net: avoid assigning .0 and .255 IPs for higher VM ID

Those occasionally cause issues if OS within a VM (especially without
qubes tools installed) assumes /24 netmask.
It wasn't an issue before, because max QID was 254.

qubes/tests/vm/mix/net.py

@@ -27,6 +27,8 @@
 
 import qubes.tests
 import qubes.tests.vm.qubesvm
+from qubes.vm.mix.net import vmid_to_ipv4
+
 
 class TC_00_NetVMMixin(
         qubes.tests.vm.qubesvm.QubesVMTestsMixin, qubes.tests.QubesTestCase):
@@ -154,3 +156,23 @@ def test_170_provides_network_netvm(self):
         self.assertPropertyValue(vm2, 'netvm', None, None, '')
         self.assertPropertyValue(vm2, 'netvm', '', None, '')
         self.assertPropertyValue(vm, 'provides_network', False, False, 'False')
+
+    def test_200_vmid_to_ipv4(self):
+        testcases = (
+            (1,   '0.1'),
+            (2,   '0.2'),
+            (254, '0.254'),
+            (255, '1.1'),
+            (256, '1.2'),
+            (257, '1.3'),
+            (508, '1.254'),
+            (509, '2.1'),
+            (510, '2.2'),
+            (511, '2.3'),
+            (512, '2.4'),
+            (513, '2.5'),
+        )
+        for vmid, ip in testcases:
+            with self.subTest(str(vmid)):
+                self.assertEqual(ipaddress.IPv4Address('1.1.' + ip),
+                                 vmid_to_ipv4('1.1', vmid))

qubes/vm/mix/net.py

@@ -101,6 +101,19 @@ def __str__(self):
         return ' '.join(self)
 
 
+def vmid_to_ipv4(prefix, vmid):
+    # avoid .0 and .255 addresses, it may trip some heuristics
+    # if OS assumes /24 netmask
+    # preserve unchanged IPs for low vmid
+    if vmid < 255:
+        return ipaddress.IPv4Address('{}.{}.{}'.format(
+            prefix, (vmid >> 8) & 0xff, vmid & 0xff))
+    # don't reserve first .1 for vmid 0, as it is invalid
+    vmid -= 1
+    return ipaddress.IPv4Address('{}.{}.{}'.format(
+        prefix, vmid // 254, (vmid % 254) + 1))
+
+
 class NetVMMixin(qubes.events.Emitter):
     ''' Mixin containing network functionality '''
     mac = qubes.property('mac', type=str,
@@ -180,13 +193,9 @@ def get_ip_for_vm(vm):
         '''
         import qubes.vm.dispvm  # pylint: disable=redefined-outer-name
         if isinstance(vm, qubes.vm.dispvm.DispVM):
-            return ipaddress.IPv4Address('10.138.{}.{}'.format(
-                (vm.dispid >> 8) & 0xff, vm.dispid & 0xff))
+            return vmid_to_ipv4('10.138', vm.dispid)
 
-        # VM technically can get address which ends in '.0'. This currently
-        # does not happen, because qid < 253, but may happen in the future.
-        return ipaddress.IPv4Address('10.137.{}.{}'.format(
-            (vm.qid >> 8) & 0xff, vm.qid & 0xff))
+        return vmid_to_ipv4('10.137', vm.qid)
 
     @staticmethod
     def get_ip6_for_vm(vm):