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Marco Braga
Marco Braga

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Use private S3 Bucket when installing OpenShift in AWS with manual authentication mode with STS

#openshift #aws #kubernetes #security

In this article, I will share the hands-on steps to replace the default public endpoint used by AWS IAM OpenID Connect (OIDC) from a public S3 Bucket to a CloudFront Distribution URL, when installing an OpenShift cluster with STS support.

Table Of Contents

Summary

Quick recap

The endpoint identifier, that also names the OpenID Connector resource (IAM OIDC), should be public access as it's used by IAM managed service to retrieve the public keys (JWKS) used on ProjectedServiceAccountToken (JSON web tokens projected to the pod). This way external systems, like IAM, can validate and accept the Kubernetes-issued OIDC tokens.

The ccoctl is the utility used to automate the OIDC setup to install an OpenShift cluster in AWS with manual authentication mode with STS.

Currently, the default ccoctl deployment creates one public S3 Bucket per cluster with JWKS objects, directly exposing the Bucket's URL as the OIDC discovery endpoint. In some AWS Accounts, public buckets or objects are unwanted or blocked. This is the main motivation to explore this topic and share some other options to use a Bucket in a more restrictive mode.

If you would like to know more about this topic, I highly advise you read:

Goal

We will walk through those steps to:

  • create one CloudFront Distribution to be used as the public endpoint for OIDC
  • create one private S3 Bucket
  • create one origin access identity (OAI) to access the S3 from CloudFront Distribution
  • patch the JWKS files with the new CloudFront Distribution URL when setting up the manual-STS during the OpenShift installation
  • create the OIDC identity provider using the CloudFront URL
  • create the IAM Roles with Trusted Policy allowing Federated OIDC service ARN with proper service account restrictions
  • create the OpenShift cluster with STS support with no public buckets

Steps

Requirements

  • OpenShift installer client (openshift-installer)
  • OpenShift client (oc)
  • ccoctl utility
  • AWS credentials with permissions to install a cluster with manual-STS support
  • aws-cli
  • jq
  • yq

Setup

  • Adjust and export the environment variables 
export CLUSTER_NAME="my-sts"
export BASE_DOMAIN="devcluster.example.com"

export CLUSTER_REGION=us-east-1
export VERSION=4.10.16
export PULL_SECRET_FILE=${HOME}/.openshift/pull-secret-latest.json
export SSH_PUB_KEY_FILE="${HOME}/.ssh/id_rsa.pub"

export WORKDIR="${CLUSTER_NAME}"
export DIR_CCO="${WORKDIR}/cco"
export DIR_INSTALLER="${WORKDIR}/installer"
export OIDC_BUCKET_NAME="${CLUSTER_NAME}-oidc"
export OIDC_BUCKET_CONTENT="${WORKDIR}/bucket-content"

mkdir -p ${WORKDIR}/{cco,installer,bucket-content}
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  • Install the clients (optional): oc, openshift-installer and ccoctl 
# oc and openshift-install
oc adm release extract \
    --tools quay.io/openshift-release-dev/ocp-release:${VERSION}-x86_64 \
    -a ${PULL_SECRET_FILE}

tar xvfz openshift-client-linux-${VERSION}.tar.gz
tar xvfz openshift-install-linux-${VERSION}.tar.gz

# ccoctl
RELEASE_IMAGE=$(./openshift-install version \
    | awk '/release image/ {print $3}')
CCO_IMAGE=$(./oc adm release info \
    --image-for='cloud-credential-operator' \
    ${RELEASE_IMAGE})

./oc image extract ${CCO_IMAGE} \
    --file="/usr/bin/ccoctl" \
    -a ${PULL_SECRET_FILE}

chmod 775 ccoctl
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You must now be able to see the client's binaries in your current directory.

