IAM 角色临时凭证可以在 cloudformation 模板中使用吗？

Question

我正在构建一个堆栈，它需要访问私有 S3 存储桶才能下载我的应用程序的最新版本。我正在使用IAM roles，这是一个相对较新的 AWS 功能，它允许为 EC2 实例分配特定角色，然后与 IAM 策略相结合。不幸的是，这些角色带有在实例化时生成的临时 API 凭证。这并不严重，但它迫使我做这个 cloud-init 脚本之类的事情（简化为相关部分）：

#!/bin/sh

# Grab our credentials from the meta-data and parse the response
CREDENTIALS=$(curl -s http://169.254.169.254/latest/meta-data/iam/security-credentials/s3access)
S3_ACCESS_KEY=$(echo $CREDENTIALS | ruby -e "require 'rubygems'; require 'json'; puts JSON[STDIN.read]['AccessKeyId'];")
S3_SECRET_KEY=$(echo $CREDENTIALS | ruby -e "require 'rubygems'; require 'json'; puts JSON[STDIN.read]['SecretAccessKey'];")
S3_TOKEN=$(echo $CREDENTIALS | ruby -e "require 'rubygems'; require 'json'; puts JSON[STDIN.read]['Token'];")

# Create an executable script to pull the file
cat << EOF > /tmp/pullS3.rb
require 'rubygems'
require 'aws-sdk'
AWS.config(
    :access_key_id     => "$S3_ACCESS_KEY",
    :secret_access_key => "$S3_SECRET_KEY",
    :session_token     => "$S3_TOKEN")
s3 = AWS::S3.new()
myfile = s3.buckets['mybucket'].objects["path/to/my/file"]
File.open("/path/to/save/myfile", "w") do |f|
    f.write(myfile.read)
end
EOF

# Downloading the file
ruby /tmp/pullS3.rb

首先：这很有效，而且效果很好。尽管如此，我很想使用 CloudFormation 对源访问的现有支持。具体来说，cfn-init 支持使用authentication resources 来获取受保护的数据，包括 S3 存储桶。是否可以从 cfn-init 中获取这些密钥，或者可能将 IAM 角色与身份验证资源联系起来？

我想另一种选择是将我的来源置于其他经过身份验证的服务之后，但目前这不是一个可行的选择。

另一个有希望的线索是AWS::IAM::AccessKey resource,，但文档不建议它可以与角色一起使用。反正我要试试。

Answer 1

我设法让这个工作。我使用的是这个交易所的代码： https://forums.aws.amazon.com/message.jspa?messageID=319465

代码不使用 IAM 策略 - 它使用 AWS::S3::BucketPolicy。

成云代码sn-p：

"Resources" : {     

"CfnUser" : {
  "Type" : "AWS::IAM::User",
  "Properties" : {
    "Path": "/",
    "Policies": [{
      "PolicyName": "root",
      "PolicyDocument": { "Statement":[{
        "Effect"   : "Allow",
        "Action"   : [
          "cloudformation:DescribeStackResource",
          "s3:GetObject"
        ],
        "Resource" :"*"
      }]}
    }]
  }
},

"CfnKeys" : {
  "Type" : "AWS::IAM::AccessKey",
  "Properties" : {
    "UserName" : {"Ref": "CfnUser"}
  }
},

"BucketPolicy" : {
  "Type" : "AWS::S3::BucketPolicy",
  "Properties" : {
    "PolicyDocument": {
      "Version"      : "2008-10-17",
      "Id"           : "CfAccessPolicy",
      "Statement"    : [{
        "Sid"        : "ReadAccess",
        "Action"     : ["s3:GetObject"],
        "Effect"     : "Allow",
        "Resource"   : { "Fn::Join" : ["", ["arn:aws:s3:::<MY_BUCKET>/*"]]},
        "Principal"  : { "AWS": {"Fn::GetAtt" : ["CfnUser", "Arn"]} }
      }]
    },
    "Bucket" : "<MY_BUCKET>"
  }
},

"WebServer": {  
  "Type": "AWS::EC2::Instance",
  "DependsOn" : "BucketPolicy",
  "Metadata" : {

    "AWS::CloudFormation::Init" : {
      "config" : {

        "sources" : {
          "/etc/<MY_PATH>" : "https://s3.amazonaws.com/<MY_BUCKET>/<MY_FILE>"
        }

      }
    },

    "AWS::CloudFormation::Authentication" : {
      "S3AccessCreds" : {
        "type" : "S3",
        "accessKeyId" : { "Ref" : "CfnKeys" },
        "secretKey" : {"Fn::GetAtt": ["CfnKeys", "SecretAccessKey"]},
        "buckets" : [ "<MY_BUCKET>" ]
      }
    }
  },

  "Properties": {
    "ImageId" : "<MY_INSTANCE_ID>", 
    "InstanceType" : { "Ref" : "WebServerInstanceType" },
    "KeyName" : {"Ref": "KeyName"},
    "SecurityGroups" : [ "<MY_SECURITY_GROUP>" ],

    "UserData"       : { "Fn::Base64" : { "Fn::Join" : ["", [
      "#!/bin/bash\n",

      "# Helper function\n",
      "function error_exit\n",
      "{\n",
      "  cfn-signal -e 1 -r \"$1\" '", { "Ref" : "WaitHandle" }, "'\n",
      "  exit 1\n",
      "}\n",

      "# Install Webserver Packages etc \n",
      "cfn-init -v --region ", { "Ref" : "AWS::Region" },
      "    -s ", { "Ref" : "AWS::StackName" }, " -r WebServer ",
      "         --access-key ", { "Ref" : "CfnKeys" },
      "         --secret-key ", {"Fn::GetAtt": ["CfnKeys", "SecretAccessKey"]}, " || error_exit 'Failed to run cfn-init'\n",

      "# All is well so signal success\n",
      "cfn-signal -e 0 -r \"Setup complete\" '", { "Ref" : "WaitHandle" }, "'\n"

    ]]}}        
  }
}

显然将 MY_BUCKET、MY_FILE、MY_INSTANCE_ID、MY_SECURITY_GROUP 替换为您的值。

Answer 2

我不确定何时添加了支持，但您可以同时使用 IAM 角色来验证 AWS::CloudFormation::Init 中 files 和 sources 部分的 S3 下载。

只需使用roleName 代替accessKeyId 和secretKey（详见AWS::CloudFormation::Authentication），例如：

"Metadata": {
    "AWS::CloudFormation::Init": {
        "download": {
            "files": {
                "/tmp/test.txt": {
                    "source": "http://myBucket.s3.amazonaws.com/test.txt"
                }
             }
        }
    },
    "AWS::CloudFormation::Authentication": {
        "default" : {
            "type": "s3",
            "buckets": [ "myBucket" ],
            "roleName": { "Ref": "myRole" }
        }
    }
}

用aws-cfn-bootstrap-1.3-11测试