verify_cert: enforce maximum number of signatures.

Cherry-picked from e473ee1 and modified
by Brian Smith. The main modifications were:

1. Maintain API compatibility with webpki 0.22.0.
2. (In `build_chain_inner`), stop immediately on fatal error, without
   considering any more paths. The point of having such fatal errors
   is to fail ASAP and avoid unneeded work in the failure case.

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

@@ -76,6 +76,7 @@ untrusted = "0.7.1"
 
 [dev-dependencies]
 base64 = "0.9.1"
+rcgen = "0.11.1"
 
 [profile.bench]
 opt-level = 3

src/verify_cert.rs

@@ -25,15 +25,58 @@ pub fn build_chain(
     cert: &Cert,
     time: time::Time,
 ) -> Result<(), Error> {
-    build_chain_inner(
+    let result = build_chain_inner(
         required_eku_if_present,
         supported_sig_algs,
         trust_anchors,
         intermediate_certs,
         cert,
         time,
         0,
-    )
+        &mut 0,
+    );
+    result.map_err(|error| {
+        match error {
+            ErrorOrInternalError::Error(e) => e,
+            // Eat internal errors,
+            ErrorOrInternalError::InternalError(_) => Error::UnknownIssuer,
+        }
+    })
+}
+
+/// Errors that we cannot report externally since `Error` wasn't declared
+/// non-exhaustive, but which we need to differentiate internally (at least
+/// for testing).
+enum InternalError {
+    MaximumSignatureChecksExceeded,
+}
+
+enum ErrorOrInternalError {
+    Error(Error),
+    InternalError(InternalError),
+}
+
+impl ErrorOrInternalError {
+    fn is_fatal(&self) -> bool {
+        match self {
+            ErrorOrInternalError::Error(_) => false,
+            ErrorOrInternalError::InternalError(InternalError::MaximumSignatureChecksExceeded) => {
+                true
+            }
+        }
+    }
+}
+
+impl From<InternalError> for ErrorOrInternalError {
+    fn from(value: InternalError) -> Self {
+        Self::InternalError(value)
+    }
+}
+
+impl From<Error> for ErrorOrInternalError {
+    fn from(error: Error) -> Self {
+        Self::Error(error)
+    }
 }
 
 fn build_chain_inner(
@@ -44,7 +87,8 @@ fn build_chain_inner(
     cert: &Cert,
     time: time::Time,
     sub_ca_count: usize,
-) -> Result<(), Error> {
+    signatures: &mut usize,
+) -> Result<(), ErrorOrInternalError> {
     let used_as_ca = used_as_ca(&cert.ee_or_ca);
 
     check_issuer_independent_properties(
@@ -62,7 +106,7 @@ fn build_chain_inner(
             const MAX_SUB_CA_COUNT: usize = 6;
 
             if sub_ca_count >= MAX_SUB_CA_COUNT {
-                return Err(Error::UnknownIssuer);
+                return Err(Error::UnknownIssuer.into());
             }
         }
         UsedAsCa::No => {
@@ -75,7 +119,7 @@ fn build_chain_inner(
     match loop_while_non_fatal_error(trust_anchors, |trust_anchor: &TrustAnchor| {
         let trust_anchor_subject = untrusted::Input::from(trust_anchor.subject);
         if cert.issuer != trust_anchor_subject {
-            return Err(Error::UnknownIssuer);
+            return Err(Error::UnknownIssuer.into());
         }
 
         let name_constraints = trust_anchor.name_constraints.map(untrusted::Input::from);
@@ -88,14 +132,17 @@ fn build_chain_inner(
 
         // 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(())
     }) {
         Ok(()) => {
             return Ok(());
         }
-        Err(..) => {
+        Err(e) => {
+            if e.is_fatal() {
+                return Err(e);
+            }
             // If the error is not fatal, then keep going.
         }
     }
@@ -105,7 +152,7 @@ fn build_chain_inner(
             cert::parse_cert(untrusted::Input::from(*cert_der), EndEntityOrCa::Ca(&cert))?;
 
         if potential_issuer.subject != cert.issuer {
-            return Err(Error::UnknownIssuer);
+            return Err(Error::UnknownIssuer.into());
         }
 
         // Prevent loops; see RFC 4158 section 5.2.
@@ -114,7 +161,7 @@ fn build_chain_inner(
             if potential_issuer.spki.value() == prev.spki.value()
                 && potential_issuer.subject == prev.subject
             {
-                return Err(Error::UnknownIssuer);
+                return Err(Error::UnknownIssuer.into());
             }
             match &prev.ee_or_ca {
                 EndEntityOrCa::EndEntity => {
@@ -143,6 +190,7 @@ fn build_chain_inner(
             &potential_issuer,
             time,
             next_sub_ca_count,
+            signatures,
         )
     })
 }
@@ -151,10 +199,16 @@ fn check_signatures(
     supported_sig_algs: &[&SignatureAlgorithm],
     cert_chain: &Cert,
     trust_anchor_key: untrusted::Input,
-) -> Result<(), Error> {
+    signatures: &mut usize,
+) -> Result<(), ErrorOrInternalError> {
     let mut spki_value = trust_anchor_key;
     let mut cert = cert_chain;
     loop {
+        *signatures += 1;
+        if *signatures > 100 {
+            return Err(InternalError::MaximumSignatureChecksExceeded.into());
+        }
+
         signed_data::verify_signed_data(supported_sig_algs, spki_value, &cert.signed_data)?;
 
         // TODO: check revocation
@@ -352,8 +406,8 @@ fn check_eku(
 
 fn loop_while_non_fatal_error<V>(
     values: V,
-    f: impl Fn(V::Item) -> Result<(), Error>,
-) -> Result<(), Error>
+    mut f: impl FnMut(V::Item) -> Result<(), ErrorOrInternalError>,
+) -> Result<(), ErrorOrInternalError>
 where
     V: IntoIterator,
 {
@@ -362,10 +416,87 @@ where
             Ok(()) => {
                 return Ok(());
             }
-            Err(..) => {
+            Err(e) => {
+                if e.is_fatal() {
+                    return Err(e);
+                }
                 // If the error is not fatal, then keep going.
             }
         }
     }
-    Err(Error::UnknownIssuer)
+    Err(Error::UnknownIssuer.into())
+}
+
+#[cfg(test)]
+mod tests {
+
+    #[test]
+    #[cfg(feature = "alloc")]
+    fn test_too_many_signatures() {
+        use super::*;
+        use crate::ECDSA_P256_SHA256;
+        use crate::{EndEntityCert, Time};
+        use alloc::{string::ToString, vec::Vec};
+        use core::convert::TryFrom;
+
+        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();
+
+        // TODO: Use `build_chain` when `Error` is maid non-exhaustive.
+        let result = build_chain_inner(
+            EKU_SERVER_AUTH,
+            &[&ECDSA_P256_SHA256],
+            anchors,
+            intermediate_certs,
+            cert.inner(),
+            time,
+            0,
+            &mut 0,
+        );
+
+        assert!(matches!(
+            result,
+            Err(ErrorOrInternalError::InternalError(
+                InternalError::MaximumSignatureChecksExceeded
+            ))
+        ));
+    }
 }