Create the Installer manifests

  • Create the installer configuration

This is the only mandatory change: credentialsMode: Manual

cat <<EOF > ${DIR_INSTALLER}/install-config.yaml
apiVersion: v1
baseDomain: ${BASE_DOMAIN}
credentialsMode: Manual
compute:
- name: worker
  replicas: 2
metadata:
  name: "${CLUSTER_NAME}"
platform:
  aws:
    region: ${CLUSTER_REGION}
    defaultMachinePlatform:
      zones:
      - ${CLUSTER_REGION}a
publish: External
pullSecret: '$(cat ${PULL_SECRET_FILE} |awk -v ORS= -v OFS= '{$1=$1}1')'
sshKey: |
  $(cat ${SSH_PUB_KEY_FILE})
EOF

echo "# Backup install-config.yaml (Optional)"
cp -v ${DIR_INSTALLER}/install-config.yaml \
    ${DIR_INSTALLER}/install-config-bkp.yaml
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  • Create the Installer manifests 
./openshift-install create manifests \
    --dir ${DIR_INSTALLER}
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  • Set the CLUSTER_ID environment variable 
CLUSTER_ID="$(yq -r .status.infrastructureName \
    ${DIR_INSTALLER}/manifests/cluster-infrastructure-02-config.yml)"
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Create the Origin Access Identity

Steps to create the Origin Access Identity (OAI) to be used to access the bucket through CloudFront Distribution:

  • Create the OAI and set the variable OAI_CLODUFRONT_ID: 
aws cloudfront create-cloud-front-origin-access-identity \
    --cloud-front-origin-access-identity-config \
    CallerReference="${OIDC_BUCKET_NAME}",Comment="OAI-${OIDC_BUCKET_NAME}"

OAI_CLODUFRONT_ID=$(aws cloudfront \
    list-cloud-front-origin-access-identities \
    --query "CloudFrontOriginAccessIdentityList.Items[?Comment==\`OAI-${OIDC_BUCKET_NAME}\`].Id" \
    --output text)
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Create the Bucket

  • Create the private Bucket 
aws s3api create-bucket \
    --bucket ${OIDC_BUCKET_NAME} \
    --acl private
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  • Create the Bucket Policy document, that allows OAI to retrieve objects 
cat <<EOF | envsubst > ${WORKDIR}/oidc-bucket-policy.json
{
    "Version": "2008-10-17",
    "Id": "PolicyForCloudFrontPrivateContent",
    "Statement": [
        {
            "Sid": "1",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity ${OAI_CLODUFRONT_ID}"
            },
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::${OIDC_BUCKET_NAME}/*"
        }
    ]
}
EOF
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  • Apply the policy to the Bucket and block public access 
aws s3api put-bucket-policy \
    --bucket ${OIDC_BUCKET_NAME} \
    --policy file://${WORKDIR}/oidc-bucket-policy.json

aws s3api put-public-access-block \
    --bucket ${OIDC_BUCKET_NAME} \
    --public-access-block-configuration \
    BlockPublicAcls=true,IgnorePublicAcls=true,BlockPublicPolicy=true,RestrictPublicBuckets=true
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Create CloudFront Distribution

  • Create the Distribution document 
cat <<EOF | envsubst > ${WORKDIR}/oidc-cloudfront.json
{
    "DistributionConfig": {
        "CallerReference": "${CLUSTER_NAME}",
        "Aliases": {
            "Quantity": 0
        },
        "Origins": {
            "Quantity": 1,
            "Items": [
            {
                "Id": "${OIDC_BUCKET_NAME}.s3.${CLUSTER_REGION}.amazonaws.com",
                "DomainName": "${OIDC_BUCKET_NAME}.s3.${CLUSTER_REGION}.amazonaws.com",
                "OriginPath": "",
                "CustomHeaders": {
                    "Quantity": 0
                },
                "S3OriginConfig": {
                    "OriginAccessIdentity": "origin-access-identity/cloudfront/${OAI_CLODUFRONT_ID}"
                },
                "ConnectionAttempts": 3,
                "ConnectionTimeout": 10,
                "OriginShield": {
                "Enabled": false
                }
            }
            ]
        },
        "DefaultCacheBehavior": {
            "TargetOriginId": "${OIDC_BUCKET_NAME}.s3.${CLUSTER_REGION}.amazonaws.com",
            "TrustedSigners": {
                "Enabled": false,
                "Quantity": 0
            },
            "TrustedKeyGroups": {
                "Enabled": false,
                "Quantity": 0
            },
            "ViewerProtocolPolicy": "https-only",
            "AllowedMethods": {
                "Quantity": 2,
                "Items": [
                    "HEAD",
                    "GET"
                ],
                "CachedMethods": {
                    "Quantity": 2,
                    "Items": [
                        "HEAD",
                        "GET"
                    ]
                }
            },
            "SmoothStreaming": false,
            "Compress": false,
            "LambdaFunctionAssociations": {
                "Quantity": 0
            },
            "FunctionAssociations": {
                "Quantity": 0
            },
            "FieldLevelEncryptionId": "",
            "CachePolicyId": "4135ea2d-6df8-44a3-9df3-4b5a84be39ad"
        },
        "CacheBehaviors": {
            "Quantity": 0
        },
        "CustomErrorResponses": {
            "Quantity": 0
        },
        "Comment": "${CLUSTER_NAME}",
        "Logging": {
            "Enabled": false,
            "IncludeCookies": false,
            "Bucket": "",
            "Prefix": ""
        },
        "PriceClass": "PriceClass_All",
        "Enabled": true,
        "ViewerCertificate": {
            "CloudFrontDefaultCertificate": true
        }
    },
    "Tags": {
        "Items": [
            {
                "Key": "Name",
                "Value": "${CLUSTER_NAME}"
            }
        ]
    }
}
EOF
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  • Create the CloudFront Distribution with Tags 
aws cloudfront create-distribution-with-tags \
    --distribution-config-with-tags \
    file://${WORKDIR}/oidc-cloudfront.json
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  • Wait until the Distribution has been created

