IAM Policy

• Access in AWS is managed by creating or using policies and attaching them to IAM identities (Users, UserGroups, Roles).
• A policy is an object in AWS that when associated with an identity or resource defines their permissions
• AWS evaluates these policies when an IAM Principal (user or role) makes a request.
• Most of the Policies in AWS are stored as JSON documents
• Refer Here for the IAM Policy Language and grammar.

policy = {
    <version_block?>
    <id_block?>
    <statement_block>
}


<version_block> = "Version" : ("2008-10-17" | "2012-10-17")

<id_block> = "Id" : <policy_id_string>

<statement_block> = "Statement" : [ <statement>, <statement>, ... ]

<statement> = { 
    <sid_block?>,
    <principal_block?>,
    <effect_block>,
    <action_block>,
    <resource_block>,
    <condition_block?>
}

<sid_block> = "Sid" : <sid_string>

<effect_block> = "Effect" : ("Allow" | "Deny")  

<principal_block> = ("Principal" | "NotPrincipal") : ("*" | <principal_map>)

<principal_map> = { <principal_map_entry>, <principal_map_entry>, ... }

<principal_map_entry> = ("AWS" | "Federated" | "Service" | "CanonicalUser") :   
    [<principal_id_string>, <principal_id_string>, ...]

<action_block> = ("Action" | "NotAction") : 
    ("*" | [<action_string>, <action_string>, ...])

<resource_block> = ("Resource" | "NotResource") : 
    ("*" | [<resource_string>, <resource_string>, ...])

<condition_block> = "Condition" : { <condition_map> }
<condition_map> = { 
  <condition_type_string> : { <condition_key_string> : <condition_value_list> },
  <condition_type_string> : { <condition_key_string> : <condition_value_list> }, ...
}  
<condition_value_list> = [<condition_value>, <condition_value>, ...]
<condition_value> = ("string" | "number" | "Boolean")

• Version Block: Optional

"Version" : ("2008-10-17" | "2012-10-17")

• Id Block: Optional. It is recommended to use UUID(GUID) Refer Here

"Id": "<policy_id_string>"

"Id":"Admin_Policy"

"Id":"cd3ad3d9-2776-4ef1-a904-4c229d1642ee"

• Statement:
  • This is the main element of the policy.
  • This can contian a single statement or an array of staments
    "Statement": [{...}, {...}]
  • Each statement will have the following
    <statement> = {
<sid_block?>,
<principal_block?>,
<effect_block>,
<action_block>,
<resource_block>,
<condition_block?>
}
• First Policy to give full access to some resource (in this example i have taken my s3 bucket)

{
    "Version": "2012-10-17",
    "Id": "FirstPolicyForLearning",
    "Statement": [
        {
            "Effect": "Allow", 
            "Action":  "*",     
            "Resource": "arn:aws:s3:::khajadatascience.com"
        }

    ]
}

• Lets add a second policy json to allow any thing apart from delete

{
    "Version": "2012-10-17",
    "Id": "SecondPolicyForLearning",
    "Statement": [
        {
            "Effect": "Allow", 
            "Action":  "*",     
            "Resource": "arn:aws:s3:::khajadatascience.com"
        },
        {
            "Effect": "Deny", 
            "Action":  "s3:DeleteBucket",    
            "Resource": "arn:aws:s3:::khajadatascience.com"
        }

    ]
}

• Refer Here
• Write a simple json policy which allows all actions on s3 and denies all actions on ec2

{
    "Version": "2012-10-17",
    "Id": "SecondPolicyForLearning",
    "Statement": [
        {
            "Effect": "Allow", 
            "Action":  "s3:*",
            "Resource": "*"
        },
        {
            "Effect": "Deny",
            "Action": "ec2:*",
            "Resource": "*"
        }
    ]
}