self.assertEqual(len(parent_random), 16)
+
+            self.assertNotEqual(child_random, parent_random)
+
     def test_parse_cert(self):
         # note that this uses an 'unofficial' function in _ssl.c,
         # provided solely for this test, to exercise the certificate

Misc/NEWS

@@ -38,6 +38,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #18747: Re-seed OpenSSL's pseudo-random number generator after fork.
+  A pthread_atfork() child handler is used to seeded the PRNG with pid, time
+  and some stack data.
+
 - Issue #8865: Concurrent invocation of select.poll.poll() now raises a
   RuntimeError exception.  Patch by Christian Schubert.
 

Modules/_ssl.c

@@ -18,6 +18,11 @@
 
 #ifdef WITH_THREAD
 #include "pythread.h"
+
+#ifdef HAVE_PTHREAD_ATFORK
+#  include <pthread.h>
+#endif
+
 #define PySSL_BEGIN_ALLOW_THREADS_S(save) \
     do { if (_ssl_locks_count>0) { (save) = PyEval_SaveThread(); } } while (0)
 #define PySSL_END_ALLOW_THREADS_S(save) \
@@ -2936,7 +2941,69 @@ Queries the entropy gather daemon (EGD) on the socket named by 'path'.\n\
 Returns number of bytes read.  Raises SSLError if connection to EGD\n\
 fails or if it does not provide enough data to seed PRNG.");
 
+/* Seed OpenSSL's PRNG at fork(), http://bugs.python.org/issue18747
+ *
+ * The child handler seeds the PRNG from pseudo-random data like pid, the
+ * current time (nanoseconds, miliseconds or seconds) and an uninitialized
+ * array. The array contains stack variables that are impossible to predict
+ * on most systems, e.g. function return address (subject to ASLR), the
+ * stack protection canary and automatic variables.
+ * The code is inspired by Apache's ssl_rand_seed() function.
+ *
+ * Note:
+ * The code uses pthread_atfork() until Python has a proper atfork API. The
+ * handlers are not removed from the child process.
+ */
+
+#if defined(HAVE_PTHREAD_ATFORK) && defined(WITH_THREAD)
+#define PYSSL_RAND_ATFORK 1
+
+static void
+PySSL_RAND_atfork_child(void)
+{
+    struct {
+        char stack[128];    /* uninitialized (!) stack data, 128 is an
+                               arbitrary number. */
+        pid_t pid;          /* current pid */
+        _PyTime_timeval tp; /* current time */
+    } seed;
+
+#ifdef WITH_VALGRIND
+    VALGRIND_MAKE_MEM_DEFINED(seed.stack, sizeof(seed.stack));
 #endif
+    seed.pid = getpid();
+    _PyTime_gettimeofday(&(seed.tp));
+
+#if 0
+    fprintf(stderr, "PySSL_RAND_atfork_child() seeds %i bytes in pid %i\n",
+            (int)sizeof(seed), seed.pid);
+#endif
+    RAND_add((unsigned char *)&seed, sizeof(seed), 0.0);
+}
+
+static int
+PySSL_RAND_atfork(void)
+{
+    static int registered = 0;
+    int retval;
+
+    if (registered)
+        return 0;
+
+    retval = pthread_atfork(NULL,                     /* prepare */
+                            NULL,                     /* parent */
+                            PySSL_RAND_atfork_child); /* child */
+    if (retval != 0) {
+        PyErr_SetFromErrno(PyExc_OSError);
+        return -1;
+    }
+    registered = 1;
+    return 0;
+}
+#endif /* HAVE_PTHREAD_ATFORK */
+
+#endif /* HAVE_OPENSSL_RAND */
+
 
 PyDoc_STRVAR(PySSL_get_default_verify_paths_doc,
 "get_default_verify_paths() -> tuple\n\
@@ -3549,5 +3616,10 @@ PyInit__ssl(void)
     if (r == NULL || PyModule_AddObject(m, "_OPENSSL_API_VERSION", r))
         return NULL;
 
+#ifdef PYSSL_RAND_ATFORK
+    if (PySSL_RAND_atfork() == -1)
+        return NULL;
+#endif
+
     return m;
 }

configure

@@ -9806,6 +9806,17 @@ $as_echo "#define HAVE_BROKEN_PTHREAD_SIGMASK 1" >>confdefs.h
 
             ;;
         esac
+fi
+done
+
+      for ac_func in pthread_atfork
+do :
+  ac_fn_c_check_func "$LINENO" "pthread_atfork" "ac_cv_func_pthread_atfork"
+if test "x$ac_cv_func_pthread_atfork" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_PTHREAD_ATFORK 1
+_ACEOF
+
 fi
 done
 

configure.ac

@@ -2512,6 +2512,7 @@ if test "$posix_threads" = "yes"; then
             [Define if pthread_sigmask() does not work on your system.])
             ;;
         esac])
+      AC_CHECK_FUNCS(pthread_atfork)
 fi
 
 

pyconfig.h.in

@@ -633,6 +633,9 @@
 /* Define if your compiler supports function prototype */
 #undef HAVE_PROTOTYPES
 
+/* Define to 1 if you have the `pthread_atfork' function. */
+#undef HAVE_PTHREAD_ATFORK
+
 /* Defined for Solaris 2.6 bug in pthread header. */
 #undef HAVE_PTHREAD_DESTRUCTOR