  • Get the CloudFront Distribution URL

CLOUDFRONT_URI=$(aws cloudfront list-distributions \
    --query "DistributionList.Items[?Comment==\`${CLUSTER_NAME}\`].DomainName" \
    --output text)

echo ${CLOUDFRONT_URI}
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Make sure you can see the URL.

Generate the OIDC configuration and keys

  • Generate the key pair used to create the service account tokens 
./ccoctl aws create-key-pair \
    --output-dir ${DIR_CCO}
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  • Generate the OpenID Connect configuration 
./ccoctl aws create-identity-provider \
    --name=${CLUSTER_NAME} \
    --region=${CLUSTER_REGION} \
    --public-key-file=${DIR_CCO}/serviceaccount-signer.public \
    --output-dir=${DIR_CCO}/ \
    --dry-run
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  • Update the S3 Bucket URL endpoint to the CloudFront Distribution endpoint:

A. Patch the issuer URL onto the OIDC configuration file /.well-known/openid-configuration

mkdir -p ${OIDC_BUCKET_CONTENT}/.well-known
cat ${DIR_CCO}/02-openid-configuration \
    | sed "s/https:\/\/${CLUSTER_NAME}[a-z.-].*\//https:\/\/${CLOUDFRONT_URI}\//" \
    | sed "s/https:\/\/${CLUSTER_NAME}[a-z.-].*/https:\/\/${CLOUDFRONT_URI}\",/" \
    > ${OIDC_BUCKET_CONTENT}/.well-known/openid-configuration
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B. Copy the keys.json

cp -v ${DIR_CCO}/03-keys.json \
    ${OIDC_BUCKET_CONTENT}/keys.json
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C. Patch the issuer URL onto Authentication custom resource in cluster-authentication-02-config.yaml

sed -i "s/https:\/\/[a-z.-].*/https:\/\/${CLOUDFRONT_URI}/" \
    ${DIR_CCO}/manifests/cluster-authentication-02-config.yaml
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D. Update the IdP OIDC object configuration

sed -i "s/https:\/\/[a-z.-].*/https:\/\/${CLOUDFRONT_URI}\",/" \
    ${DIR_CCO}/04-iam-identity-provider.json
jq . ${DIR_CCO}/04-iam-identity-provider.json
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  • Upload the Bucket content 
aws s3 sync ${OIDC_BUCKET_CONTENT}/ \
    s3://${OIDC_BUCKET_NAME}
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  • Make sure you can access the content through the public URL

NOTE: CloudFront can take some time to deploy the Distribution. Please be sure the Distribution has been deployed and is available before running this step (Status=Enabled). You can access the CloudFront Console to check it.

curl https://${CLOUDFRONT_URI}/keys.json
curl https://${CLOUDFRONT_URI}/.well-known/openid-configuration
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Create the OpenID Connector identity provider

  • Create the IdP OIDC 
aws iam create-open-id-connect-provider \
    --cli-input-json file://${DIR_CCO}/04-iam-identity-provider.json \
    > ${DIR_CCO}/04-iam-identity-provider-object.json
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  • Get the OpenID Connect ARN 
OIDC_ARN=$(jq -r .OpenIDConnectProviderArn \
    ${DIR_CCO}/04-iam-identity-provider-object.json)

echo ${OIDC_ARN}
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Create the IAM Roles

Now let’s extract the CredentialRequests which contain the definition of IAM Roles permissions, besides the service account information which will allowed to assume the Role.

