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26 September 2018

Oslo Policy Deep Dive (part 2)

by Juan Antonio Osorio Robles

In the previous blog post I covered all you need to know to write your own policies and understand where they come from.

Here, We’ll go through some examples of how you would change the policy for a service, and how to take that new policy into use.

For this, I’ve created a repository to try things out and hopefully get you practicing this kind of thing. Of course, things will be slightly different in your environment, depending on how you’re running OpenStack. But you should get the basic idea.

We’ll use Barbican as a test service to do basic policy changes. The configuration that I’m providing is not meant for production, but it makes it easier to make changes and test things out. It’s a very minimal and simple barbican configuration that has the “unauthenticated” context enabled. This means that it doesn’t rely on keystone, and it will use whatever roles and project you provide in the REST API.

The default policy & how to change it

As mentioned in the previous blog post, nowadays, the default policy in “shipped” as part of the codebase. For some services, folks might still package the policy.json file. However, for our test service (Barbican), this is not the case.

You can easily overwrite the default policy by providing a policy.json file yourself. By default, oslo.policy will read the project’s base directory, and try to get the policy.json file from there. For barbican, this will be /etc/barbican/policy.json. For keystone, /etc/keystone/policy.json.

It is worth noting that this file is configurable by setting the policy_file setting in your service’s configuration, which is under the oslo_policy group of the configuration file.

If you have a running service, and you add or modify the policy.json file, the changes will immediately take effect. No need to restart nor reload your service.

The way this works is that olso.policy will attempt to read the file’s modification time (using os.path.getmtime(filename)), and cache that. If on a subsequent read, the modification time has changed, it’ll re-read the policy file and load the new rules.

It is also worth noting that when using policy.json, you don’t need to provide the whole policy, only the rules and aliases you’re planning to change.

If you need to get the policy of a specific service, it’s fairly straightforward given the tools that oslo.policy provides. All you need to do is the following:

oslopolicy-policy-generator --namespace $SERVICE_NAME

It is important to note that this will get you the effective policy that’s being executed. So, any changes that you make to the policy will be reflected in the output of this command.

If you want to get a sample file for the default policy with all the documentation for each option, you’ll do the following:

oslopolicy-sample-generator --namespace $SERVICE_NAME

So, in order to output Barbican’s effective policy, we’ll do the following:

oslopolicy-policy-generator --namespace barbican

Note that this outputs the policy in yaml format, and oslo.policy reads policy.json by default, so you’ll have to tranform such file into json to take it into use.

Setting up the testing environment

NOTE: If you just plan to read through this and not actually do the exercises, you may skip this section.

Lets clone the repository first:

git clone https://github.com/JAORMX/barbican-policy-tests.git
cd barbican-policy-tests

Now that we’re in the repo, you’ll notice several scripts there. To provide you with a consistent environemnt, I decided to rely on containeeeers!!! So, in order to continue, you’ll need to have Docker installed in your system.

(Maybe in the future I’ll update this to run with Podman and Buildah)

To build the minimal barbican container, execute the following:

./build-barbican-container-image.sh

You can verify that you have the barbican-minimal image with the latest tag by running docker images.

To test that the image was built correctly and you can run barbican, execute the following:

./run-barbican-simple.sh

You will notice barbican is running, and can see the name of its container with docker ps. You’ll notice its listening on the port 9311 on localhost.

Exercises

Preface

In the following exercises, we’ll do some changes to the Barbican policy. To do this, it’s worth understanding some things about the service and the policy itself.

Barbican is Secret Storage as a service. To simplify things, we’ll focus on the secret storage side of things.

There are the operations you can do on a secret:

Barbican also assumes 5 keystone roles, and bases its policy on the usage of these roles:

The Barbican default policy also comes with some useful aliases as defaults:

{
"admin": "role:admin",
"observer": "role:observer",
"creator": "role:creator",
"audit": "role:audit",
"service_admin": "role:key-manager:service-admin",
...
}

So this makes overwriting specific roles fairly straight forward.

Scenario #1

The Keystone default roles proposal proposes the usage of three roles, which should also work with all OpenStack services. These roles are: reader, member and admin.

