cedar-policy-symcc

Symbolic Cedar Compiler (SymCC)

With this library, you can

• Compile Cedar policies to logical constraints in SMT-LIB.
• Formally verify a number of useful properties about your Cedar policies with concrete counterexamples.

Our symbolic compiler and verifiers have been formally modeled and verified in Lean to guarantee trustworthy verification results.

SymCC currently supports formally verifying the following properties:

• Policy never errors (CedarSymCompiler::check_never_errors).
• Policy set always allows (CedarSymCompiler::check_always_allows).
• Policy set always denies (CedarSymCompiler::check_always_denies).
• Policy set subsumption (CedarSymCompiler::check_implies).
• Policy set equivalence (CedarSymCompiler::check_equivalent).
• Policy set disjointness (CedarSymCompiler::check_disjoint).

For each of them, we also have the CedarSymCompiler::check_*_with_counterexample counterparts that produce a counterexample (a synthesized request and entity store) if the property is not true.

Setup

To get started, first download or compile the cvc5-1.3.1 SMT solver. The following example assumes that you have set the following environment variable:

CVC5=<path to cvc5 1.3.1 executable>

Example

To verify that a policy set does not always allow every well-formed request:

use tokio;
use std::str::FromStr;
use cedar_policy::{Schema, PolicySet, Authorizer, Decision};
use cedar_policy_symcc::{solver::LocalSolver, CedarSymCompiler, SymEnv, WellTypedPolicies};

#[tokio::main]
async fn main() {
    // Parse Cedar schema
    let schema = Schema::from_cedarschema_str(r#"
        entity User;
        entity Document { owner: User };
        action view appliesTo {
            principal: [User],
            resource: [Document]
        };
    "#).unwrap().0;

    // Parse Cedar policy set
    let policy_set = PolicySet::from_str(r#"
        permit(principal, action == Action::"view", resource)
        when { resource.owner == principal };
    "#).unwrap();

    // Initialize the symbolic compiler
    let cvc5 = LocalSolver::cvc5().unwrap();
    let mut compiler = CedarSymCompiler::new(cvc5).unwrap();

    // Iterate through all request environments and check the property
    for req_env in schema.request_envs() {
        // Encode the request environment symbolically
        let sym_env = SymEnv::new(&schema, &req_env).unwrap();

        // Validate/type check the policy set
        let typed_policies = WellTypedPolicies::from_policies(&policy_set, &req_env, &schema).unwrap();

        // Verify that `policy_set` does not always allow any request
        let always_denies = compiler.check_always_allows(&typed_policies, &sym_env).await.unwrap();
        assert!(!always_denies);

        // Similar to above, but returns a counterexample (a synthesized request
        // and entity store) which is denied by the policy set.
        let cex = compiler.check_always_allows_with_counterexample(&typed_policies, &sym_env).await.unwrap().unwrap();
        let resp = Authorizer::new().is_authorized(&cex.request, &policy_set, &cex.entities);
        assert!(resp.decision() == Decision::Deny);
    }
}

To learn more about what you can do with SymCC, see the documentation of the CedarSymCompiler type.

Development

To build and test this crate, run the following commands from the root of the repository:

cargo build -p cedar-policy-symcc
CVC5=<absolute path to cvc5 1.3.1 executable> cargo test -p cedar-policy-symcc

Structure of this crate:

• symcc is the core library. It maps directly to the Lean model.
• lib.rs is the frontend for symcc, and does not directly correspond to the Lean, but it provides an interface in terms of cedar-policy types rather than cedar-policy-core types.