crypto/tls: remove RSA KEX ciphers from the default list

Removes the RSA KEX based ciphers from the default list. This can be
reverted using the tlsrsakex GODEBUG.

Fixes #63413

Change-Id: Id221be3eb2f6c24b91039d380313f0c87d339f98
Reviewed-on: https://go-review.googlesource.com/c/go/+/541517
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

doc/godebug.md

@@ -155,6 +155,10 @@ Go 1.22 changed the default minimum TLS version supported by both servers
 and clients to TLS 1.2. The default can be reverted to TLS 1.0 using the
 [`tls10server` setting](/pkg/crypto/tls/#Config).
 
+Go 1.22 changed the default TLS cipher suites used by clients and servers when
+not explicitly configured, removing the cipher suites which used RSA based key
+exchange. The default can be revert using the [`tlsrsakex` setting](/pkg/crypto/tls/#Config).
+
 ### Go 1.21
 
 Go 1.21 made it a run-time error to call `panic` with a nil interface value,

src/crypto/tls/cipher_suites.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"hash"
 	"internal/cpu"
+	"internal/godebug"
 	"runtime"
 
 	"golang.org/x/crypto/chacha20poly1305"
@@ -335,10 +336,35 @@ var disabledCipherSuites = []uint16{
 }
 
 var (
-	defaultCipherSuitesLen = len(cipherSuitesPreferenceOrder) - len(disabledCipherSuites)
-	defaultCipherSuites    = cipherSuitesPreferenceOrder[:defaultCipherSuitesLen]
+	defaultCipherSuitesLen int
+	defaultCipherSuites    []uint16
 )
 
+// rsaKexCiphers contains the ciphers which use RSA based key exchange,
+// which we disable by default.
+var rsaKexCiphers = map[uint16]bool{
+	TLS_RSA_WITH_RC4_128_SHA:        true,
+	TLS_RSA_WITH_3DES_EDE_CBC_SHA:   true,
+	TLS_RSA_WITH_AES_128_CBC_SHA:    true,
+	TLS_RSA_WITH_AES_256_CBC_SHA:    true,
+	TLS_RSA_WITH_AES_128_CBC_SHA256: true,
+	TLS_RSA_WITH_AES_128_GCM_SHA256: true,
+	TLS_RSA_WITH_AES_256_GCM_SHA384: true,
+}
+
+var rsaKEXgodebug = godebug.New("tlsrsakex")
+
+func init() {
+	rsaKexEnabled := rsaKEXgodebug.Value() == "1"
+	for _, c := range cipherSuitesPreferenceOrder[:len(cipherSuitesPreferenceOrder)-len(disabledCipherSuites)] {
+		if !rsaKexEnabled && rsaKexCiphers[c] {
+			continue
+		}
+		defaultCipherSuites = append(defaultCipherSuites, c)
+	}
+	defaultCipherSuitesLen = len(defaultCipherSuites)
+}
+
 // defaultCipherSuitesTLS13 is also the preference order, since there are no
 // disabled by default TLS 1.3 cipher suites. The same AES vs ChaCha20 logic as
 // cipherSuitesPreferenceOrder applies.

src/crypto/tls/common.go

@@ -670,7 +670,9 @@ type Config struct {
 	// the list is ignored. Note that TLS 1.3 ciphersuites are not configurable.
 	//
 	// If CipherSuites is nil, a safe default list is used. The default cipher
-	// suites might change over time.
+	// suites might change over time. In Go 1.22 RSA key exchange based cipher
+	// suites were removed from the default list, but can be re-added with the
+	// GODEBUG setting tlsrsakex=1.
 	CipherSuites []uint16
 
 	// PreferServerCipherSuites is a legacy field and has no effect.

src/crypto/tls/tls_test.go

@@ -1288,7 +1288,8 @@ func TestClientHelloInfo_SupportsCertificate(t *testing.T) {
 			SignatureSchemes:  []SignatureScheme{PKCS1WithSHA1},
 			SupportedVersions: []uint16{VersionTLS13, VersionTLS12},
 			config: &Config{
-				MaxVersion: VersionTLS12,
+				CipherSuites: []uint16{TLS_RSA_WITH_AES_128_GCM_SHA256},
+				MaxVersion:   VersionTLS12,
 			},
 		}, ""}, // Check that mutual version selection works.
 
@@ -1385,6 +1386,9 @@ func TestClientHelloInfo_SupportsCertificate(t *testing.T) {
 		{rsaCert, &ClientHelloInfo{
 			CipherSuites:      []uint16{TLS_RSA_WITH_AES_128_GCM_SHA256},
 			SupportedVersions: []uint16{VersionTLS12},
+			config: &Config{
+				CipherSuites: []uint16{TLS_RSA_WITH_AES_128_GCM_SHA256},
+			},
 		}, ""}, // static RSA fallback
 	}
 	for i, tt := range tests {

src/internal/godebugs/table.go

@@ -47,6 +47,7 @@ var All = []Info{
 	{Name: "tarinsecurepath", Package: "archive/tar"},
 	{Name: "tls10server", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "tlsmaxrsasize", Package: "crypto/tls"},
+	{Name: "tlsrsakex", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "x509sha1", Package: "crypto/x509"},
 	{Name: "x509usefallbackroots", Package: "crypto/x509"},
 	{Name: "zipinsecurepath", Package: "archive/zip"},

src/net/http/client_test.go

@@ -946,7 +946,7 @@ func testResponseSetsTLSConnectionState(t *testing.T, mode testMode) {
 
 	c := ts.Client()
 	tr := c.Transport.(*Transport)
-	tr.TLSClientConfig.CipherSuites = []uint16{tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA}
+	tr.TLSClientConfig.CipherSuites = []uint16{tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA}
 	tr.TLSClientConfig.MaxVersion = tls.VersionTLS12 // to get to pick the cipher suite
 	tr.Dial = func(netw, addr string) (net.Conn, error) {
 		return net.Dial(netw, ts.Listener.Addr().String())
@@ -959,7 +959,7 @@ func testResponseSetsTLSConnectionState(t *testing.T, mode testMode) {
 	if res.TLS == nil {
 		t.Fatal("Response didn't set TLS Connection State.")
 	}
-	if got, want := res.TLS.CipherSuite, tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA; got != want {
+	if got, want := res.TLS.CipherSuite, tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA; got != want {
 		t.Errorf("TLS Cipher Suite = %d; want %d", got, want)
 	}
 }

src/runtime/metrics/doc.go

@@ -311,6 +311,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the crypto/tls
 		package due to a non-default GODEBUG=tlsmaxrsasize=... setting.
 
+	/godebug/non-default-behavior/tlsrsakex:events
+		The number of non-default behaviors executed by the crypto/tls
+		package due to a non-default GODEBUG=tlsrsakex=... setting.
+
 	/godebug/non-default-behavior/x509sha1:events
 		The number of non-default behaviors executed by the crypto/x509
 		package due to a non-default GODEBUG=x509sha1=... setting.