Policy as Code: Intro to Sentinel

In the last few years, we've seen an explosion of DSLs and tools that focus on enabling Infrastructure as Code. The Infrastructure as Code approach allows the best practices of software development like code reuse, versioning, and peer review to be applied to new domains such as infrastructure management. In helping organizations adopt these tools, we found a common shortcoming was the ability to impose specific compliance or security policies on infrastructure changes. In trying to apply the "As Code" approach to this problem, we created a new language called Sentinel, focusing specifically on policy applications. Sentinel is meant to be read and written by non-programmers, specific compliance and security officers who are unfamiliar with traditional programming languages and constructs. We discuss the design principles of the Sentinel language, it's basic syntax, runtime implementation, and some of the use cases we've deployed it for.

Transcript

1. 
   Policy as Code:
   Intro to Sentinel
   @armon
   

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2. 
   Armon Dadgar
   @armon
   

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3. @armon
   

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4. “POLICY AS CODE”
   

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5. “INFRASTRUCTURE AS CODE”
   

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6. • Before: Artisanal and Handcrafted
   • Point-and-Click UX
   • SSH + Manual Setup
   • Rsync between machines
   • etc
   INFRASTRUCTURE BEFORE
   

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7. • After: Automated and Standardized
   • Single source of truth
   • Automated provisioning
   • Enables Scale
   INFRASTRUCTURE AS CODE
   

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9. • Learn from Software Development
   • Versioning (Rollbacks)
   • Peer Review
   • Abstraction / Encapsulation
   • Code Reuse
   • Automation and Leverage
   BENEFITS
   

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10. PROVISIONING A DATACENTER
    

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11. Developers
    IT Operations
    Networking
    Security
    Procurement
    Sysadmins
    Finance
    Legal
    

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12. • Replace ticketing workflows with APIs
    • Public Cloud consolidated many concerns
    • Private Cloud brings similar APIs
    • Enables Infrastructure as Code
    • Huge increase in leverage
    UBIQUITOUS APIS
    

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13. PARADOX OF AUTOMATION
    

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14. SANITY CHECKING
    IT Ops Procurement
    “Please provision 5000 VMs”
    

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15. SANITY CHECKING
    IT Ops Procurement
    “Are you sure 5000?”
    

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16. SANITY CHECKING
    IT Ops Cloud
    “Please provision 5000 VMs”
    

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17. SANITY CHECKING
    IT Ops Cloud
    “Done!”
    

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18. • Paradox of Automation
    • Accidental Error
    • Compliance Bypass
    • Malicious Intent
    SCALING AUTOMATION
    

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19. • Governs Infrastructure as Code
    • Defines a sandbox to automate in
    • Codify business regulation and “sanity checking”
    POLICY AS CODE
    

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20. • Separate teams for Procurement / IT Operations / etc
    • Bulkhead against failures
    • Separate authors
    • Policy as Code (Compliance / Security)
    • Infrastructure as Code (Operators / Developers)
    SEPARATION OF RESPONSIBILITY
    

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21. POLICY AS CODE
    

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22. • Changes only during business hours
    • Min / Max number of service instances
    • Instance types allowed
    • Public Cloud regions
    • Ensure resources are tagged (billing, tenancy)
    • Certified containers / AMIs only
    EXAMPLE POLICIES
    

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23. • Security / Compliance teams define and enforce
    • Not familiar with “As Code” approach
    • Need to test policies
    ORGANIZATIONAL CHALLENGES
    

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24. SENTINEL
    

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25. • “Policy as Code Framework”
    • Sentinel Language Specification*
    • Golang Embedded Runtime
    • Simulator tool
    • Import SDK
    WHAT IS SENTINEL
    * https://docs.hashicorp.com/sentinel/language/spec
    

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26. • Non-programmer friendly
    • Easy to Embed
    • Simple
    • Debuggable
    • Go Friendly
    DESIGN GOALS
    

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27. • Language must be read and written by those with little to no
    programming experience
    • Traditional constructs should be available, but not needed for
    most policies
    NON-PROGRAMMER FRIENDLY
    

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28. • Impacts Runtime more than Language
    • No support for unsafe operations (arbitrary memory access)
    • Must be easy to sandbox
    • Active Enforcement
    • Prevent Violation A Priori
    • Passive Enforcement
    • Detect Violation Post Hoc
    EASY TO EMBED
    

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29. • Simple types (string, int, bool) at the expense of performance or
    exactness
    • Policies are typically small and don’t need to much flexibility
    • Ideally only one way to do something
    • “Zen of Python”
    SIMPLE
    

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30. • Easy to debug and test for those not familiar
    • Often the sharpest corner when learning
    DEBUGABBLE
    

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31. • Majority of HashiCorp tooling is Go
    • Simple to integration
    • Minimal changes to tooling / process
    • Performance implications
    GO FRIENDLY
    

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32. NOT INVENTED HERE?
    

