tls: Fix OCSP stapling bug when certificate names overlap other certs

https://caddy.community/t/random-ocsp-response-errors-for-random-clients/2473?u=matt

Certificates are keyed by name in the cache, optimized for fast lookups
during TLS handshakes using SNI. A more "correct" way that is truly a
1:1 would be to cache certificates by a hash of the leaf's DER bytes,
but this involves an extra index to maintain. So instead of that, we
simply choose to prevent overlap when keying certificates by server
name. This avoids the ambiguity when updating OCSP staples, for instance.

caddytls/certificates.go

@@ -14,7 +14,9 @@ import (
 )
 
 // certCache stores certificates in memory,
-// keying certificates by name.
+// keying certificates by name. Certificates
+// should not overlap in the names they serve,
+// because a name only maps to one certificate.
 var certCache = make(map[string]Certificate)
 var certCacheMu sync.RWMutex
 
@@ -27,6 +29,8 @@ type Certificate struct {
 
 	// Names is the list of names this certificate is written for.
 	// The first is the CommonName (if any), the rest are SAN.
+	// This should be the exact list of keys by which this cert
+	// is accessed in the cache, careful to avoid overlap.
 	Names []string
 
 	// NotAfter is when the certificate expires.
@@ -164,17 +168,9 @@ func makeCertificate(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 	if err != nil {
 		return cert, err
 	}
-	if len(tlsCert.Certificate) == 0 {
-		return cert, errors.New("certificate is empty")
-	}
-	cert.Certificate = tlsCert
 
-	// Parse leaf certificate, extract relevant metadata, and staple OCSP
-	leaf, err := x509.ParseCertificate(tlsCert.Certificate[0])
-	if err != nil {
-		return cert, err
-	}
-	err = fillCertFromLeaf(&cert, leaf)
+	// Extract relevant metadata and staple OCSP
+	err = fillCertFromLeaf(&cert, tlsCert)
 	if err != nil {
 		return cert, err
 	}
@@ -186,9 +182,19 @@ func makeCertificate(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 	return cert, nil
 }
 
-// fillCertFromLeaf populates cert.Names and cert.NotAfter
-// using data in leaf.
-func fillCertFromLeaf(cert *Certificate, leaf *x509.Certificate) error {
+// fillCertFromLeaf populates metadata fields on cert from tlsCert.
+func fillCertFromLeaf(cert *Certificate, tlsCert tls.Certificate) error {
+	if len(tlsCert.Certificate) == 0 {
+		return errors.New("certificate is empty")
+	}
+	cert.Certificate = tlsCert
+
+	// the leaf cert should be the one for the site; it has what we need
+	leaf, err := x509.ParseCertificate(tlsCert.Certificate[0])
+	if err != nil {
+		return err
+	}
+
 	if leaf.Subject.CommonName != "" {
 		cert.Names = []string{strings.ToLower(leaf.Subject.CommonName)}
 	}
@@ -210,7 +216,9 @@ func fillCertFromLeaf(cert *Certificate, leaf *x509.Certificate) error {
 	if len(cert.Names) == 0 {
 		return errors.New("certificate has no names")
 	}
+
 	cert.NotAfter = leaf.NotAfter
+
 	return nil
 }
 
@@ -231,7 +239,6 @@ func cacheCertificate(cert Certificate) {
 	if _, ok := certCache[""]; !ok {
 		// use as default - must be *appended* to end of list, or bad things happen!
 		cert.Names = append(cert.Names, "")
-		certCache[""] = cert
 	}
 	for len(certCache)+len(cert.Names) > 10000 {
 		// for simplicity, just remove random elements
@@ -243,7 +250,20 @@ func cacheCertificate(cert Certificate) {
 			break
 		}
 	}
-	for _, name := range cert.Names {
+	for i := 0; i < len(cert.Names); i++ {
+		name := cert.Names[i]
+		if _, ok := certCache[name]; ok {
+			// do not allow certificates to overlap in the names they serve;
+			// this ambiguity causes problems because it is confusing while
+			// maintaining certificates; see OCSP maintenance code and
+			// https://caddy.community/t/random-ocsp-response-errors-for-random-clients/2473?u=matt.
+			log.Printf("[NOTICE] There is already a certificate loaded for %s, "+
+				"so certificate for %v will not service that name",
+				name, cert.Names)
+			cert.Names = append(cert.Names[:i], cert.Names[i+1:]...)
+			i--
+			continue
+		}
 		certCache[name] = cert
 	}
 	certCacheMu.Unlock()

caddytls/maintain.go

@@ -246,6 +246,16 @@ func UpdateOCSPStaples() {
 			log.Printf("[INFO] Advancing OCSP staple for %v from %s to %s",
 				cert.Names, lastNextUpdate, cert.OCSP.NextUpdate)
 			for _, n := range cert.Names {
+				// BUG: If this certificate has names on it that appear on another
+				// certificate in the cache, AND the other certificate is keyed by
+				// that name in the cache, then this method of 'queueing' the staple
+				// update will cause this certificate's new OCSP to be stapled to
+				// a different certificate! See:
+				// https://caddy.community/t/random-ocsp-response-errors-for-random-clients/2473?u=matt
+				// This problem should be avoided if names on certificates in the
+				// cache don't overlap with regards to the cache keys.
+				// (This is isn't a bug anymore, since we're careful when we add
+				// certificates to the cache by skipping keying when key already exists.)
 				updated[n] = ocspUpdate{rawBytes: cert.Certificate.OCSPStaple, parsed: cert.OCSP}
 			}
 		}