fork: reseed Python/SSL PRNGs

Mac's SSL seems to have a pthread_atfork handler or similar that does
this for us, no clue if Linux is the same.

docs/api.rst

@@ -575,10 +575,19 @@ Router Class
           object destructors called, including TLS usage by native extension
           code, triggering many new variants of all the issues above.
 
+        * Pseudo-Random Number Generator state that is easily observable by
+          network peers to be duplicate, violating requirements of
+          cryptographic protocols through one-time state reuse. In the worst
+          case, children continually reuse the same state due to repeatedly
+          forking from a static parent.
+
         :py:meth:`fork` cleans up Mitogen-internal objects, in addition to
-        locks held by the :py:mod:`logging` package. You must arrange for your
-        program's state, including any third party packages in use, to be
-        cleaned up by specifying an `on_fork` function.
+        locks held by the :py:mod:`logging` package, reseeds
+        :py:func:`random.random`, and the OpenSSL PRNG via
+        :py:func:`ssl.RAND_add`, but only if the :py:mod:`ssl` module is
+        already loaded. You must arrange for your program's state, including
+        any third party packages in use, to be cleaned up by specifying an
+        `on_fork` function.
 
         The associated stream implementation is
         :py:class:`mitogen.fork.Stream`.

mitogen/fork.py

@@ -28,6 +28,8 @@
 
 import logging
 import os
+import random
+import sys
 import threading
 
 import mitogen.core
@@ -37,6 +39,17 @@ import mitogen.parent
 LOG = logging.getLogger('mitogen')
 
 
+def fixup_prngs():
+    """
+    Add 256 bits of /dev/urandom to OpenSSL's PRNG in the child, and re-seed
+    the random package with the same data.
+    """
+    s = os.urandom(256 / 8)
+    random.seed(s)
+    if 'ssl' in sys.modules:
+        sys.modules['ssl'].RAND_add(s, 75.0)
+
+
 def break_logging_locks():
     """
     After fork, ensure any logging.Handler locks are recreated, as a variety of
@@ -87,6 +100,7 @@ class Stream(mitogen.parent.Stream):
         mitogen.core.Latch._on_fork()
         mitogen.core.Side._on_fork()
         break_logging_locks()
+        fixup_prngs()
         if self.on_fork:
             self.on_fork()
         mitogen.core.set_block(childfp.fileno())

tests/fork_test.py

@@ -1,5 +1,10 @@
 
+import ctypes
 import os
+import random
+import ssl
+import struct
+import sys
 
 import mitogen
 import unittest2
@@ -8,12 +13,44 @@ import testlib
 import plain_old_module
 
 
+IS_64BIT = struct.calcsize('P') == 8
+PLATFORM_TO_PATH = {
+    ('darwin', False): '/usr/lib/libssl.dylib',
+    ('darwin', True): '/usr/lib/libssl.dylib',
+    ('linux2', False): '/usr/lib/libssl.so',
+    ('linux2', True): '/usr/lib/x86_64-linux-gnu/libssl.so',
+}
+
+c_ssl = ctypes.CDLL(PLATFORM_TO_PATH[sys.platform, IS_64BIT])
+c_ssl.RAND_pseudo_bytes.argtypes = [ctypes.c_char_p, ctypes.c_int]
+c_ssl.RAND_pseudo_bytes.restype = ctypes.c_int
+
+
+def random_random():
+    return random.random()
+
+
+def RAND_pseudo_bytes(n=32):
+    buf = ctypes.create_string_buffer(n)
+    assert 1 == c_ssl.RAND_pseudo_bytes(buf, n)
+    return buf[:]
+
+
 class ForkTest(testlib.RouterMixin, unittest2.TestCase):
     def test_okay(self):
         context = self.router.fork()
         self.assertNotEqual(context.call(os.getpid), os.getpid())
         self.assertEqual(context.call(os.getppid), os.getpid())
 
+    def test_random_module_diverges(self):
+        context = self.router.fork()
+        self.assertNotEqual(context.call(random_random), random_random())
+
+    def test_ssl_module_diverges(self):
+        context = self.router.fork()
+        self.assertNotEqual(context.call(RAND_pseudo_bytes),
+                            RAND_pseudo_bytes())
+
 
 if __name__ == '__main__':
     unittest2.main()