crypto/tls: check verifiedChains roots when resuming sessions

When resuming TLS sessions, on the server and client verify that the
chains stored in the session state (verifiedChains) are still acceptable
with regards to the Config by checking for the inclusion of the root in
either ClientCAs (server) or RootCAs (client). This prevents resuming
a session with a certificate chain that would be rejected during a full
handshake due to an untrusted root.

Updates #77113
Updates #77217
Updates CVE-2025-68121

Change-Id: I11fe00909ef1961c24ecf80bf5b97f7b1121d359
Reviewed-on: https://go-review.googlesource.com/c/go/+/737700
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Coia Prant <coiaprant@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

common.go

@@ -1881,13 +1881,28 @@ func fipsAllowChain(chain []*x509.Certificate) bool {
 	return true
 }
 
-// anyUnexpiredChain reports if at least one of verifiedChains is still
-// unexpired. If verifiedChains is empty, it returns false.
-func anyUnexpiredChain(verifiedChains [][]*x509.Certificate, now time.Time) bool {
+// anyValidVerifiedChain reports if at least one of the chains in verifiedChains
+// is valid, as indicated by none of the certificates being expired and the root
+// being in opts.Roots (or in the system root pool if opts.Roots is nil). If
+// verifiedChains is empty, it returns false.
+func anyValidVerifiedChain(verifiedChains [][]*x509.Certificate, opts x509.VerifyOptions) bool {
 	for _, chain := range verifiedChains {
-		if len(chain) != 0 && !slices.ContainsFunc(chain, func(cert *x509.Certificate) bool {
-			return now.Before(cert.NotBefore) || now.After(cert.NotAfter) // cert is expired
+		if len(chain) == 0 {
+			continue
+		}
+		if slices.ContainsFunc(chain, func(cert *x509.Certificate) bool {
+			return opts.CurrentTime.Before(cert.NotBefore) || opts.CurrentTime.After(cert.NotAfter)
 		}) {
+			continue
+		}
+		// Since we already validated the chain, we only care that it is
+		// rooted in a CA in CAs, or in the system pool. On platforms where
+		// we control chain validation (e.g. not Windows or macOS) this is a
+		// simple lookup in the CertPool internal hash map. On other
+		// platforms, this may be more expensive, depending on how they
+		// implement verification of just root certificates.
+		root := chain[len(chain)-1]
+		if _, err := root.Verify(opts); err == nil {
 			return true
 		}
 	}

handshake_client.go

@@ -414,7 +414,12 @@ func (c *Conn) loadSession(hello *clientHelloMsg) (
 			// application from a faulty ClientSessionCache implementation.
 			return nil, nil, nil, nil
 		}
-		if !anyUnexpiredChain(session.verifiedChains, c.config.time()) {
+		opts := x509.VerifyOptions{
+			CurrentTime: c.config.time(),
+			Roots:       c.config.RootCAs,
+			KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		}
+		if !anyValidVerifiedChain(session.verifiedChains, opts) {
 			// No valid chains, delete the entry.
 			c.config.ClientSessionCache.Put(cacheKey, nil)
 			return nil, nil, nil, nil

handshake_server.go

@@ -510,8 +510,13 @@ func (hs *serverHandshakeState) checkForResumption() error {
 	if sessionHasClientCerts && c.config.time().After(sessionState.peerCertificates[0].NotAfter) {
 		return nil
 	}
+	opts := x509.VerifyOptions{
+		CurrentTime: c.config.time(),
+		Roots:       c.config.ClientCAs,
+		KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
+	}
 	if sessionHasClientCerts && c.config.ClientAuth >= VerifyClientCertIfGiven &&
-		!anyUnexpiredChain(sessionState.verifiedChains, c.config.time()) {
+		!anyValidVerifiedChain(sessionState.verifiedChains, opts) {
 		return nil
 	}
 

handshake_server_tls13.go

@@ -464,8 +464,13 @@ func (hs *serverHandshakeStateTLS13) checkForResumption() error {
 		if sessionHasClientCerts && c.config.time().After(sessionState.peerCertificates[0].NotAfter) {
 			continue
 		}
+		opts := x509.VerifyOptions{
+			CurrentTime: c.config.time(),
+			Roots:       c.config.ClientCAs,
+			KeyUsages:   []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
+		}
 		if sessionHasClientCerts && c.config.ClientAuth >= VerifyClientCertIfGiven &&
-			!anyUnexpiredChain(sessionState.verifiedChains, c.config.time()) {
+			!anyValidVerifiedChain(sessionState.verifiedChains, opts) {
 			continue
 		}