AWS S3 IAM policy for role for restricting few instances to connect to S3 bucket based in instance tag or instance id

Question

I have a AWS S3 already associated with all the instances for read privileges to all S3 buckets. Now I need to add a policy to the roles for write privileges(Put object) so that a few of these instances can have write permissions to certain folders in the S3. Is there any way to achieve it through instance tag(better option for me) or instance id.

I tried adding an IAM policy but when I set the condition, my instances are not getting the required privileges.

The IAM policy I used is:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "Stmt1456567757624",
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::testbucket/testfolder1/*",
      "Condition": {
        "ArnEquals": {
          "aws:SourceArn": "arn:aws:ec2:eu-west-1:<accountno>:instance/<instanceid1>"
        }
      }
    },
    {
      "Sid": "Stmt1456567757625",
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::testbucket/testfolder2/*",
      "Condition": {
        "ArnEquals": {
          "aws:SourceArn": "arn:aws:ec2:eu-west-1:<accountno>:instance/<instanceid2>"
        }
      }
    }
  ]
}

Answer 1

Here's an alternative, based on hints given in Granting access to S3 resources based on role name...

Instead of using aws:SourceArn, use aws:userid!

The Request Information That You Can Use for Policy Variables documentation has a table showing various values of aws:userid including:

For Role assigned to an Amazon EC2 instance, it is set to role-id:ec2-instance-id

Therefore, you could use the Role ID of the role that is used to launch the Amazon EC2 instance to permit access OR the Instance ID.

For example, this one is based on a Role ID:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SID123",
            "Effect": "Allow",
            "Action": [
                "s3:*"
            ],
            "Resource": [
                "*"
            ],
            "Condition": {
                "StringLike": {
                    "aws:userid": [
                        "AROAIIPEUJOUGITIU5BB6*"
                    ]
                }
            }
        }
    ]
}

Of course, if you are going to assign permission based on a Role ID, then you can just as easily grant permissions within the Role itself.

This one is based on an Instance ID:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SID123",
            "Effect": "Allow",
            "Action": [
                "s3:*"
            ],
            "Resource": [
                "*"
            ],
            "Condition": {
                "StringLike": {
                    "aws:userid": [
                        "*:i-03c9a5f3fae4b630a"
                    ]
                }
            }
        }
    ]
}

The Instance ID will remain with the instance, but a new one will be assigned if a new instance is launched, even from the same Amazon Machine Image (AMI).

Answer 2

The IAM Policy Elements Reference documentation says:

aws:SourceArn – To check the source of the request, using the Amazon Resource Name (ARN) of the source. (This value is available for only some services.)

However, the documentation does not state which services can use it.

There are examples available for its use with SQS and SNS, with a sourceARN of an Amazon S3 bucket and also using sourceARN with Lambda. However, it does not appear to be supported with Amazon EC2.