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© 2015, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
Frank Chen, CourseraBrennan Saeta, Coursera
October 2015
CMP406
Amazon ECS at CourseraPowering a general-purpose near-line execution
microservice, while defending against untrusted code
What to Expect from the Session
• Techniques for a unified near-line, batch, and scheduled micro-service powered by Amazon ECS
• Security vulnerabilities and countermeasures when running untrusted code in Docker with Amazon ECS
• Reasons to modify the Amazon ECS agent
Session Outline
• Introduction to Coursera• Near-line, batch and scheduled job execution framework
• Motivations and background• Amazon ECS benefits and limitations• Iguazú and its architecture
• Evaluating programming assignments• System requirements• Security threat model• Attacks and defenses
Education at Scale
15 million learners worldwide
2.5 millioncourse completions
1,300+courses
125+partners
A unified execution framework
Batch Processing Enables…
ReportingInstructor Reports• Grade exports• Learner demographics• Course progress
statistics
Internal Reports• Business metrics• Payments
reconciliation
Scheduled Processing Enables…
Marketing• Recommendation emails
• Targeted marketing / reactivation emails
Nearline Processing Enables…
Pedagogical Innovations• Peer-review matching & analysis
• Auto-graded programming assignments
The early days…
January 2012
Bad Old Days of Batch Processing @ Coursera
Cascade• PHP-based job runner• Originally ran in screen sessions • Polled APIs for new jobs• Forced restarts on regular basis
due to unidentified memory leaks• Fragile and unreliable
The early days…
Bad Old Days of Batch Processing @ Coursera
Saturn• Scala scheduled batch job runner
• Powered by Quartz Scheduler library• Better than Cascade, but…• All jobs ran on same JVM, causing
interference
The not-so early days?
Looking for something better…
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What We Wanted
Reliable Easy Development Easy Deployment
High Efficiency Low Ops Load Cost Effective
What Else Did We Look At?
Home-grown Tech
• Tried, but proved to be unreliable
• Difficult to handle coordination and synchronization
• Powerful, but hard to productionize
• Needs developers with experience
• Designed for GCE first
• Not a managed service, higher Ops load
Amazon ECS to the Rescue
Amazon re:Invent 2014 – Dr. Werner Vogels introducing Amazon ECS
Screenshot from https://www.youtube.com/watch?v=LE5uBqNp2Ds by Amazon Web Services
Amazon ECS to the Rescue
Little maintenance
Integrated with rest of AWS
Easy to develop for
Amazon ECS to the Rescue
Little maintenance
Integrated with rest of AWS
Easy to develop for
Amazon ECS to the Rescue
Little maintenance
Integrated with rest of AWS
Easy to develop for
However…
Amazon ECS is a great building block, but we still need to build tools around it
for our purposes.
What We Built: Iguazú
Marissa Strniste (https://www.flickr.com/photos/mstrniste/5999464924) CC-BY-2.0
• Batch Job Scheduler for Amazon ECS• Immediately• Deferred (run once at X time)• Scheduled recurring (cron-like)
• Programmatically accessible internally viaour standard APIs and clients
• Named for Iguazú falls• World’s largest waterfall by volume• We hope Iguazú handles a similar volume of jobs
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Iguazú Frontend
Iguazú Scheduler
Iguazú Backend
Iguazú: Architecture
CassandraServices Services
Iguazú Admin
ECS Workers
SQS
ECS API
Devs
Users
Developing Iguazú Jobs
class Job extends AbstractJob with StrictLogging { override val reservedCpu = 1024 // 1 CPU core override val reservedMemory = 1024 // 1 GB RAM
def run(parameters: JsValue) = { logger.info("I am running my job! ") expensiveComputationHere() }}
Running Jobs from Other Services
// invoking a job with one function call// from another service via Naptime RPC/REST framework
val invocationId = IguazuJobInvocationClient .create(IguazuJobInvocationRequest( jobName = "exportQuizGrades", parameters = quizParams))
Iguazú: Developer / Ops User Interface
Deploying Jobs
Easy Deployment1. Developers Merge into master. Done!
Jenkins Build Steps:2. Builds zip package from master3. Prepares Docker image with zip file4. Pushes image into Docker registry5. Registers updated jobs with
Amazon ECS API
Logs
• Logs are in /var/lib/docker/containers/*
• Upload into log analysis service (Sumologic)
• Wrapper prints out job name and job IDat the start for easy searching
• Good enough for now
Metrics
• Using third-party metrics collector (Datadog)
• Metrics for both jobs and container instances
• So long as the worker machines can talk to Internet, things will work out pretty well
Since April 2015…
65 jobs in production
>1000 runs per day
44 different scheduled jobs
EvaluatingProgramming Assignments
Programming Assignments at Coursera
The Security Challenge
Compiling and running untrusted, arbitrary code in Amazon EC2
Would you like to compile and run C code from randompeople on the Internet on your servers?
