verify_cert: enforce maximum number of signatures.

Pathbuilding complexity can be quadratic, particularly when the set of
intermediates all have subjects matching a trust anchor. In these cases
we need to bound the number of expensive signature validation operations
that are performed to avoid a DoS on CPU usage.

This commit implements a simple maximum signature check limit inspired
by the approach taken in the Golang x509 package. No more than 100
signatures will be evaluated while pathbuilding. This limit works in
practice for Go when processing real world certificate chains and so
should be appropriate for our use case as well.

Cargo.toml

@@ -82,6 +82,7 @@ untrusted = "0.7.1"
 
 [dev-dependencies]
 base64 = "0.13"
+rcgen = { version = "0.11.1", default-features = false }
 
 [profile.bench]
 opt-level = 3

src/end_entity.rs

@@ -98,6 +98,7 @@ impl<'a> EndEntityCert<'a> {
             &self.inner,
             time,
             0,
+            &mut 0_usize,
         )
     }
 
@@ -130,6 +131,7 @@ impl<'a> EndEntityCert<'a> {
             &self.inner,
             time,
             0,
+            &mut 0_usize,
         )
     }
 

src/error.rs

@@ -81,6 +81,9 @@ pub enum Error {
     /// and as recommended by RFC6125.
     MalformedExtensions,
 
+    /// The maximum number of signature checks has been reached. Path complexity is too great.
+    MaximumSignatureChecksExceeded,
+
     /// The certificate contains an unsupported critical extension.
     UnsupportedCriticalExtension,
 

src/verify_cert.rs

@@ -17,6 +17,7 @@ use crate::{
     der, signed_data, subject_name, time, Error, SignatureAlgorithm, TrustAnchor,
 };
 
+#[allow(clippy::too_many_arguments)]
 pub(crate) fn build_chain(
     required_eku_if_present: KeyPurposeId,
     supported_sig_algs: &[&SignatureAlgorithm],
@@ -25,6 +26,7 @@ pub(crate) fn build_chain(
     cert: &Cert,
     time: time::Time,
     sub_ca_count: usize,
+    signatures: &mut usize,
 ) -> Result<(), Error> {
     let used_as_ca = used_as_ca(&cert.ee_or_ca);
 
@@ -79,15 +81,17 @@ pub(crate) fn build_chain(
 
         // TODO: check_distrust(trust_anchor_subject, trust_anchor_spki)?;
 
-        check_signatures(supported_sig_algs, cert, trust_anchor_spki)?;
+        check_signatures(supported_sig_algs, cert, trust_anchor_spki, signatures)?;
 
         Ok(())
     });
 
     // If the error is not fatal, then keep going.
-    if result.is_ok() {
-        return Ok(());
-    }
+    match result {
+        Ok(()) => return Ok(()),
+        err @ Err(Error::MaximumSignatureChecksExceeded) => return err,
+        _ => {}
+    };
 
     loop_while_non_fatal_error(intermediate_certs, |cert_der| {
         let potential_issuer =
@@ -132,6 +136,7 @@ pub(crate) fn build_chain(
             &potential_issuer,
             time,
             next_sub_ca_count,
+            signatures,
         )
     })
 }
@@ -140,10 +145,16 @@ fn check_signatures(
     supported_sig_algs: &[&SignatureAlgorithm],
     cert_chain: &Cert,
     trust_anchor_key: untrusted::Input,
+    signatures: &mut usize,
 ) -> Result<(), Error> {
     let mut spki_value = trust_anchor_key;
     let mut cert = cert_chain;
     loop {
+        *signatures += 1;
+        if *signatures > 100 {
+            return Err(Error::MaximumSignatureChecksExceeded);
+        }
+
         signed_data::verify_signed_data(supported_sig_algs, spki_value, &cert.signed_data)?;
 
         // TODO: check revocation
@@ -341,16 +352,85 @@ fn check_eku(
 
 fn loop_while_non_fatal_error<V>(
     values: V,
-    f: impl Fn(V::Item) -> Result<(), Error>,
+    mut f: impl FnMut(V::Item) -> Result<(), Error>,
 ) -> Result<(), Error>
 where
     V: IntoIterator,
 {
     for v in values {
         // If the error is not fatal, then keep going.
-        if f(v).is_ok() {
-            return Ok(());
+        match f(v) {
+            Ok(()) => return Ok(()),
+            err @ Err(Error::MaximumSignatureChecksExceeded) => return err,
+            _ => {}
         }
     }
     Err(Error::UnknownIssuer)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    #[cfg(feature = "alloc")]
+    fn test_too_many_signatures() {
+        use std::convert::TryFrom;
+
+        use crate::{EndEntityCert, Time, ECDSA_P256_SHA256};
+
+        let alg = &rcgen::PKCS_ECDSA_P256_SHA256;
+
+        let make_issuer = || {
+            let mut ca_params = rcgen::CertificateParams::new(Vec::new());
+            ca_params
+                .distinguished_name
+                .push(rcgen::DnType::OrganizationName, "Bogus Subject");
+            ca_params.is_ca = rcgen::IsCa::Ca(rcgen::BasicConstraints::Unconstrained);
+            ca_params.key_usages = vec![
+                rcgen::KeyUsagePurpose::KeyCertSign,
+                rcgen::KeyUsagePurpose::DigitalSignature,
+                rcgen::KeyUsagePurpose::CrlSign,
+            ];
+            ca_params.alg = alg;
+            rcgen::Certificate::from_params(ca_params).unwrap()
+        };
+
+        let ca_cert = make_issuer();
+        let ca_cert_der = ca_cert.serialize_der().unwrap();
+
+        let mut intermediates = Vec::with_capacity(101);
+        let mut issuer = ca_cert;
+        for _ in 0..101 {
+            let intermediate = make_issuer();
+            let intermediate_der = intermediate.serialize_der_with_signer(&issuer).unwrap();
+            intermediates.push(intermediate_der);
+            issuer = intermediate;
+        }
+
+        let mut ee_params = rcgen::CertificateParams::new(vec!["example.com".to_string()]);
+        ee_params.is_ca = rcgen::IsCa::ExplicitNoCa;
+        ee_params.alg = alg;
+        let ee_cert = rcgen::Certificate::from_params(ee_params).unwrap();
+        let ee_cert_der = ee_cert.serialize_der_with_signer(&issuer).unwrap();
+
+        let anchors = &[TrustAnchor::try_from_cert_der(&ca_cert_der).unwrap()];
+        let time = Time::from_seconds_since_unix_epoch(0x1fed_f00d);
+        let cert = EndEntityCert::try_from(&ee_cert_der[..]).unwrap();
+        let intermediates_der: Vec<&[u8]> = intermediates.iter().map(|x| x.as_ref()).collect();
+        let intermediate_certs: &[&[u8]] = intermediates_der.as_ref();
+
+        let result = build_chain(
+            EKU_SERVER_AUTH,
+            &[&ECDSA_P256_SHA256],
+            anchors,
+            intermediate_certs,
+            cert.inner(),
+            time,
+            0,
+            &mut 0_usize,
+        );
+
+        assert!(matches!(result, Err(Error::MaximumSignatureChecksExceeded)));
+    }
+}