CodePipeline ECS Blue/Green Deployment cross account fails with PermissionError

I'm trying to set up CodePipeline with an ECS blue/green deployment where the deployment is in a different AWS account.

I've been using the two guides for ECS Blue/Green and CodePipeline cross-account deployments. CodePipeline lives in Account A along with its KMS Key, S3 artifact bucket and ECR repository. The ECS cluster lives in Account B with the CodeDeploy setup.

The ECR, KMS key and S3 buckets have cross-account permissions (these give a different error when wrong). The cluster starts up and runs, and CodeDeploy works correctly when invoked inside Account B.

A role in Account B has been created for CodePipeline to assume and it has granted Account A permission to assume the role. This role currently has the AWSCodeDeployRoleForECS policy (I intend to reduce this once it works)

CodePipeline fails with an unhelpful message of

   "code": "PermissionError",
   "message": "The provided role does not have sufficient permissions to access CodeDeploy"
}```

The codepipeline role does have permission to access codedeploy as it's in the canned AWS policy. I can only assume there's some missing permission but I cannot find out what from this message.

I've discovered the answer after tracing through CloudTrail. There were two permissions missing from the CodePipeline deployment role which I can't find documented, they are ecs:RegisterTaskDefinition and iam:PassRole for the ECS Container role. CodeDeploy assumes a different role during deployment that also needs these permissions, but it looks like CodePipeline needs them to start the deployment.

The documentation I was working off had an example for CodeDeploy cross-account, but this was CodeDeploy to EC2 rather than to ECS.

My final permissions for the role assumed in Account B by CodePipeline looks like:

{
  "Version": "2012-10-17",
  "Statement": [
    {
        "Action": [
            "codedeploy:CreateDeployment",
            "codedeploy:GetDeployment",
            "codedeploy:GetDeploymentConfig",
            "codedeploy:GetApplicationRevision",
            "codedeploy:RegisterApplicationRevision",
            "codedeploy:GetApplication",
            "ecs:RegisterTaskDefinition"
        ],
        "Resource": "*",
        "Effect": "Allow"
    },
    {
        "Action": [
            "s3:GetObject*",
            "s3:PutObject",
            "s3:PutObjectAcl"
        ],
        "Resource": "arn:aws:s3:::deployment_intermediate_bucket/*",
        "Effect": "Allow"
    },
    {
        "Action": [ "s3:ListBucket"],
        "Resource": "arn:aws:s3:::deployment_intermediate_bucket",
        "Effect": "Allow"
    },
    {
        "Effect": "Allow",
        "Action": [
            "kms:DescribeKey",
            "kms:GenerateDataKey*",
            "kms:Encrypt",
            "kms:ReEncrypt*",
            "kms:Decrypt"
        ],
        "Resource": [
            "deployment_kms_key_arn"
        ]
    },
    {
        "Action": [
            "iam:PassRole"
        ],
        "Effect": "Allow",
        "Resource": "ecs_container_role_arn"
    }
  ]
}

I'm going to reduce this down to the minimum required.