Add example for accessing S3 resources across different AWS accounts …

…with IAM service enforced (#248)

* Add docs on how to share s3 resources to another account

* Add test script, README and makefile targets

multi-account-multi-region-s3-access/Makefile

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+# Enforce IAM to test cross-account cross-region access
+export ENFORCE_IAM=1
+export DEBUG=1
+
+SHELL := /bin/bash
+
+usage:       ## Show this help
+	@fgrep -h "##" $(MAKEFILE_LIST) | fgrep -v fgrep | sed -e 's/\\$$//' | sed -e 's/##//'
+
+install:     ## Install dependencies
+	@which localstack || pip install localstack
+	@which awslocal || pip install awscli-local
+
+run:         ## Run the cross-account cross-region experiment of copying data from one S3 bucket to another
+	./run.sh
+
+start:       ## Start LocalStack
+	localstack start -d
+
+stop:        ## Stop LocalStack
+	@echo
+	localstack stop
+
+ready:       ## Wait for LocalStack to be ready
+	@echo Waiting on the LocalStack container...
+	@localstack wait -t 30 && echo Localstack is ready to use! || (echo Gave up waiting on LocalStack, exiting. && exit 1)
+
+logs:        ## Retrieve logs from LocalStack
+	@localstack logs > logs.txt
+
+test-ci:     ## Run CI test
+	make start install ready run; return_code=`echo $$?`;\
+	make logs; make stop; exit $$return_code;
+
+.PHONY: usage install start run stop ready logs test-ci

multi-account-multi-region-s3-access/README.md

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+# Localstack Demo: Access S3 resources from different account and different region
+
+Simple demo script to showcase the accessing of S3 resources from a different AWS account using bucket policies and IAM users with specific IAM policies attached to their identities.
+The script uses a couple of AWS profiles to achieve that:
+
+* Admin user of account A with account ID `000000000001`.
+
+* Admin user of account B with account ID `000000000002`.
+
+* Account A user that creates the S3 bucket and subsequent resources inside the bucket.
+
+* Account B user that copies the resources from account A user's S3 bucket `source` into a bucket `target` it owns.
+
+## Prerequisites
+
+* LocalStack
+* Docker
+* Python 3.6+ / Python Pip
+* `make`
+
+## Installing
+
+To install the dependencies:
+
+```shell
+make install
+```
+
+## Starting LocalStack
+
+Make sure that LocalStack is started:
+
+```shell
+LOCALSTACK_AUTH_TOKEN=... make start
+```
+
+## Running
+
+Run the sample demo script:
+
+```shell
+make run
+```
+
+## License
+
+This code is available under the Apache 2.0 license.
+

multi-account-multi-region-s3-access/bucket/main.go

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+package main
+
+import "fmt"
+
+func helloworld() string {
+	return "Hello World!!"
+}
+
+func main() {
+	fmt.Println(helloworld())
+}

multi-account-multi-region-s3-access/bucket/main_test.go

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+package main
+
+import "testing"
+
+func TestHelloWorld(t *testing.T) {
+	if helloworld() != "Hello World!!" {
+		t.Fatal("Test fail")
+	}
+}
+

multi-account-multi-region-s3-access/run.sh

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+#!/bin/bash
+set -euxo pipefail
+export AWS_SECRET_ACCESS_KEY=test
+
+# Create `ls-a` and `ls-b` IAM users using the root accounts of each user
+AWS_ACCESS_KEY_ID=000000000001 awslocal iam create-user --user-name ls-a
+AWS_ACCESS_KEY_ID=000000000002 awslocal iam create-user --user-name ls-b
+
+# Create IAM policies for each of the IAM users using the root accounts of each user
+AWS_ACCESS_KEY_ID=000000000001 awslocal iam create-policy --policy-name pa --policy-document '{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Action":"s3:*","Resource":"*"}]}'
+AWS_ACCESS_KEY_ID=000000000002 awslocal iam create-policy --policy-name pb --policy-document '{"Version":"2012-10-17","Statement":[{"Effect":"Allow","Action":"s3:*","Resource":"*"}]}'
+
+# Attach the IAM policies to each IAM user using the root accounts of each user 
+AWS_ACCESS_KEY_ID=000000000001 awslocal iam attach-user-policy --user-name ls-a --policy-arn arn:aws:iam::000000000001:policy/pa
+AWS_ACCESS_KEY_ID=000000000002 awslocal iam attach-user-policy --user-name ls-b --policy-arn arn:aws:iam::000000000002:policy/pb
+
+# Create access keys for each user using the root accounts of each user
+CREDENTIALS_A=$(AWS_ACCESS_KEY_ID=000000000001 awslocal iam create-access-key --user-name ls-a)
+CREDENTIALS_B=$(AWS_ACCESS_KEY_ID=000000000002 awslocal iam create-access-key --user-name ls-b)
+
+# Retrieve the access key id of each user
+# In LocalStack, the secret access key is not strictly enforced
+# But the access key id is
+USER_ACCESS_KEY_ID_A=`jq -r .AccessKey.AccessKeyId <<< $CREDENTIALS_A`
+USER_ACCESS_KEY_ID_B=`jq -r .AccessKey.AccessKeyId <<< $CREDENTIALS_B`
+
+# Create `source` bucket in `ls-a` user's account
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_A awslocal s3 mb s3://source
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_A awslocal s3 sync ./bucket s3://source
+
+# Attach a bucket policy so that user `ls-b` can access it
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_A awslocal s3api put-bucket-policy --bucket source --policy file://source_bucket_policy.json
+
+# Attempt to access bucket `source` using `ls-a` user
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_B awslocal s3 ls s3://source
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_B awslocal s3api list-object-versions --bucket source --prefix main.go
+
+# Sync buckets `source` and `target` using `ls-b` user
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_B awslocal s3 mb s3://target
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_B awslocal s3 sync s3://source s3://target
+AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_B awslocal s3api list-object-versions --bucket target --prefix main.go
+
+# Fail the script if somehow user A can access the resources of bucket `target`
+echo "Check if s3api list-object-versions commnands fails as expected"
+if AWS_ACCESS_KEY_ID=$USER_ACCESS_KEY_ID_A awslocal s3api list-object-versions --bucket target --prefix main.go; then
+    exit 1
+fi

multi-account-multi-region-s3-access/source_bucket_policy.json

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+{
+    "Version": "2012-10-17",
+    "Statement": [
+        {
+            "Effect": "Allow",
+            "Action": [
+                "s3:Get*"
+            ],
+            "Principal": { "AWS": "arn:aws:iam::000000000002:user/ls-b" },
+            "Resource": "arn:aws:s3:::source/*"
+        },
+        {
+            "Effect": "Allow",
+            "Action": [
+                "s3:List*"
+            ],
+            "Principal": { "AWS": "arn:aws:iam::000000000002:user/ls-b" },
+            "Resource": "arn:aws:s3:::source"
+        }
+    ]
+}