Allow S3 bucket operations based on EC2 role - amazon-web-services

Question

Our EC2s are secured using IAM roles. When trying to run an AWS console command such as aws s3 cp I am seeing:
fatal error: An error occurred (403) when calling the HeadObject operation: Forbidden
If allowed based on specific users that are given keys, there are no issues. This just isn't working with roles.
Here is the bucket ACL:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "Public",
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::my-bucket/*"
},
{
"Sid": "Devs",
"Effect": "Deny",
"NotPrincipal": {
"AWS": "arn:aws:iam::1234567890:user/DevUser"
},
"Action": "s3:*",
"Resource": "arn:aws:s3:::my-bucket/something-privileged/*"
},
{
"Sid": "EC2s",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::1234567890:role/EC2Role"
},
"Action": "s3:*",
"Resource": "arn:aws:s3:::my-bucket/something-privileged/*"
},
]
}
As you can see, we want the public to generally be able to fetch objects that we link to. This works.
We want devs to be able to access a specific hidden folder in the bucket using their AWS keys. This works.
We want EC2s to be able to run aws-cli commands on that same hidden folder using only the assigned security role. This does not work.
I also tried "Effect": "Deny", "NotPrincipal": { ... } on the EC2 statement but that didn't work either.
What's wrong with this ACL?

Answer 1

You have a Deny statement in this where the principal is not that specific IAM user. In any AWS privilege a deny will always override an allow which is the scenario happening here.
To allow this here you will need to include the IAM role arn in the NotPrincipal statement as well. This would look like the below statement.
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "Public",
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::my-bucket/*"
},
{
"Sid": "Devs",
"Effect": "Deny",
"NotPrincipal": {
"AWS": ["arn:aws:iam::1234567890:user/DevUser", "arn:aws:iam::1234567890:role/EC2Role"]
},
"Action": "s3:*",
"Resource": "arn:aws:s3:::my-bucket/something-privileged/*"
}
]
}