policy: allow spiffe ids in MeshTLSAuthentication (#11882)

This change enables the use of SPIFFE identities in `MeshTLSAuthentication`.
To make that happen validation of the identities field on the CRD has been moved
to the policy controller admission webhook. Apart from a more clear expression
of the constraints that a SPIFFE ID needs to meet, this approach allows for
richer error messages. Note that the DNS validation is still based on a regex.

Signed-off-by: Zahari Dichev <zaharidichev@gmail.com>

Cargo.lock

@@ -1181,6 +1181,7 @@ dependencies = [
  "linkerd-policy-controller-k8s-status",
  "openssl",
  "parking_lot",
+ "regex",
  "serde",
  "serde_json",
  "thiserror",

charts/linkerd-crds/templates/policy/meshtls-authentication.yaml

@@ -49,7 +49,6 @@ spec:
                   minItems: 1
                   items:
                     type: string
-                    pattern: '^(\*|[a-z0-9]([-a-z0-9]*[a-z0-9])?)(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*$'
                 identityRefs:
                   type: array
                   minItems: 1

policy-controller/Cargo.toml

@@ -33,6 +33,7 @@ serde_json = "1"
 thiserror = "1"
 tokio-stream = { version = "0.1", features = ["sync"] }
 tracing = "0.1"
+regex = "1"
 
 [dependencies.clap]
 version = "4"

policy-controller/src/admission.rs

@@ -1,3 +1,4 @@
+use super::validation;
 use crate::k8s::{
     labels,
     policy::{
@@ -296,7 +297,12 @@ fn validate_identity_ref(id: &NamespacedTargetRef) -> Result<()> {
 #[async_trait::async_trait]
 impl Validate<MeshTLSAuthenticationSpec> for Admission {
     async fn validate(self, _ns: &str, _name: &str, spec: MeshTLSAuthenticationSpec) -> Result<()> {
-        // The CRD validates identity strings, but does not validate identity references.
+        for id in spec.identities.iter().flatten() {
+            if let Err(err) = validation::validate_identity(id) {
+                bail!("id {} is invalid: {}", id, err);
+            }
+        }
+
         for id in spec.identity_refs.iter().flatten() {
             validate_identity_ref(id)?;
         }

policy-controller/src/lib.rs

@@ -2,6 +2,7 @@
 #![forbid(unsafe_code)]
 mod admission;
 pub mod index_list;
+mod validation;
 pub use self::admission::Admission;
 use anyhow::Result;
 use linkerd_policy_controller_core::inbound::{

