feat(keybroker-server): Use Rego to appraise EARs

Use Rego to implement the appraisal policy for attestation results.

A default policy is hardcoded (see keybroker-server/src/policy.rego).

The user may provide their own policy to override the default using the
`--policy` command line switch.

The user must provide a JSON file containing one or more RIM reference values.

Fix #13

Signed-off-by: Thomas Fossati <thomas.fossati@linaro.org>

rust-keybroker/keybroker-server/Cargo.toml

@@ -22,3 +22,6 @@ clap = { version = "=4.3.24", features = ["derive", "std"] }
 keybroker-common = { path = "../keybroker-common" }
 veraison-apiclient = { git = "https://github.com/veraison/rust-apiclient.git" }
 ear = { git = "https://github.com/veraison/rust-ear.git" }
+regorus = "0.2.5"
+serde_json = "1.0.128"
+anyhow = "1.0.89"

rust-keybroker/keybroker-server/src/error.rs

@@ -37,6 +37,14 @@ pub enum Error {
     /// that are transacted through the API between the client and the server, if the client provides faulty data.
     #[error(transparent)]
     Base64Decode(#[from] base64::DecodeError),
+
+    /// Represents errors from the use of the policy evaluation library.
+    #[error(transparent)]
+    Policy(#[from] anyhow::Error),
+
+    /// Represents errors from the use of the JSON serialisation and deserialisation library.
+    #[error(transparent)]
+    Json(#[from] serde_json::Error),
 }
 
 /// Errors happening within the verification process logic.

rust-keybroker/keybroker-server/src/main.rs

@@ -13,6 +13,7 @@ use keystore::KeyStore;
 mod challenge;
 mod error;
 mod keystore;
+pub mod policy;
 mod verifier;
 
 #[post("/key/{keyid}")]
@@ -90,6 +91,9 @@ async fn submit_evidence(
 
     let verifier_base = data.args.verifier.clone();
 
+    let optional_policy = data.args.policy.clone();
+    let rims = data.args.rims.clone();
+
     // We are in an async context, but the verifier client is synchronous, so spawn
     // it as a blocking task.
     let handle = task::spawn_blocking(move || {
@@ -102,6 +106,8 @@ async fn submit_evidence(
             content_type_str,
             &challenge.challenge_value,
             &evidence_bytes,
+            optional_policy,
+            &rims,
         )
     });
     let result = handle.await.unwrap();
@@ -167,6 +173,14 @@ struct Args {
     /// Set the server verbosity
     #[arg(short, long, default_value_t = false)]
     verbose: bool,
+
+    /// Rego file containing the appraisal policy for (EAR) attestation results
+    #[arg(long, default_value = None)]
+    policy: Option<String>,
+
+    /// File containing a JSON array with base64-encoded known-good RIM values
+    #[arg(long, default_value = "rims.json")]
+    rims: String,
 }
 
 struct ServerState {

rust-keybroker/keybroker-server/src/policy.rego

@@ -0,0 +1,23 @@
+package arm_cca
+
+default allow := false
+
+allow if {
+    input.eat_profile == "tag:github.com,2023:veraison/ear"
+
+    # platform part
+    prec := input.submods.CCA_SSD_PLATFORM
+    prec["ear.status"] == "affirming"
+
+    # realm part
+    rrec := input.submods.CCA_REALM
+    rrec["ear.status"] == "warning"
+
+    rtv := rrec["ear.trustworthiness-vector"]
+    rtv["instance-identity"] == 2
+
+    # check RIM value against known-good-values
+    rclaims := rrec["ear.veraison.annotated-evidence"]
+    rim := rclaims["cca-realm-initial-measurement"]
+    rim in data["reference-values"]
+}

rust-keybroker/keybroker-server/src/policy.rs

@@ -0,0 +1,75 @@
+// Copyright 2024 Contributors to the Veraison project.
+// SPDX-License-Identifier: Apache-2.0
+
+use regorus::{self, Value};
+
+use crate::error::Result;
+
+// Evaluate an EAR claims-set against the appraisal policy and known-good RIM values
+pub(crate) fn rego_eval(
+    custom_policy: Option<String>,
+    rims: &str,
+    ear_claims: &str,
+) -> Result<Value> {
+    const POLICY: &str = include_str!("policy.rego");
+
+    // Create engine.
+    let mut engine = regorus::Engine::new();
+
+    engine.set_rego_v1(true);
+    engine.set_strict_builtin_errors(false);
+
+    // Add the appraisal policy
+    match custom_policy {
+        None => {
+            engine.add_policy(String::from("policy.rego"), String::from(POLICY))?;
+        }
+        Some(file) => {
+            engine.add_policy_from_file(file)?;
+        }
+    }
+
+    // Load the configured known good RIM values
+    engine.add_data(Value::from_json_file(rims)?)?;
+
+    // Set the EAR claims-set to be appraised
+    engine.set_input(Value::from_json_str(ear_claims)?);
+
+    let results = engine.eval_rule("data.arm_cca.allow".to_string())?;
+
+    Ok(results)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rego_eval_ear_default_policy_ok() {
+        let ear_claims = include_str!("../../../testdata/ear-claims-ok.json");
+        let rims = stringify_testdata_path("rims-matching.json");
+
+        let results = rego_eval(None, &rims, ear_claims).expect("successful eval");
+
+        assert_eq!(results.to_string(), "true");
+    }
+
+    #[test]
+    fn rego_eval_default_policy_unmatched_rim() {
+        let ear_claims = include_str!("../../../testdata/ear-claims-ok.json");
+        let rims = stringify_testdata_path("rims-not-matching.json");
+
+        let results = rego_eval(None, &rims, ear_claims).expect("successful eval");
+
+        assert_eq!(results.to_string(), "false");
+    }
+
+    fn stringify_testdata_path(s: &str) -> String {
+        let mut test_data = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+
+        test_data.push("../../testdata");
+        test_data.push(s);
+
+        test_data.into_os_string().into_string().unwrap()
+    }
+}

