Prioritize ssm creds for linux

[sidlovskyy]

Summary

In Linux environment credentials priority is set to:

1. Env vars (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY).
2. Shared credentials file
3. EC2 role credentials.
4. Rotating shared credentials file located at /rotatingcreds/credentials

When ECS Agent (in case of ECS Anywhere) is installed on EC2 instance of other AWS account with IAM Role attached, EC2 Role credentials are prioritized over rotating credentials which means SSM Agent credentials won't be used.
As far as there is no way to change credentials providers priority it's not possible to make ECS Agent use SSM Agent rotating credentials. It makes usage of ECS Anywhere hardly possible when external instance is part of other AWS Account with EC2 IAM Role credentials attached. It may not be possible to modify EC2 Role credentials due to policies in external AWS Account.

It makes sense to prioritize rotating credentials over EC2 Role credentials in case ECS Agent is installed on external instance. Please note, similar behavior is implemented for Windows:

amazon-ecs-agent/ecs-agent/credentials/instancecreds/instancecreds_windows.go

Line 54 in b702281

credProviders = append(credProviders[:1], append([]credentials.Provider{providers.NewRotatingSharedCredentialsProvider()}, credProviders[1:]...)...)

Alternatively, it would be great if credentials providers priority could be re-configured by additional config variable.

Implementation details

In Linux environment when ECS_EXTERNAL variable is set to true RotatingSharedCredentialsProvider is prioritized over EC2RoleProvider so that SSM Agent credentials are prioritized over EC2 Role credentials.

Testing

New tests cover the changes: no

Description for the changelog

Prioritize RotatingSharedCredentialsProvider over EC2RoleProvider instance credentials provider for external instances in Linux

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[amogh09]

Thanks for the contribution @sidlovskyy . I see that this change was made for Windows in #3184. I am trying to find out why Linux was not considered in-scope at that time.

[amogh09]

Looks like this change makes GetCredentials implementation for Linux identical to that of Windows. I think we should merge the two into a single instancecreds/instancecreds.go file.

[amogh09]

In the meanwhile I will reproduce this issue on my end. Once the PR is ready I can verify that the change works by testing it in the same environment.

[amogh09]

I was able to reproduce the issue on my end.

[amogh09]

Thanks for the contribution @sidlovskyy . I see that this change was made for Windows in #3184. I am trying to find out why Linux was not considered in-scope at that time.

I talked to some folks about this. We don't have a good reason as to why #3184 did not make the same change for Linux. It makes sense for the instance credentials fetch logic to be platform agnostic.

[sidlovskyy]

Thanks for the contribution @sidlovskyy . I see that this change was made for Windows in #3184. I am trying to find out why Linux was not considered in-scope at that time.

I talked to some folks about this. We don't have a good reason as to why #3184 did not make the same change for Linux. It makes sense for the instance credentials fetch logic to be platform agnostic.

Thank you, should I update PR to include platform agnostic instancecreds.go that is the same as existing windows implementation?