policy-controller/src/validation.rs

@@ -0,0 +1,245 @@
+use anyhow::Result;
+use thiserror::Error;
+
+use regex::Regex;
+
+const SCHEME_PREFIX: &str = "spiffe://";
+const VALID_TRUST_DOMAIN_CHARS: &str = "abcdefghijklmnopqrstuvwxyz0123456789-._";
+const VALID_PATH_SEGMENT_CHARS: &str =
+    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._";
+
+const DNS_LIKE_IDENTITY_REGEX: &str =
+    r"^(\*|[a-z0-9]([-a-z0-9]*[a-z0-9])?)(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*$";
+
+#[derive(Debug, Error, PartialEq, Clone)]
+pub enum IdError {
+    /// The trust domain name of SPIFFE ID cannot be empty.
+    #[error("SPIFFE trust domain is missing")]
+    MissingTrustDomain,
+
+    /// A trust domain name can only contain chars in a limited char set.
+    #[error(
+        "SPIFFE trust domain characters are limited to lowercase letters, numbers, dots, dashes, and \
+         underscores"
+    )]
+    BadTrustDomainChar,
+
+    /// A path segment can only contain chars in a limited char set.
+    #[error(
+        "SPIFFE path segment characters are limited to letters, numbers, dots, dashes, and underscores"
+    )]
+    BadPathSegmentChar,
+
+    /// Path cannot contain empty segments, e.g '//'
+    #[error("SPIFFE path cannot contain empty segments")]
+    EmptySegment,
+
+    /// Path cannot contain dot segments, e.g '/.', '/..'
+    #[error("SPIFFE path cannot contain dot segments")]
+    DotSegment,
+
+    /// Path cannot have a trailing slash.
+    #[error("SPIFFE path cannot have a trailing slash")]
+    TrailingSlash,
+
+    #[error(
+        "identity must be a valid SPIFFE id or a DNS SAN, matching the regex: {}",
+        DNS_LIKE_IDENTITY_REGEX
+    )]
+    Invalid,
+}
+
+// Validates that an ID is either in DNS or SPIFFE form. SPIFFE
+// validation is based on https://github.com/spiffe/spiffe/blob/27b59b81ba8c56885ac5d4be73b35b9b3305fd7a/standards/SPIFFE-ID.md.
+// Implementation is based on: https://github.com/maxlambrecht/rust-spiffe/blob/3d3614f70d0d7a4b9190ab9650e224f2ac362368/spiffe/src/spiffe_id/mod.rs
+pub(crate) fn validate_identity(id: &str) -> Result<(), IdError> {
+    if let Some(rest) = id.strip_prefix(SCHEME_PREFIX) {
+        let i = rest.find('/').unwrap_or(rest.len());
+
+        if i == 0 {
+            return Err(IdError::MissingTrustDomain);
+        }
+
+        let td = &rest[..i];
+        if td.chars().any(|c| !VALID_TRUST_DOMAIN_CHARS.contains(c)) {
+            return Err(IdError::BadTrustDomainChar);
+        }
+
+        let path = &rest[i..];
+
+        if !path.is_empty() {
+            validate_path(path)?;
+        }
+
+        Ok(())
+    } else {
+        let regex = Regex::new(DNS_LIKE_IDENTITY_REGEX).expect("should_compile");
+        if !regex.is_match(id) {
+            return Err(IdError::Invalid);
+        }
+        Ok(())
+    }
+}
+
+/// Validates that a path string is a conformant path for a SPIFFE ID.
+/// See https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE-ID.md#22-path
+fn validate_path(path: &str) -> Result<(), IdError> {
+    let chars = path.char_indices().peekable();
+    let mut segment_start = 0;
+
+    for (idx, c) in chars {
+        if c == '/' {
+            match &path[segment_start..idx] {
+                "/" => return Err(IdError::EmptySegment),
+                "/." | "/.." => return Err(IdError::DotSegment),
+                _ => {}
+            }
+            segment_start = idx;
+            continue;
+        }
+
+        if !VALID_PATH_SEGMENT_CHARS.contains(c) {
+            return Err(IdError::BadPathSegmentChar);
+        }
+    }
+
+    match &path[segment_start..] {
+        "/" => return Err(IdError::TrailingSlash),
+        "/." | "/.." => return Err(IdError::DotSegment),
+        _ => {}
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn valid_dns() {
+        assert!(validate_identity("system.local").is_ok())
+    }
+
+    #[test]
+    fn valid_dns_all() {
+        assert!(validate_identity("*").is_ok())
+    }
+
+    #[test]
+    fn valid_dns_prefix() {
+        assert!(validate_identity("*.system.local").is_ok())
+    }
+
+    #[test]
+    fn invalid_dns_suffix() {
+        let err = validate_identity("system.local.*").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_dns_trailing_dot() {
+        let err = validate_identity("system.local.").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_dns_leading_dot() {
+        let err = validate_identity(".system.local").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_dns_double_dots() {
+        let err = validate_identity("system..local").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn valid_spiffe_no_path() {
+        assert!(validate_identity("spiffe://trustdomain").is_ok())
+    }
+
+    #[test]
+    fn valid_spiffe_with_path() {
+        assert!(validate_identity("spiffe://trustdomain/path/element").is_ok())
+    }
+
+    #[test]
+    fn invalid_spiffe_scheme() {
+        let err = validate_identity("http://domain.test/path/element").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_spiffe_wrong_scheme() {
+        let err = validate_identity("spiffe:/path/element").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_spiffe_empty_trust_domain() {
+        let err = validate_identity("spiffe:///path/element").unwrap_err();
+        assert_eq!(err, IdError::MissingTrustDomain);
+    }
+
+    #[test]
+    fn invalid_spiffe_no_slashes_in_scheme() {
+        let err = validate_identity("spiffe:path/element").unwrap_err();
+        assert_eq!(err, IdError::Invalid);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_query() {
+        let err = validate_identity("spiffe://domain.test/path/element?query=").unwrap_err();
+        assert_eq!(err, IdError::BadPathSegmentChar);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_fragment() {
+        let err = validate_identity("spiffe://domain.test/path/element#fragment-1").unwrap_err();
+        assert_eq!(err, IdError::BadPathSegmentChar);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_str_port() {
+        let err = validate_identity("spiffe://domain.test:8080/path/element").unwrap_err();
+        assert_eq!(err, IdError::BadTrustDomainChar);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_user_info() {
+        let err = validate_identity("spiffe://user:password@test.org/path/element").unwrap_err();
+        assert_eq!(err, IdError::BadTrustDomainChar);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_trailing_slash() {
+        let err = validate_identity("spiffe://test.org/").unwrap_err();
+        assert_eq!(err, IdError::TrailingSlash);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_with_empty_segment() {
+        let err = validate_identity("spiffe://test.org//").unwrap_err();
+        assert_eq!(err, IdError::EmptySegment);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_str_with_path_with_trailing_slash() {
+        let err = validate_identity("spiffe://test.org/path/other/").unwrap_err();
+        assert_eq!(err, IdError::TrailingSlash);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_str_with_dot_segment() {
+        let err = validate_identity("spiffe://test.org/./other").unwrap_err();
+        assert_eq!(err, IdError::DotSegment);
+    }
+
+    #[test]
+    fn invalid_spiffe_uri_str_with_double_dot_segment() {
+        let err = validate_identity("spiffe://test.org/../other").unwrap_err();
+        assert_eq!(err, IdError::DotSegment);
+    }
+}