1st Generation SystemClass graders in separate AWS acct
Custom grader systemson cloud providers
Course grader under the instructor’s desk
Learners Coursera Servers Queue Service
1st Generation System: Weaknesses
No Auto Scaling No standard security Graders crashed
1st Generation System: Weaknesses
No Auto Scaling No standard security Graders crashed
1st Generation System: Weaknesses
No Auto Scaling No standard security Graders crashed
Design Goals
Cost Savings No Maintenance Near Real-time Secure Infrastructure
Design Goals
Cost Savings No Maintenance Near Real-time Secure Infrastructure
Design Goals
Cost Savings No Maintenance Near Real-time Secure Infrastructure
Design Goals
Cost Savings No Maintenance Near Real-time Secure Infrastructure
Threat Model
Prevent submitted code from:• impacting the evaluation of other submissions.• disrupting the grading environment (e.g., DoS)• affecting the rest of the Coursera learning platformAdditional goals:• Minimize exfiltration of information
• Test cases, solutions, etc…• Minimize risk of submissions changing own scores• Avoid turning into bitcoin miners or part of botnet
Threat Model - Assumptions
• Run arbitrary binaries
• Instructor grading scripts may have vulnerabilities• ∴ Grading code is untrusted
• Unknown vulnerabilities in Docker and Linux name-spacing and/or container implementation
Attack / Vulnerability Classes
Divided into 2 main categories:• Assuming basic containers are secure, prevent any
negative impacts to running arbitrary code.
• Assuming basic container technology is vulnerable, mitigate negative impacts as much as possible.
What We Built: GrID
Patrick Hoesly (https://www.flickr.com/photos/zooboing/5665221326/) CC-BY-2.0
• Service + architecture for gradingprogramming assignments
• Builds on Amazon ECS and Iguazú
• Named for Tron’s “digital frontier”• Backronym: Grading Inside Docker
High-level GrID Architecture
Learners
GrID
Iguazú
S3 Bucket
ECS APIs
Grading MachinesVPC Firewalls
Coursera Production Account Coursera GrID Grading Account
High-level GrID Architecture
Learners
GrID
Iguazú
S3 Bucket
ECS APIs
Grading MachinesVPC Firewalls
Coursera Production Account Coursera GrID Grading Account
High-level GrID Architecture
Learners
GrID
Iguazú
S3 Bucket
ECS API
Grading MachinesVPC Firewalls
Production Acct GrID Grading Account
High-level GrID Architecture
Learners
GrID
Iguazú
S3 Bucket
ECS API
Grading Machines
VPC Firewalls
Production Acct GrID Grading Account
Attacks: Resource Exhaustion
Defenses:• Docker / CGroups:
• CPU quotas• Memory limits• Swap limits
• Hard timeouts for container execution• btrfs limits
• file system storage quotas• IOPS throttling
Attacks: Kernel Resource Exhaustion
Defenses:• Open file limits per container
(nofile)
• nproc Process limits
• Limit kernel memory per cgroup
• Limit execution time
Attacks: Network attacks
Attacks:• Bitcoin mining• DoS attacks on third-party systems• Access Amazon S3 and other AWS
APIs
Defense:• Deny network access
Modifying the ECS Agent: Network Modes
• NetworkDisabled too restrictive• Some graders require local loopback• Feature also deprecated
• --net=none + deny net_admin + audit network• Isolation via Docker creating an
independent network stack for each container
• github.com/coursera/amazon-ecs-agent
Attacks: Namespace / Container Vulnerabilities
• App Armor & Mandatory Access Control• Required modifying the Amazon ECS Agent• Allows auditing or denying access to a
variety of subsystems
• Drop capabilities• No need for NET_BIND_SERVICE,
CAP_FOWNER
• No root within container
Attacks: Root escalations within the container
• We modify instructor grader images before allowing them to be run
• Clears setuid• Inserts C wrapper to drop privileges from
root and redirect stdin/stdout/stderr• Required Amazon ECS Agent
modification• Grant root privileges• Map Docker socket into Docker
containers to run Docker in Docker!
Attacks: If all else fails…
• Utilizes VPC security measures to further restrict network access
• No public internet access• Security group to restrict
inbound/outbound access• Network flow logs for auditing
• Separate AWS account• Run in an Auto Scaling group
• Regularly terminate all grading EC2 instances
Other Security Measures
• Utilize AWS CloudTrail for audit logs
• Third-party security monitoring (Threat Stack)
• No one should log in, so any TTY is an alert
• Penetration testing by third-party red team (Synack)
Technique: Co-process• Environment has no network, but has to
get submissions in and results out
• Python co-process watches Amazon ECS / Docker
• Python co-process then:• Mounts a shared folder containing submission• Reads back the grade from the shared folder
after container exits• Monitors and cleans up
Future Improvements
• Priority queues for different grading priorities
• Re-grades vs on-demand grades• Better instructor tooling
• Automated “unit-testing” for new graders• Better simulation of production
environment on instructor machines• Support scheduling GPUs
Lessons Learned
• Run the latest kernels• Latest security patches• btrfs wedging on older kernels
• Default Ubuntu 14.04 kernel not new enough!
• Carefully monitor disk usage• Docker-in-docker can’t clean up after
itself (yet).• Reliable deploy tooling pays for itself
Related Sessions
Also from Coursera:• BDT404 - Building and Managing Large-Scale ETL Data
Flows with AWS Data Pipeline and Dataduct - Friday
Containers and Amazon ECS:• CMP302 - Amazon EC2 Container Service: Distributed
Applications at Scale – Next timeslot in Venetian H
Thank you!
Questions?
Also, we are hiring!www.coursera.org/jobs
tech.coursera.org
Brennan Saetagithub/saeta
Frank Chengithub/frankchn
Remember to complete your evaluations!