Since Stein, Neutron rbac should allow security group shared with projects.  The create rbac policy here failed silently and show security group with id caught the error. (trace pasted below).

<testcase classname="neutron_tempest_plugin.api.test_security_groups.RbacSharedSecurityGroupTest" id="2a81795c-2a35-1
<failure>Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/neutron_tempest_plugin/api/test_security_groups.py", line 189, in test_regular_c
self.client.show_security_group(sg['id'])
File "/usr/lib/python3.6/site-packages/neutron_tempest_plugin/services/network/json/network_client.py", line 133, in _s
resp, body = self.get(uri)
File "/root/.rally/verification/verifier-a6db9203-2d68-4c79-a00f-d023307751cf/repo/tempest/lib/common/rest_client.py",
return self.request('GET', url, extra_headers, headers)
File "/root/.rally/verification/verifier-a6db9203-2d68-4c79-a00f-d023307751cf/repo/tempest/lib/common/rest_client.py",
self._error_checker(resp, resp_body)
File "/root/.rally/verification/verifier-a6db9203-2d68-4c79-a00f-d023307751cf/repo/tempest/lib/common/rest_client.py",
raise exceptions.NotFound(resp_body, resp=resp)
tempest.lib.exceptions.NotFound: Object not found
Details: {'type': 'HTTPNotFound', 'message': 'The resource could not be found.', 'detail': ''}
</failure>