crypto/tls: disable ExportKeyingMaterial without EMS

Fixes #43922

Change-Id: Idaad7daa6784807ae3a5e4d944e88e13d01fd0b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/544155
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>

common.go

@@ -304,11 +304,13 @@ type ConnectionState struct {
 // ExportKeyingMaterial returns length bytes of exported key material in a new
 // slice as defined in RFC 5705. If context is nil, it is not used as part of
 // the seed. If the connection was set to allow renegotiation via
-// Config.Renegotiation, this function will return an error.
+// Config.Renegotiation, or if the connections supports neither TLS 1.3 nor
+// Extended Master Secret, this function will return an error.
 //
-// There are conditions in which the returned values might not be unique to a
-// connection. See the Security Considerations sections of RFC 5705 and RFC 7627,
-// and https://mitls.org/pages/attacks/3SHAKE#channelbindings.
+// Exporting key material without Extended Master Secret or TLS 1.3 was disabled
+// in Go 1.22 due to security issues (see the Security Considerations sections
+// of RFC 5705 and RFC 7627), but can be re-enabled with the GODEBUG setting
+// tlsunsafeekm=1.
 func (cs *ConnectionState) ExportKeyingMaterial(label string, context []byte, length int) ([]byte, error) {
 	return cs.ekm(label, context, length)
 }

conn.go

@@ -15,6 +15,7 @@ import (
 	"errors"
 	"fmt"
 	"hash"
+	"internal/godebug"
 	"io"
 	"net"
 	"sync"
@@ -1599,6 +1600,8 @@ func (c *Conn) ConnectionState() ConnectionState {
 	return c.connectionStateLocked()
 }
 
+var ekmgodebug = godebug.New("tlsunsafeekm")
+
 func (c *Conn) connectionStateLocked() ConnectionState {
 	var state ConnectionState
 	state.HandshakeComplete = c.isHandshakeComplete.Load()
@@ -1620,7 +1623,15 @@ func (c *Conn) connectionStateLocked() ConnectionState {
 		}
 	}
 	if c.config.Renegotiation != RenegotiateNever {
-		state.ekm = noExportedKeyingMaterial
+		state.ekm = noEKMBecauseRenegotiation
+	} else if c.vers != VersionTLS13 && !c.extMasterSecret {
+		state.ekm = func(label string, context []byte, length int) ([]byte, error) {
+			if ekmgodebug.Value() == "1" {
+				ekmgodebug.IncNonDefault()
+				return c.ekm(label, context, length)
+			}
+			return noEKMBecauseNoEMS(label, context, length)
+		}
 	} else {
 		state.ekm = c.ekm
 	}

prf.go

@@ -252,13 +252,20 @@ func (h *finishedHash) discardHandshakeBuffer() {
 	h.buffer = nil
 }
 
-// noExportedKeyingMaterial is used as a value of
+// noEKMBecauseRenegotiation is used as a value of
 // ConnectionState.ekm when renegotiation is enabled and thus
 // we wish to fail all key-material export requests.
-func noExportedKeyingMaterial(label string, context []byte, length int) ([]byte, error) {
+func noEKMBecauseRenegotiation(label string, context []byte, length int) ([]byte, error) {
 	return nil, errors.New("crypto/tls: ExportKeyingMaterial is unavailable when renegotiation is enabled")
 }
 
+// noEKMBecauseNoEMS is used as a value of ConnectionState.ekm when Extended
+// Master Secret is not negotiated and thus we wish to fail all key-material
+// export requests.
+func noEKMBecauseNoEMS(label string, context []byte, length int) ([]byte, error) {
+	return nil, errors.New("crypto/tls: ExportKeyingMaterial is unavailable when neither TLS 1.3 nor Extended Master Secret are negotiated; override with GODEBUG=tlsunsafeekm=1")
+}
+
 // ekmFromMasterSecret generates exported keying material as defined in RFC 5705.
 func ekmFromMasterSecret(version uint16, suite *cipherSuite, masterSecret, clientRandom, serverRandom []byte) func(string, []byte, int) ([]byte, error) {
 	return func(label string, context []byte, length int) ([]byte, error) {