crypto/tls: restrict RSA keys in certificates to <= 8192 bits

Extremely large RSA keys in certificate chains can cause a client/server
to expend significant CPU time verifying signatures. Limit this by
restricting the size of RSA keys transmitted during handshakes to <=
8192 bits.

Based on a survey of publicly trusted RSA keys, there are currently only
three certificates in circulation with keys larger than this, and all
three appear to be test certificates that are not actively deployed. It
is possible there are larger keys in use in private PKIs, but we target
the web PKI, so causing breakage here in the interests of increasing the
default safety of users of crypto/tls seems reasonable.

Thanks to Mateusz Poliwczak for reporting this issue.

Fixes #61460
Fixes CVE-2023-29409

Change-Id: Ie35038515a649199a36a12fc2c5df3af855dca6c
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1912161
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/515257
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>

src/crypto/tls/handshake_client.go

@@ -936,6 +936,10 @@ func (hs *clientHandshakeState) sendFinished(out []byte) error {
 	return nil
 }
 
+// maxRSAKeySize is the maximum RSA key size in bits that we are willing
+// to verify the signatures of during a TLS handshake.
+const maxRSAKeySize = 8192
+
 // verifyServerCertificate parses and verifies the provided chain, setting
 // c.verifiedChains and c.peerCertificates or sending the appropriate alert.
 func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
@@ -947,6 +951,10 @@ func (c *Conn) verifyServerCertificate(certificates [][]byte) error {
 			c.sendAlert(alertBadCertificate)
 			return errors.New("tls: failed to parse certificate from server: " + err.Error())
 		}
+		if cert.cert.PublicKeyAlgorithm == x509.RSA && cert.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)
+		}
 		activeHandles[i] = cert
 		certs[i] = cert.cert
 	}

src/crypto/tls/handshake_client_test.go

@@ -2721,3 +2721,81 @@ func testTLS13OnlyClientHelloCipherSuite(t *testing.T, ciphers []uint16) {
 		t.Fatalf("handshake failed: %s", err)
 	}
 }
+
+// discardConn wraps a net.Conn but discards all writes, but reports that they happened.
+type discardConn struct {
+	net.Conn
+}
+
+func (dc *discardConn) Write(data []byte) (int, error) {
+	return len(data), nil
+}
+
+// largeRSAKeyCertPEM contains a 8193 bit RSA key
+const largeRSAKeyCertPEM = `-----BEGIN CERTIFICATE-----
+MIIInjCCBIWgAwIBAgIBAjANBgkqhkiG9w0BAQsFADASMRAwDgYDVQQDEwd0ZXN0
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+u58=
+-----END CERTIFICATE-----`
+
+func TestHandshakeRSATooBig(t *testing.T) {
+	testCert, _ := pem.Decode([]byte(largeRSAKeyCertPEM))
+
+	c := &Conn{conn: &discardConn{}, config: testConfig.Clone()}
+
+	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)
+	}
+
+	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)
+	}
+}

src/crypto/tls/handshake_server.go

@@ -864,6 +864,10 @@ 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 len(certs) == 0 && requiresClientCert(c.config.ClientAuth) {