security: preserve permissions on renewal conf (#4430)

Ensure that permissions are preserved when renewal data is written to
conf files. This allows users to limit access to the file, if they wish.

Testing done:
 * `tox -e py27`
 * `tox -e lint`
 * Manual Testing
    * Got a new certificate. Restricted the permissions on the renewal
      conf. Renewed the certificate. Verified that the new renewal conf
      permissions matched.

certbot/storage.py

@@ -4,6 +4,7 @@ import glob
 import logging
 import os
 import re
+import stat
 
 import configobj
 import parsedatetime
@@ -117,10 +118,20 @@ def write_renewal_config(o_filename, n_filename, archive_dir, target, relevant_d
     # TODO: add human-readable comments explaining other available
     #       parameters
     logger.debug("Writing new config %s.", n_filename)
+
+    # Ensure that the file exists
+    open(n_filename, 'a').close()
+
+    # Copy permissions from the old version of the file, if it exists.
+    if os.path.exists(o_filename):
+        current_permissions = stat.S_IMODE(os.lstat(o_filename).st_mode)
+        os.chmod(n_filename, current_permissions)
+
     with open(n_filename, "wb") as f:
         config.write(outfile=f)
     return config
 
+
 def rename_renewal_config(prev_name, new_name, cli_config):
     """Renames cli_config.certname's config to cli_config.new_certname.
 

certbot/tests/storage_test.py

@@ -3,6 +3,7 @@
 import datetime
 import os
 import shutil
+import stat
 import unittest
 
 import configobj
@@ -758,13 +759,16 @@ class RenewableCertTests(BaseRenewableCertTest):
         with open(temp, "w") as f:
             f.write("[renewalparams]\nuseful = value # A useful value\n"
                     "useless = value # Not needed\n")
+        os.chmod(temp, 0o640)
         target = {}
         for x in ALL_FOUR:
             target[x] = "somewhere"
         archive_dir = "the_archive"
         relevant_data = {"useful": "new_value"}
+
         from certbot import storage
         storage.write_renewal_config(temp, temp2, archive_dir, target, relevant_data)
+
         with open(temp2, "r") as f:
             content = f.read()
         # useful value was updated
@@ -775,6 +779,9 @@ class RenewableCertTests(BaseRenewableCertTest):
         self.assertTrue("useless" not in content)
         # check version was stored
         self.assertTrue("version = {0}".format(certbot.__version__) in content)
+        # ensure permissions are copied
+        self.assertEqual(stat.S_IMODE(os.lstat(temp).st_mode),
+                         stat.S_IMODE(os.lstat(temp2).st_mode))
 
     def test_update_symlinks(self):
         from certbot import storage