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33. • JavaScript, Python, Lua, Ruby
    • Non-programmer friendliness
    • Embeddable
    • Go friendliness
    EXISTING LANGUAGES
    

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34. • Sentinel has dynamic types
    • Wanted static typing
    • Much friendlier to non-programmers
    • External integrations more complex
    DESIGN DECISIONS: DYNAMIC TYPING
    

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35. import "time"
    deployment_window = rule {
    time.hour > 8 and time.hour < 17
    }
    main = rule { deployment_window }
    First Class Rules
    

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36. • Equivalent to functions
    • Lazy
    • Memoized
    • Performance for redundant rules
    • More Friendly
    DESIGN DECISIONS: FIRST CLASS RULES
    

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37. import "external"
    external_ready = rule { external.expensive_call() }
    internal_override = rule { … }
    main = rule {
    external_ready or
    (!external_ready and interal_override)
    }
    

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38. • Dedicated Existential Operators
    • any: expression is true for any term
    • all: expression is true for all terms
    • “All artifacts must come from a trusted source”
    • “Any of the tags must be the tenant ID”
    DESIGN DECISIONS: EXISTENTIAL OPERATORS
    

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39. • Plain English Operators
    • and, or, contains, in, is, matches, not
    • Traditional Operators
    • Easier for non-programmers
    DESIGN DECISIONS: SIMPLE OPERATORS
    

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40. • Avoid Exceptions / Panic
    • Must propagate errors
    • “Infectious” Undefined
    • Can be escaped by boolean logic with else
    DESIGN DECISIONS: UNDEFINED VALUE
    

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41. import “time”
    # valid_bar will be undefined
    obj = null
    valid_bar = rule { obj.foo.bar == 42 }
    valid_time = rule { time.hour > 8 }
    main = rule { (valid_bar or valid_time) else false }
    

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42. BASIC SYNTAX
    

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43. main = rule {
    all obj.items as item {
    item matches "my-item-[a-z0-9]+"
    }
    }
    

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44. sum = func(list) {
    count = 0
    for list as elem {
    count += elem
    }
    return count
    }
    main = rule {
    sum(obj.items) is 42
    }
    

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45. allowed_size = ["m3.small", "m4.small", "c1.large"]
    allowed_regions = {“stage”:“east","prod":“west"}
    valid_provider = rule { all providers as p {
    p.region is allowed_regions[env.region]
    }

    }
    valid_resource = rule { all resources as r {
    r.type is "instance" and r.size in allowed_size
    }
    }
    main = rule { valid_provider and valid_resource}
    

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46. RUNTIME IMPLEMENTATION
    

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47. • Lexer / Parser
    • Semantic Checker
    • Evaluator / Interpreter
    • Standard Library
    KEEP IT SIMPLE
    

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48. • Learn From Go
    • Maintain the position offsets (line, column)
    • Support a “fmt” command
    • Canonical Format
    LEXER / PARSER
    

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49. • Early warning to users
    • Main rules exist
    • Imports valid
    SEMANTIC CHECKING
    

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50. • Rule Memoization
    • Policy Caching
    • Import Recycling
    • Sandboxing
    • Execution time limited
    • Memory limited
    • Stack depth limited
    EVALUATOR
    

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51. • Avoid User Functions
    • Built in Standard Library
    • Time, Math, CIDR parsing, Strings, etc
    STANDARD LIBRARY
    

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52. ENFORCEMENT LEVELS
    "I'm sorry, Dave.
    I'm afraid I can't do that"
    

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53. • Different Levels
    • Runtime aware
    • Advisory
    • Soft Mandatory
    • Hard Mandatory
    ENFORCEMENT LEVELS
    

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54. • Policy is allowed to fail
    • Warning is showed to user
    • “Warn if user provisions deprecated instance type”
    ENFORCEMENT LEVEL: ADVISORY
    

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55. • Action is blocked when policy fails
    • Action is allowed when an override is specified
    • Override is tied to additional permissions
    • User has explicitly acknowledged out of policy action
    • “Do not allow changes outside of business hours (unless there is
    an outage)”
    ENFORCEMENT LEVEL: SOFT MANDATORY
    