  • Extract CredentialRequests from the release image 
./oc adm release extract \
    --credentials-requests \
    --cloud=aws \
    --to=${DIR_CCO}/credrequests \
    ${RELEASE_IMAGE}
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  • Create IAM Roles 
./ccoctl aws create-iam-roles \
    --name=${CLUSTER_NAME} \
    --region=${CLUSTER_REGION}\
    --credentials-requests-dir=${DIR_CCO}/credrequests \
    --identity-provider-arn=${OIDC_ARN} \
    --output-dir ${DIR_CCO}
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  • Copy the manifests to the installer directory 
cp -rvf ${DIR_CCO}/manifests/* ${DIR_INSTALLER}/manifests
cp -rvf ${DIR_CCO}/tls ${DIR_INSTALLER}/
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Create the Cluster

  • Create a cluster 
./openshift-install create cluster \
    --dir ${DIR_INSTALLER} \
    --log-level debug
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Done! o/

Post-install review

Installer overview

  • Install logs 
INFO Access the OpenShift web-console here: https://console-openshift-console.apps.my-sts.devcluster.openshift.com

DEBUG Time elapsed per stage:                      
DEBUG            cluster: 6m26s                    
DEBUG          bootstrap: 50s                      
DEBUG Bootstrap Complete: 9m43s                    
DEBUG                API: 2m1s                     
DEBUG  Bootstrap Destroy: 55s                      
DEBUG  Cluster Operators: 9m40s                    
INFO Time elapsed: 27m58s
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  • Check the service account issuer URL on the Authentication resource 
$ oc get authentication cluster -o json \
    | jq .spec.serviceAccountIssuer
"https://d15diimhmpdwiy.cloudfront.net"
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  • Check if all Cluster Operators are available 
# COs Available
$ oc get co  -o json \
    | jq -r ".items[].status.conditions[] | select(.type==\"Available\").status" \
    | sort |uniq -c
32 True

# COs Degraded
$ oc get co -o json \
    | jq -r ".items[].status.conditions[] | select(.type==\"Degraded\").status" \
    | sort |uniq -c
32 False
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Component tests

Let’s test the credentials provided to the Machine-API Controller.

  • Check operator state 
$ oc get co machine-api
NAME          VERSION   AVAILABLE   PROGRESSING   DEGRADED   SINCE   MESSAGE
machine-api   4.10.16   True        False         False      17m
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  • Check the credentials presented to the component 
$ oc get secrets aws-cloud-credentials \
    -n openshift-machine-api \
    -o jsonpath='{.data.credentials}' \
    | base64 -d
[default]
role_arn = arn:aws:iam::${ACCOUNT_ID}:role/oidc-def-openshift-machine-api-aws-cloud-credentials
web_identity_token_file = /var/run/secrets/openshift/serviceaccount/token
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  • Get the ProjectedServiceAccountToken 
# Get Token path from AWS credentials mounted to pod
TOKEN_PATH=$(oc get secrets aws-cloud-credentials \
    -n openshift-machine-api \
    -o jsonpath='{.data.credentials}' |\
    base64 -d |\
    grep ^web_identity_token_file |\
    awk '{print$3}')

# Get Controler's pod
CAPI_POD=$(oc get pods -n openshift-machine-api \
    -l api=clusterapi \
    -o jsonpath='{.items[*].metadata.name}')

# Extract tokens from pod
TOKEN=$(oc exec -n openshift-machine-api ${CAPI_POD} \
    -c machine-controller -- cat ${TOKEN_PATH})
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  • Inspect the token - Key ID 
$ echo $TOKEN | awk -F. '{ print $1 }' | base64 -d 2>/dev/null | jq .alg
"RS256"
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  • Inspect the token - Issuer URI 
$ echo $TOKEN | awk -F. '{ print $2 }' | base64 -d 2>/dev/null | jq .iss
"https://d15diimhmpdwiy.cloudfront.net"
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Test the token with AssumeRoleWithWebIdentity

  • Extract the IAM Role ARN from the Secret 
IAM_ROLE=$(oc get secrets aws-cloud-credentials \
    -n openshift-machine-api \
    -o jsonpath='{.data.credentials}' |\
    base64 -d |\
    grep ^role_arn |\
    awk '{print$3}')
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  • Assume the IAM Role with the previously extracted token 
aws sts assume-role-with-web-identity \
    --role-arn "${IAM_ROLE}" \
    --role-session-name "my-session" \
    --web-identity-token "${TOKEN}"
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The temporary credentials should be returned, otherwise, the controller may have issues accessing the AWS services.

