There will come a day when we can provision users and provide the necessary authorization to required enterprise resources with easy, but until that day comes there will be hacks like this one.
This is very much Proof of Concept (PoC), I have not tested this beyond my network, and there would be a lot of angles to consider with any type of deployment, security (sanitization and additional logic), IAM SSH Public Keys were designed for CodeCommit, and other unknowns.
My first attempt was to create a PAM module that would authenticate SSH users via aws-sdk-cpp libraries, accessing the API using instance profile / EC2 IAM roles, allowing EC2 instances to obtain credentials to access the information. This seemed like a good route, but trying to hack a PAM module together that would be portable enough for distribution would be a challenge, static versions of aws-sdk-cpp, etc. I will revisit this in the future, but for now, I will move on.
On my second attempt, I create a Bash script that would produce the correct output for SSH AuthorizedKeysCommand directive using SSH public keys from AWS IAM service via awscli. As the EC2 instance can utilize the instance profile role via aws command. This didn’t come without headaches, permission issues, etc.