Prohibit file injection writing to host filesystem

This is a refinement of the previous fix in commit 2427d4a,
which does the file name canonicalization as the root user.
This is required so that guest images could not for example,
protect malicious symlinks in a directory only readable by root.

Note this requires adding the 'readlink' binary to the
nova sudoers file.

Fixes bug: 1031311, CVE-2012-3447
Change-Id: I7f7cdeeffadebae7451e1e13f73f1313a7df9c5c

nova/tests/test_virt.py

@@ -18,6 +18,7 @@
 from nova import exception
 from nova import flags
 from nova import test
+from nova import utils
 from nova.virt import disk
 from nova.virt import driver
 
@@ -86,6 +87,17 @@ def test_swap_is_usable(self):
 
 
 class TestVirtDisk(test.TestCase):
+    def setUp(self):
+        super(TestVirtDisk, self).setUp()
+
+        real_execute = utils.execute
+
+        def nonroot_execute(*cmd_parts, **kwargs):
+            kwargs.pop('run_as_root', None)
+            return real_execute(*cmd_parts, **kwargs)
+
+        self.stubs.Set(utils, 'execute', nonroot_execute)
+
     def test_check_safe_path(self):
         ret = disk._join_and_check_path_within_fs('/foo', 'etc',
                                                       'something.conf')

nova/tests/test_xenapi.py

@@ -519,9 +519,13 @@ def _tee_handler(cmd, **kwargs):
             self._tee_executed = True
             return '', ''
 
+        def _readlink_handler(cmd_parts, **kwargs):
+            return os.path.realpath(cmd_parts[2]), ''
+
         fake_utils.fake_execute_set_repliers([
             # Capture the tee .../etc/network/interfaces command
             (r'tee.*interfaces', _tee_handler),
+            (r'readlink -nm.*', _readlink_handler),
         ])
         self._test_spawn(glance_stubs.FakeGlance.IMAGE_MACHINE,
                          glance_stubs.FakeGlance.IMAGE_KERNEL,

nova/virt/disk.py

@@ -275,7 +275,9 @@ def _join_and_check_path_within_fs(fs, *args):
     mounted guest fs.  Trying to be clever and specifying a
     path with '..' in it will hit this safeguard.
     '''
-    absolute_path = os.path.realpath(os.path.join(fs, *args))
+    absolute_path, _err = utils.execute('readlink', '-nm',
+                                        os.path.join(fs, *args),
+                                        run_as_root=True)
     if not absolute_path.startswith(os.path.realpath(fs) + '/'):
         raise exception.Invalid()
     return absolute_path