Lets take this into use in Barbican, and replace our already existing observer role, for reader.

In this case, we can take the alias into use, by doing very minimal changes, we can replace the usage of observer entirely.

I have already defined this role in the aforementioned repo, lets take a look:

{
"observer": "role:reader"
}

And that’s it!

Now in the barbican policy, every instance of the “rule:observer” assertion will actually reference the “reader” role.

Testing scenario #1

There is already a script that runs barbican and takes this policy into use. Lets run it, and verify that we can effectively use the reader role instead of the observer role:

# Run the container
./run-barbican-with-reader-role.sh

# Create a sample secret
./create-secret.sh

# Attempt to list the available secrets with the reader role. This
# operation should succeed.
./list-secrets.sh reader

# Attempt to list the available secrets with the observer role. This
# operation should fail.
./list-secrets.sh observer

# Once you're done, you can stop the container

Scenario #2

Barbican’s audit role is meant to only read a very minimal set of things from the barbican’s entities. For some, this role might not be very useful, and it also doesn’t fit with Keystone’s set of default roles, so lets delete it!

As before, I have already defined a policy for this purpose:

{
"audit": "!"
}

As you can see, this replace the audit alias, and any attempt to use it will be rejected in the policy, effectively dissallowing the audit role use.

Testing scenario #2

There is already a script that runs barbican and takes this policy into use. Lets run it, and verify that we can effectively no longer use the audit role:

# run the container
./run-barbican-without-audit-policy.sh

# create a secret
./create-secret.sh

# Attempt to view the secret metadata with the creator role. This
# operation should succeed.
curl -H 'X-Project-Id: 1234' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<some ID> | python -m json.tool

# Attempt to view the secret metadata with the audit role. This
# operation should fail.
curl -H 'X-Project-Id: 1234' -H 'X-Roles: audit' \
    http://localhost:9311/v1/secrets/<some ID> | python -m json.tool

# Once you're done, you can stop the container

Scenario #3

Now that we have tried a couple of things and it has gone fine. Lets put it all together and replicate the Keystone default role recommendation.

Here’s what we’ll do: As before, we’ll replace the observer role with reader. We’ll also replace the creator role with member, and finally, we’ll remove the audit role.

Here’s the policy file:

{
"observer": "role:reader",
"creator": "role:member",
"audit": "!"
}

This time, we’ll change the policy file in-place, as this is something you might need to do or automate in your own deployment.

Testing scenario #3

Here, we’ll run a minimal container that doesn’t take any specific policy into use. We’ll log into it, modify the policy.json file, and test out the results.

# Run the container
./run-barbican-simple.sh

# Open a bash session in the container
docker exec -ti (docker ps | grep barbican-minimal | awk '{print $1}') bash

# (In the container) Create the new policy file
cat <<EOF > /etc/barbican/policy.json
{
"observer": "role:reader",
"creator": "role:member",
"audit": "!"
}
EOF

# (In the container) Exit the container
exit

# Attempt to create a sample secret with the creator role. This operation
# should fail
./create-secret.sh creator

# Attempt to create a sample secret with the member role. This operation
# should succeed
./create-secret.sh member

# Attempt to list the available secrets with the observer role. This
# operation should fail.
./list-secrets.sh observer

# Attempt to list the available secrets with the reader role. This
# operation should succeed.
./list-secrets.sh reader

# Attempt to view the secret metadata with the audit role. This
# operation should fail.
curl -H 'X-Project-Id: 1234' -H 'X-Roles: audit' \
    http://localhost:9311/v1/secrets/<some ID> | python -m json.tool

# Attempt to view the secret metadata with the creator role. This
# operation should fail.
curl -H 'X-Project-Id: 1234' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<some ID> | python -m json.tool

# Attempt to view the secret metadata with the member role. This
# operation should succeed.
curl -H 'X-Project-Id: 1234' -H 'X-Roles: member' \
    http://localhost:9311/v1/secrets/<some ID> | python -m json.tool

# Once you're done, you can stop the container

Scenario #4

For our last case, lets assume that for some reason you need a “super-admin” role that is able to read everybody’s secret metadata. There is no equivalent of this role in Barbican, so we’ll have to modify more things in order to get this to work.

