From 814bfd937238cbd211ea30805c36ae682cfd7b48 Mon Sep 17 00:00:00 2001
From: Kashyap Chamarthy <kchamart@redhat.com>
Date: Fri, 22 Jun 2018 12:11:56 +0200
Subject: [PATCH] conf: libvirt: Make `/dev/urandom` the default for
 'rng_dev_path'
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Since libvirt 1.3.4, any RNG (Random Number Generator) device path (that
returns random numbers when read!) is accepted.  However, the
recommended source of entropy is `/dev/urandom` (it is non-blocking; and
doesn't have the same limitations of `dev/random`, which is a legacy
interface).

Therefore, make `/dev/urandom` the default RNG for 'rng_dev_path' config
attribute; adjust the relevant tests.  Also update the documention to
reflect this change.

Change-Id: Ia39402a045ffb1943463b5741655d84071613e8c
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
---
 nova/conf/libvirt.py                             | 16 +++++++++++++---
 nova/tests/unit/virt/libvirt/test_config.py      |  4 ++--
 nova/tests/unit/virt/libvirt/test_driver.py      |  4 ++--
 ...efault-for-rng_dev_path-150a76b0ea74cbc2.yaml |  6 ++++++
 4 files changed, 23 insertions(+), 7 deletions(-)
 create mode 100644 releasenotes/notes/urandom-as-default-for-rng_dev_path-150a76b0ea74cbc2.yaml

diff --git a/nova/conf/libvirt.py b/nova/conf/libvirt.py
index 6316d0c4b99b..94c52e318c5e 100644
--- a/nova/conf/libvirt.py
+++ b/nova/conf/libvirt.py
@@ -661,9 +661,19 @@ Possible cache modes:
   mode in production environments.
 """),
     cfg.StrOpt('rng_dev_path',
-               help='A path to a device that will be used as source of '
-                    'entropy on the host. Permitted options are: '
-                    '/dev/random or /dev/hwrng'),
+               default='/dev/urandom',
+               help="""
+The path to an RNG (Random Number Generator) device that will be used as
+the source of entropy on the host.  Since libvirt 1.3.4, any path (that
+returns random numbers when read) is accepted.  The recommended source
+of entropy is ``/dev/urandom`` -- it is non-blocking, therefore
+relatively fast; and avoids the limitations of ``/dev/random``, which is
+a legacy interface.  For more details (and comparision between different
+RNG sources), refer to the "Usage" section in the Linux kernel API
+documentation for ``[u]random``:
+http://man7.org/linux/man-pages/man4/urandom.4.html and
+http://man7.org/linux/man-pages/man7/random.7.html.
+"""),
     cfg.ListOpt('hw_machine_type',
                 help='For qemu or KVM guests, set this option to specify '
                      'a default machine type per host architecture. '
diff --git a/nova/tests/unit/virt/libvirt/test_config.py b/nova/tests/unit/virt/libvirt/test_config.py
index b68af754aabe..3a1a38a6d336 100644
--- a/nova/tests/unit/virt/libvirt/test_config.py
+++ b/nova/tests/unit/virt/libvirt/test_config.py
@@ -3271,7 +3271,7 @@ class LibvirtConfigGuestRngTest(LibvirtConfigBaseTest):
 
     def test_config_rng_driver_with_rate(self):
         obj = config.LibvirtConfigGuestRng()
-        obj.backend = '/dev/random'
+        obj.backend = '/dev/urandom'
         obj.rate_period = '12'
         obj.rate_bytes = '34'
 
@@ -3279,7 +3279,7 @@ class LibvirtConfigGuestRngTest(LibvirtConfigBaseTest):
         self.assertXmlEqual(xml, """
 <rng model='virtio'>
     <rate period='12' bytes='34'/>
-    <backend model='random'>/dev/random</backend>
+    <backend model='random'>/dev/urandom</backend>
 </rng>""")
 
 
diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py
index 8f1678ead8ad..24deda295423 100644
--- a/nova/tests/unit/virt/libvirt/test_driver.py
+++ b/nova/tests/unit/virt/libvirt/test_driver.py
@@ -5348,7 +5348,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
                               vconfig.LibvirtConfigMemoryBalloon)
 
         self.assertEqual(cfg.devices[6].model, 'random')
-        self.assertIsNone(cfg.devices[6].backend)
+        self.assertEqual(cfg.devices[6].backend, '/dev/urandom')
         self.assertIsNone(cfg.devices[6].rate_bytes)
         self.assertIsNone(cfg.devices[6].rate_period)
 
@@ -5422,7 +5422,7 @@ class LibvirtConnTestCase(test.NoDBTestCase,
                             vconfig.LibvirtConfigMemoryBalloon)
 
         self.assertEqual(cfg.devices[6].model, 'random')
-        self.assertIsNone(cfg.devices[6].backend)
+        self.assertEqual(cfg.devices[6].backend, '/dev/urandom')
         self.assertEqual(cfg.devices[6].rate_bytes, 1024)
         self.assertEqual(cfg.devices[6].rate_period, 2)
 
diff --git a/releasenotes/notes/urandom-as-default-for-rng_dev_path-150a76b0ea74cbc2.yaml b/releasenotes/notes/urandom-as-default-for-rng_dev_path-150a76b0ea74cbc2.yaml
new file mode 100644
index 000000000000..2867f0fd24ad
--- /dev/null
+++ b/releasenotes/notes/urandom-as-default-for-rng_dev_path-150a76b0ea74cbc2.yaml
@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    The default value of the configuration attribute
+    ``[libvirt]/rng_dev_path`` is now set to ``/dev/urandom``.  Refer to
+    the documentation of ``rng_dev_path`` for details.