AWS IAM Roles & Policies: How ChatGPT Helped Me Understand

I interacted with ChatGPT, and here’s how it cleared my whole doubt about IAM hierarchy:

User: “So this is hierarchy
Ec2->Role->Policy
User~>Role->Policy”

Assistant: “Exactly! You’re almost there — let’s lay it out cleanly …”
(explained how EC2 and Users assume roles and apply policies)

User: “So your are telling this is also possible
User->Policy”

Assistant: “✅ Yes, absolutely!
You’re 100% correct — and this is one of the most common setups.”
(clarified direct user→policy attachment)

🔍 IAM Hierarchy & Trust Relationships

Principal	Can Have Policy?	Can Assume Role?	Description
User	✅ Direct or via Group	✅ If `sts:AssumeRole` granted	Baseline access; can get elevated access
Group	✅	❌	Manages users with shared policies
Role	✅ Trust + Permission Policies	—	Identity assumed by users/services/accounts
Service (EC2/Lambda)	❌	✅ via instance profile	Assumes role for secure AWS access

🧭 Identity & Permission Flow

// EC2 → assumes → Role → has → Permission Policies → accesses AWS  
// User → (optional) assumes → Role → has → Permission Policies → elevated access  
// User → belongs to → Group → has → Policies → baseline access
  

📌 Policy Types: Trust vs Permission

Terraform logic in practice matches what AWS documentation says:

• Trust Policy: Attached to a Role. Defines who can assume it via sts:AssumeRole .
• Permission Policy: Defines what actions are allowed; attached to User, Group, or Role.

🌟 JSON Examples

1. EC2 → Role → Policy (S3 Access)

// Trust Policy
{ "Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"Service":"ec2.amazonaws.com"},"Action":"sts:AssumeRole"}] }

// Permission Policy
{ "Version":"2012-10-17","Statement":[{"Effect":"Allow","Action":["s3:GetObject","s3:PutObject"],"Resource":["arn:aws:s3:::my-bucket/*"]}] }
  

2. User → Assume Admin Role → Policy

// User Policy
{ "Effect":"Allow","Action":"sts:AssumeRole","Resource":"arn:aws:iam::123456789012:role/AdminRole" }

// AdminRole Trust Policy
{ "Effect":"Allow","Principal":{"AWS":"arn:aws:iam::123456789012:user/DevUser"},"Action":"sts:AssumeRole" }

// AdminRole Permission Policy
{ "Effect":"Allow","Action":"*","Resource":"*" }
  

3. User → Policy (Permanent S3 Read‑Only)

{ "Version":"2012-10-17","Statement":[{"Effect":"Allow","Action":["s3:ListBucket","s3:GetObject"],"Resource":["arn:aws:s3:::*"]}] }

4. Cross‑Account Role (Account B → A)

// Trust Policy (A trusts B)
{ "Version":"2012-10-17","Statement":[{"Effect":"Allow","Principal":{"AWS":"arn:aws:iam::111122223333:root"},"Action":"sts:AssumeRole"}] }

// Permission Policy
{ "Effect":"Allow","Action":["s3:ListBucket","s3:GetObject"],"Resource":["arn:aws:s3:::shared-bucket/*"] }
  

🔄 Two‑Way “Gate” for Role Assumption

Both the caller (e.g., user or service) and the role must explicitly allow sts:AssumeRole. The role’s trust policy defines who can assume it, and the caller’s policy must permit calling the API [oai_citation_attribution:0‡Stack Overflow](https://stackoverflow.com/questions/61807515/why-is-stsassumerole-in-the-trust-policy-and-not-the-permissions-policy-of-a-ro?utm_source=chatgpt.com)[oai_citation_attribution:1‡LinkedIn](https://www.linkedin.com/pulse/permission-policy-vs-trust-aws-rupesh-tiwari?utm_source=chatgpt.com).

🧠 Final Takeaway

• Users/Groups: Can have direct policies → permanent permissions.
• Roles: Trust + permission policies → temporary identities with scoped access.
• Services & Cross‑Account: Use roles for secure, auditable, temporary access—no long-lived credentials.

🚀 What’s Next?

Need CLI examples, Terraform/CloudFormation templates, or visual diagrams? Happy to help!