[release-branch.go1.19] crypto/tls: add GODEBUG to control max RSA key size

Add a new GODEBUG setting, tlsmaxrsasize, which allows controlling the
maximum RSA key size we will accept during TLS handshakes.

Fixes #61968

Change-Id: I52f060be132014d219f4cd438f59990011a35c96
Reviewed-on: https://go-review.googlesource.com/c/go/+/517495


Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/518536
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

src/crypto/tls/conn.go

@@ -1413,6 +1413,11 @@ func (c *Conn) closeNotify() error {
 //
 // For control over canceling or setting a timeout on a handshake, use
 // HandshakeContext or the Dialer's DialContext method instead.
+//
+// In order to avoid denial of service attacks, the maximum RSA key size allowed
+// in certificates sent by either the TLS server or client is limited to 8192
+// bits. This limit can be overridden by setting tlsmaxrsasize in the GODEBUG
+// environment variable (e.g. GODEBUG=tlsmaxrsasize=4096).
 func (c *Conn) Handshake() error {
 	return c.HandshakeContext(context.Background())
 }

src/crypto/tls/handshake_client.go

@@ -16,8 +16,10 @@ import (
 	"errors"
 	"fmt"
 	"hash"
+	"internal/godebug"
 	"io"
 	"net"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
@@ -857,9 +859,18 @@ func (hs *clientHandshakeState) sendFinished(out []byte) error {
 	return nil
 }
 
-// maxRSAKeySize is the maximum RSA key size in bits that we are willing
+// defaultMaxRSAKeySize is the maximum RSA key size in bits that we are willing
 // to verify the signatures of during a TLS handshake.
-const maxRSAKeySize = 8192
+const defaultMaxRSAKeySize = 8192
+
+func checkKeySize(n int) (max int, ok bool) {
+	if v := godebug.Get("tlsmaxrsasize"); v != "" {
+		if max, err := strconv.Atoi(v); err == nil {
+			return max, n <= max
+		}
+	}
+	return defaultMaxRSAKeySize, n <= defaultMaxRSAKeySize
+}
 
 // verifyServerCertificate parses and verifies the provided chain, setting
 // c.verifiedChains and c.peerCertificates or sending the appropriate alert.
@@ -871,9 +882,12 @@ func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
 			c.sendAlert(alertBadCertificate)
 			return errors.New("tls: failed to parse certificate from server: " + err.Error())
 		}
-		if cert.PublicKeyAlgorithm == x509.RSA && cert.PublicKey.(*rsa.PublicKey).N.BitLen() > maxRSAKeySize {
-			c.sendAlert(alertBadCertificate)
-			return fmt.Errorf("tls: server sent certificate containing RSA key larger than %d bits", maxRSAKeySize)
+		if cert.PublicKeyAlgorithm == x509.RSA {
+			n := cert.PublicKey.(*rsa.PublicKey).N.BitLen()
+			if max, ok := checkKeySize(n); !ok {
+				c.sendAlert(alertBadCertificate)
+				return fmt.Errorf("tls: server sent certificate containing RSA key larger than %d bits", max)
+			}
 		}
 		certs[i] = cert
 	}

src/crypto/tls/handshake_client_test.go

@@ -2678,19 +2678,44 @@ u58=
 -----END CERTIFICATE-----`
 
 func TestHandshakeRSATooBig(t *testing.T) {
-	testCert, _ := pem.Decode([]byte(largeRSAKeyCertPEM))
+	for _, tc := range []struct {
+		name              string
+		godebug           string
+		expectedServerErr string
+		expectedClientErr string
+	}{
+		{
+			name:              "key too large",
+			expectedServerErr: "tls: server sent certificate containing RSA key larger than 8192 bits",
+			expectedClientErr: "tls: client sent certificate containing RSA key larger than 8192 bits",
+		},
+		{
+			name:    "acceptable key (GODEBUG=tlsmaxrsasize=8193)",
+			godebug: "tlsmaxrsasize=8193",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.godebug != "" {
+				t.Setenv("GODEBUG", tc.godebug)
+			}
 
-	c := &Conn{conn: &discardConn{}, config: testConfig.Clone()}
+			testCert, _ := pem.Decode([]byte(largeRSAKeyCertPEM))
 
-	expectedErr := "tls: server sent certificate containing RSA key larger than 8192 bits"
-	err := c.verifyServerCertificate([][]byte{testCert.Bytes})
-	if err == nil || err.Error() != expectedErr {
-		t.Errorf("Conn.verifyServerCertificate unexpected error: want %q, got %q", expectedErr, err)
-	}
+			c := &Conn{conn: &discardConn{}, config: testConfig.Clone()}
+
+			err := c.verifyServerCertificate([][]byte{testCert.Bytes})
+			if tc.expectedServerErr == "" && err != nil {
+				t.Errorf("Conn.verifyServerCertificate unexpected error: %s", err)
+			} else if tc.expectedServerErr != "" && (err == nil || err.Error() != tc.expectedServerErr) {
+				t.Errorf("Conn.verifyServerCertificate unexpected error: want %q, got %q", tc.expectedServerErr, err)
+			}
 
-	expectedErr = "tls: client sent certificate containing RSA key larger than 8192 bits"
-	err = c.processCertsFromClient(Certificate{Certificate: [][]byte{testCert.Bytes}})
-	if err == nil || err.Error() != expectedErr {
-		t.Errorf("Conn.processCertsFromClient unexpected error: want %q, got %q", expectedErr, err)
+			err = c.processCertsFromClient(Certificate{Certificate: [][]byte{testCert.Bytes}})
+			if tc.expectedClientErr == "" && err != nil {
+				t.Errorf("Conn.processCertsFromClient unexpected error: %s", err)
+			} else if tc.expectedClientErr != "" && (err == nil || err.Error() != tc.expectedClientErr) {
+				t.Errorf("Conn.processCertsFromClient unexpected error: want %q, got %q", tc.expectedClientErr, err)
+			}
+		})
 	}
 }

src/crypto/tls/handshake_server.go

@@ -819,9 +819,12 @@ func (c *Conn) processCertsFromClient(certificate Certificate) error {
 			c.sendAlert(alertBadCertificate)
 			return errors.New("tls: failed to parse client certificate: " + err.Error())
 		}
-		if certs[i].PublicKeyAlgorithm == x509.RSA && certs[i].PublicKey.(*rsa.PublicKey).N.BitLen() > maxRSAKeySize {
-			c.sendAlert(alertBadCertificate)
-			return fmt.Errorf("tls: client sent certificate containing RSA key larger than %d bits", maxRSAKeySize)
+		if certs[i].PublicKeyAlgorithm == x509.RSA {
+			n := certs[i].PublicKey.(*rsa.PublicKey).N.BitLen()
+			if max, ok := checkKeySize(n); !ok {
+				c.sendAlert(alertBadCertificate)
+				return fmt.Errorf("tls: client sent certificate containing RSA key larger than %d bits", max)
+			}
 		}
 	}