rust-keybroker/keybroker-server/src/verifier.rs

@@ -2,14 +2,17 @@
 // SPDX-License-Identifier: Apache-2.0
 
 use crate::error::Result;
-use ear::{Algorithm, Ear, TrustTier};
+use crate::policy;
+use ear::{Algorithm, Ear};
 use veraison_apiclient::*;
 
 pub fn verify_with_veraison_instance(
     verifier_base_url: &str,
     media_type: &str,
     challenge: &[u8],
     evidence: &[u8],
+    policy: Option<String>,
+    rims: &str,
 ) -> Result<bool> {
     // Get the discovery URL from the base URL
     let discovery = Discovery::from_base_url(String::from(verifier_base_url))?;
@@ -60,14 +63,13 @@ pub fn verify_with_veraison_instance(
         verification_key_string.as_bytes(),
     )?;
 
-    // The simplest possible appraisal policy: accept if we have an AFFIRMING or WARNING result
-    // from every submodule.
-    // TODO: This policy is rather too "relaxed" - the simplest and strictest policy would be
-    //       to require AFFIRMING from every submodule. We have some integration issues with Veraison
-    //       today that prevent this.
-    let verified = ear.submods.iter().all(|(_module, appraisal)| {
-        appraisal.status == TrustTier::Affirming || appraisal.status == TrustTier::Warning
-    });
+    let ear_claims = serde_json::to_string(&ear)?;
 
-    Ok(verified)
+    // Appraise the received EAR using the embedded policy (see ./policy.rego)
+    // unless a custom one has been provided on the command line.  The deafault
+    // policy also wants to match the RIM value reported by the CCA token with
+    // the known-good RIM values supplied on the command line.
+    let results = policy::rego_eval(policy, rims, &ear_claims)?;
+
+    Ok(results.to_string() == "true")
 }

testdata/ear-claims-ok.json

@@ -0,0 +1,36 @@
+{
+  "eat_nonce": "bobW2XzHE7xt1D285JGmtAMRwCeov4WjnaY-nORMEyqKEZ0pb65qaZnpvz5EcbDOASRdiJQkwx6JeTs7HWsVBA==",
+  "eat_profile": "tag:github.com,2023:veraison/ear",
+  "submods": {
+    "CCA_REALM": {
+      "ear.status": "warning",
+      "ear.trustworthiness-vector": {
+        "configuration": 0,
+        "executables": 33,
+        "file-system": 0,
+        "hardware": 0,
+        "instance-identity": 2,
+        "runtime-opaque": 0,
+        "sourced-data": 0,
+        "storage-opaque": 0
+      },
+      "ear.veraison.annotated-evidence": {
+        "cca-realm-challenge": "bobW2XzHE7xt1D285JGmtAMRwCeov4WjnaY+nORMEyqKEZ0pb65qaZnpvz5EcbDOASRdiJQkwx6JeTs7HWsVBA==",
+        "cca-realm-extensible-measurements": [
+          "JNWwopbMBcvYBoxQZ8W9Rzt3Ddpq4IL+O6MKvj+aarE=",
+          "eI/AkL/GuO2QMVK6hBTnPa9bjHux55rVAqsGmbZZ7RY=",
+          "2sRqWEFdw6ANenQYUgCOnK5k9S0DufdtdvSzZE/vxBY=",
+          "MsavxiflVYXAMVU1nzMaDiJfaEDblH3Zbvq4G+JnGTk="
+        ],
+        "cca-realm-hash-algo-id": "sha-256",
+        "cca-realm-initial-measurement": "MRMUq3NiA1DPdYg0rlxl2ejC3H/r5ufZZUu+hk4wDUk=",
+        "cca-realm-personalization-value": "VGhlIHF1aWNrIGJyb3duIGZveCBqdW1wcyBvdmVyIDEzIGxhenkgZG9ncy5UaGUgcXVpY2sgYnJvd24gZm94IA==",
+        "cca-realm-public-key": "BHb5iAkb5YXtQYAa7Pq4WFSMYwV+FrDmdhILvQ0vnCngVsXUGgEw65whUXiZ3CMUayjhsGK9PqSzFf0hnxy7Uoy250ykm+Fnc3NPYaHKYQMbK789kY8vlP/EIo5QkZVErg==",
+        "cca-realm-public-key-hash-algo-id": "sha-256"
+      }
+    },
+    "CCA_SSD_PLATFORM": {
+      "ear.status": "affirming"
+    }
+  }
+}

testdata/rims-matching.json

@@ -0,0 +1,6 @@
+{
+  "reference-values": [
+    "MRMUq3NiA1DPdYg0rlxl2ejC3H/r5ufZZUu+hk4wDUk=",
+    "q3N/r5ufZZUu+iAg0rlxl2ejC3HMRMUhk4wDUk1DPdY="
+  ]
+}

testdata/rims-not-matching.json

@@ -0,0 +1,6 @@
+{
+  "reference-values": [
+    "XRMUq3NiA1DPdYg0rlxl2ejC3H/r5ufZZUu+hk4wDUk=",
+    "X3N/r5ufZZUu+iAg0rlxl2ejC3HMRMUhk4wDUk1DPdY="
+  ]
+}