How to secure an S3 bucket to an Instance's Role? - amazon-web-services

Question

Using cloudformation I have launched an EC2 instance with a role that has an S3 policy which looks like the following
{"Statement":[{"Action":"s3:*","Resource":"*","Effect":"Allow"}]}
In S3 the bucket policy is like so
{
"Version": "2012-10-17",
"Id": "MyPolicy",
"Statement": [
{
"Sid": "ReadAccess",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::123456678:role/Production-WebRole-1G48DN4VC8840"
},
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::web-deploy/*"
}
]
}
When I login to the instance and attempt to curl any object I upload into the bucket (without acl modifications) I receive and Unauthorized 403 error.
Is this the correct way to restrict access to a bucket to only instances launched with a specific role?

Answer 1

The EC2 instance role is more than sufficient to put/read to any of your S3 buckets, but you need to use the instance role, which is not done automatically by curl.
You should use for example aws s3 cp <local source> s3://<bucket>/<key>, which will automatically used the instance role.

Answer 2

There are three ways to grant access to an object in Amazon S3:
Object ACL: Specific objects can be marked as "Public", so anyone can access them.
Bucket Policy: A policy placed on a bucket to determine what access to Allow/Deny, either publicly or to specific Users.
IAM Policy: A policy placed on a User, Group or Role, granting them access to an AWS resource such as an Amazon S3 bucket.
If any of these policies grant access, the user can access the object(s) in Amazon S3. One exception is if there is a Deny policy, which overrides an Allow policy.
Role on the Amazon EC2 instance
You have granted this role to the Amazon EC2 instance:
{"Statement":[{"Action":"s3:*","Resource":"*","Effect":"Allow"}]}
This will provide credentials to the instance that can be accessed by the AWS Command-Line Interface (CLI) or any application using the AWS SDK. They will have unlimited access to Amazon S3 unless there is also a Deny policy that otherwise restricts access.
If anything, that policy is granting too much permission. It is allowing an application on that instance to do anything it wants to your Amazon S3 storage, including deleting it all! It is better to assign least privilege, only giving permission for what the applications need to do.
Amazon S3 Bucket Policy
You have also created a Bucket Policy, which allows anything that has assumed the Production-WebRole-1G48DN4VC8840 role to retrieve the contents of the web-deploy bucket.
It doesn't matter what specific permissions the role itself has -- this policy means that merely using the role to access the web-deploy bucket will allow it to read all files. Therefore, this policy alone would be sufficient to your requirement of granting bucket access to instances using the Role -- you do not also require the policy within the role itself.
So, why can't you access the content? It is because using a straight CURL does not identify your role/user. Amazon S3 receives the request and treats it as anonymous, thereby not granting access.
Try accessing the data via the CLI or programmatically via an SDK call. For example, this CLI command would download an object:
aws s3 cp s3://web-deploy/foo.txt foo.txt
The CLI will automatically grab credentials related to your role, allowing access to the objects.