Solution Review

Using CloudFront to use as an endpoint URL for OIDC was one option explored in this article, I can see many other possibilities like Lambda, on-prem web server, and so on. The most important is: that the IAM OIDC requires a public endpoint to serve the public keys and configuration.

In my opinion, CloudFront Distribution seems to have many benefits, such as low operation, low cost, no code to maintain, secure, as well as fully managed.

Let's create a matrix comparing a few available options:

# URL exposure solution Est.Cost(USD)/mo Private S3 Serverless Codeless Low-Ops Note
1 S3 0.11 No Yes Yes No Private bucket
2 CloudFront + S3 free-tier** + 0.11** Yes Yes Yes Yes Best option evaluated
3 Lambda Endpoint+S3 free-tier + 0.11 Yes Yes No No Additional code management required, and function management as well
4 ApiGW+Lambda+S3 (free-tier*2) + 0.11 Yes Yes No No Additional code management required, and function management as well
5 ALB+Lambda+S3 17,73 + free-tier + 0.11 Yes Yes No No Additional code management required, and function management as well

AWS Pricing Calculator available here.

*Estimated cost calculation (based on CloudFront Distribution metrics):

  • ~4 requests per minute (Avg) => ~172800/mo
  • ~1500KiB per minute (Avg) => ~64.8GiB/mo

**The CloudFront option will be free when enabling the cache on the requests to the origin, since all the S3 content is static. Otherwise the cost will be higher than[1]: S3 direct/public URL.

***Free tier details:

# S3 Free-tier:
S3 Free-tier: 20,000 GET Requests; 2,000 PUT, COPY, POST, or LIST Requests; and 100 GB of Data Transfer Out each month.

# CloudFront Free-tier:
1 TB of data transfer out, 10,000,000 HTTP and HTTPS Requests, plus 2,000,000 CloudFront Function invocations each month.
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Conclusion

As you can see, I didn’t find any restriction to using CloudFront as a public endpoint for IAM OIDC when setting the S3 bucket for private access only, and keeping it compliant with the S3 best practices.

I would also like to mention that there is no difference/impact in terms of cluster security, as the cluster does not access the OIDC URL. So, that change is more for the AWS account security compliance.

I can also see some advantages when writing this article, like expanding the solution when you're operating many accounts with many clusters thus simplifying the life of DevSecOps teams:

  • Centralize the management of the OIDC files into one single entry point;
  • Create your own DNS domain for the OIDC identifier;
  • Flexibility to create a 'multi-tenant' solution storing many JWKS from the different clusters in the same bucket; or it could be in different buckets using the same entry point (CloudFront) routing to different origins (S3 Buckets).

Furthermore, it would be nice to have:

  • AWS to implement the OIDC private access to the thumbprints instead of a public HTTPS*, this way it would be possible to set a couple of S3 bucket policies, allowing only OIDC ARN principal;
  • ccoctl utility to create the steps using CloudFront by default;
  • openshift-installer to embed the ccoctl steps/automation when using manual-STS;
  • openshift-installer to deploy the default IPI cluster with STS by default.

*there's a blocker from OIDC spec[1] in this suggestion, but AWS could improve the security in this access since the only OIDC client, in this case, is the STS service (access between AWS services).

[1] "The returned Issuer location MUST be a URI RFC 3986 [RFC3986] with a scheme component that MUST be HTTPS, a host component, and optionally, port and path components and no query or fragment components." [https://openid.net/specs/openid-connect-discovery-1_0.html#IssuerDiscovery]

Suggestions for the next topics:

  • Create one multi-tenant bucket with custom DNS on CloudFront to serve JWKS files from multiple clusters
  • Evaluate the following options to serve public URLs to IAM OIDC, like:

References

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