To simplify things, we’ll only modify the GET operation for secret metadata.

Please note that this is only done for learning purposes, do not try this in production.

First thing we’ll need is to retrieve the policy line that actually gets executed for secret metadata. In Barbican, it’s the secret:get policy.

From whithin the container, or if you have the barbican package installed somewhere, you can do the following in order to get this exact policy:

oslopolicy-policy-generator --namespace barbican | grep "secret:get"

This will get us the following line:

"secret:get": "rule:secret_non_private_read or rule:secret_project_creator or rule:secret_project_admin or rule:secret_acl_read"

Note that in the barbican policy, we explicitly check for most users that the user is in the same project as the project that the secret belongs to. In this case, we’ll omit this in order to enable the “super-admin” to retrieve any secret’s metadata.

Here is the final policy.json file we’ll use:

{
"super_admin": "role:super-admin",
"secret:get": "rule:secret_non_private_read or rule:secret_project_creator or rule:secret_project_admin or rule:secret_acl_read or rule:super_admin"
}

Testing scenario #4

Here, we’ll run a minimal container that doesn’t take any specific policy into use. We’ll log into it, modify the policy.json file, and test out the results.

# Run the container
./run-barbican-simple.sh

# Open a bash session in the container
docker exec -ti (docker ps | grep barbican-minimal | awk '{print $1}') bash

# (In the container) Lets verify what the current policy is for "secret:get".
# This should output the default rule.
oslopolicy-policy-generator --namespace barbican | grep "secret:get"

# (In the container) Create the new policy file
cat <<EOF > /etc/barbican/policy.json
{
"super_admin": "role:super-admin",
"secret:get": "rule:secret_non_private_read or rule:secret_project_creator or rule:secret_project_admin or rule:secret_acl_read or rule:super_admin"
}
EOF

# (In the container) Lets verify what the current policy is for "secret:get".
# This should output the updated policy.
oslopolicy-policy-generator --namespace barbican | grep "secret:get"

# (In the container) Exit the container
exit

# Lets now create a couple of secrets with the creator role in the default
# project (1234).

# This will be secret #1
./create-secret.sh creator
# This will be secret #2
./create-secret.sh creator

# Lets now create a couple of secrets with the creator role in another project
# (1111).

# This will be secret #3
./create-secret.sh creator 1111

Using the creator role and project ‘1234’, you should only be able to retrieve secrets #1 and #2, but should get an error with secret #3.

# So... this should work
curl -H 'X-Project-Id: 1234' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #1> | python -m json.tool

# this should work
curl -H 'X-Project-Id: 1234' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #2> | python -m json.tool

# ...And this should fail
curl -H 'X-Project-Id: 1234' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #3> | python -m json.tool

Using the creator role and project ‘1111’, you should only be able to retrieve secret #3, but should get an error with secrets #1 and #2

# So... this should fail
curl -H 'X-Project-Id: 1111' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #1> | python -m json.tool

# this should fail
curl -H 'X-Project-Id: 1111' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #2> | python -m json.tool

# ...And this should work
curl -H 'X-Project-Id: 1111' -H 'X-Roles: creator' \
    http://localhost:9311/v1/secrets/<secret #3> | python -m json.tool

Finally, lets try our new super-admin role. As you will notice, you don’t even need to be part of the projects to get the metadata:

# So... this should work
curl -H 'X-Project-Id: POLICY' -H 'X-Roles: super-admin' \
    http://localhost:9311/v1/secrets/<secret #1> | python -m json.tool

# this should work
curl -H 'X-Project-Id: IS' -H 'X-Roles: super-admin' \
    http://localhost:9311/v1/secrets/<secret #2> | python -m json.tool

# ...And this should work too
curl -H 'X-Project-Id: COOL' -H 'X-Roles: super-admin' \
    http://localhost:9311/v1/secrets/<secret #3> | python -m json.tool

Conclusion

You have now learned how to do simple modifications to your service’s policy!

With great power comes great responsibility… And all those things… But seriously, be careful! You might end up with unintended results.

In the next blog post, we’ll cover implied roles and how you can use them in your policies!

tags: tripleo - openstack

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