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56. • Action is blocked when policy fails
    • Override is not possible
    • “Encryption keys are at least 128 bits and non-exportable”
    ENFORCEMENT LEVEL: HARD MANDATORY
    

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57. • Multiple policies may be evaluated per action
    • Smaller policies vs Monolithic policy
    • Mandatory failures can short circuit
    RUNTIME AWARENESS
    

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58. • Sentinel Runtime is embedded
    • In Data Path
    • Active Enforcement
    • How to develop and test policies?
    EMBEDDED FRAMEWORK
    

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59. SENTINEL SIMULATOR
    

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60. Terminal
    Usage: sentinel [--version] [--help] []
    Available commands are:
    apply Execute a policy and output the result
    doc Show documentation for an import from a doc file
    fmt Format Sentinel policy to a canonical format
    test Test policies
    version Prints the Sentinel version
    

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61. Terminal
    $ sentinel apply -trace example_1.sentinel
    Pass
    Execution trace. The information below will show the values of all
    the rules evaluated and their intermediate boolean expressions. Note that
    some boolean expressions may be missing if short-circuit logic was taken.
    TRUE - example_1.sentinel:3:1 - Rule "main"
    TRUE - example_1.sentinel:3:15 - time.hour > 8
    TRUE - example_1.sentinel:3:33 - time.hour < 17
    

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62. Terminal
    $ sentinel apply -trace example_1.sentinel
    Fail
    Execution trace. The information below will show the values of all
    the rules evaluated and their intermediate boolean expressions. Note that
    some boolean expressions may be missing if short-circuit logic was taken.
    FALSE - example_1.sentinel:3:1 - Rule "main"
    FALSE - example_1.sentinel:3:15 - time.hour > 16
    

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63. • Apply allows for playing with policies during development
    • Test suites
    • Regression testing
    TESTING POLICIES
    

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64. Terminal
    $ tree
    .
    ├── example_1.sentinel
    ├── test
    │ ├── example_1
    │ │ ├── fail.json
    │ │ └── pass.json
    

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65. {
    "config":
    {
    ...
    },
    "mock":
    {
    "time": {
    "hour": 9
    }
    },
    "test": {
    "main": false
    }
    }
    pass.json
    

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66. Terminal
    $ sentinel test -v example_1.sentinel
    PASS - example_1.sentinel
    PASS - test/example_1/fail.json
    trace:
    FALSE - example_1.sentinel:3:1 - Rule "main"
    FALSE - example_1.sentinel:3:15 - time.hour >= 16
    PASS - test/example_1/pass.json
    trace:
    TRUE - example_1.sentinel:3:1 - Rule "main"
    TRUE - example_1.sentinel:3:15 - time.hour >= 16
    TRUE - example_1.sentinel:3:35 - time.hour < 17
    

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67. CONTINUOUS INTEGRATION
    

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68. IMPORT SDK
    

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69. • Infrastructure composed of many pieces
    • Many policies require external information
    • Easy to incorporate external state
    IMPORTS
    

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70. • SDK makes it easy to expose new import
    • Host systems allow imports to be specified
    • Configured in advance for security
    • Plugins use IPC
    • Runtime multiplexes and garbage collects
    IMPORT SDK
    

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71. • Integrate common systems
    • Integrate custom internal systems
    • Extend with arbitrary logic
    IMPORT SDK
    

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72. CROSS SYSTEM PROTECTION
    Submit
    Service
    Service
    Configured?
    

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73. CROSS SYSTEM PROTECTION
    Delete
    Service
    Config
    Service
    Running?
    

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74. USE CASES
    

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75. • Infrastructure as Code guardrails
    • Simplify Compliance
    • “Fool me once, shame on you.”
    POLICY AS CODE USE CASES
    

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76. • Define Sandbox
    • Limits the potential damage of automation
    • Impose “sanity check” without manual process
    INFRASTRUCTURE AS CODE GUARDRAILS
    

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77. • Compliance often expressed as Word doc
    • Codify compliance requirements
    • Active enforcement simpler than passive detection
    • “An once of prevention is worth a pound of cure.”
    SIMPLIFY COMPLIANCE
    

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78. • Policies will never be exhaustive
    • Memory is finite!
    • Codify policies to defend against repeat mistakes
    • Scaling policies easier to expanding memory
    AVOID REPEAT MISTAKES
    

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79. • Policy as Code builds upon “As Code”
    • Shared benefits as Infrastructure as Code
    • Sentinel a framework for Policy as Code
    • Language, Import SDK OSS
    • Next Step in Infrastructure Automation
    CONCLUSION
    

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80. THANKS!
    

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