crypto/tls: populate Leaf in X509KeyPair

Fixes #67065

Change-Id: I189e194de8aa94523eb64e1dd294a70cb81cbdf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/585856
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

tls.go

@@ -22,6 +22,7 @@ import (
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"internal/godebug"
 	"net"
 	"os"
 	"strings"
@@ -222,11 +223,14 @@ func (d *Dialer) DialContext(ctx context.Context, network, addr string) (net.Con
 	return c, nil
 }
 
-// LoadX509KeyPair reads and parses a public/private key pair from a pair
-// of files. The files must contain PEM encoded data. The certificate file
-// may contain intermediate certificates following the leaf certificate to
-// form a certificate chain. On successful return, Certificate.Leaf will
-// be nil because the parsed form of the certificate is not retained.
+// LoadX509KeyPair reads and parses a public/private key pair from a pair of
+// files. The files must contain PEM encoded data. The certificate file may
+// contain intermediate certificates following the leaf certificate to form a
+// certificate chain. On successful return, Certificate.Leaf will be populated.
+//
+// Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
 func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 	certPEMBlock, err := os.ReadFile(certFile)
 	if err != nil {
@@ -239,9 +243,14 @@ func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 	return X509KeyPair(certPEMBlock, keyPEMBlock)
 }
 
+var x509keypairleaf = godebug.New("x509keypairleaf")
+
 // X509KeyPair parses a public/private key pair from a pair of
-// PEM encoded data. On successful return, Certificate.Leaf will be nil because
-// the parsed form of the certificate is not retained.
+// PEM encoded data. On successful return, Certificate.Leaf will be populated.
+//
+// Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
 func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 	fail := func(err error) (Certificate, error) { return Certificate{}, err }
 
@@ -296,6 +305,12 @@ func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 		return fail(err)
 	}
 
+	if x509keypairleaf.Value() != "0" {
+		cert.Leaf = x509Cert
+	} else {
+		x509keypairleaf.IncNonDefault()
+	}
+
 	cert.PrivateKey, err = parsePrivateKey(keyDERBlock.Bytes)
 	if err != nil {
 		return fail(err)

tls_test.go

@@ -8,13 +8,19 @@ import (
 	"bytes"
 	"context"
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
 	"crypto/x509"
+	"crypto/x509/pkix"
 	"encoding/json"
+	"encoding/pem"
 	"errors"
 	"fmt"
 	"internal/testenv"
 	"io"
 	"math"
+	"math/big"
 	"net"
 	"os"
 	"reflect"
@@ -1945,3 +1951,54 @@ func TestHandshakeKyber(t *testing.T) {
 		})
 	}
 }
+
+func TestX509KeyPairPopulateCertificate(t *testing.T) {
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	keyDER, err := x509.MarshalPKCS8PrivateKey(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	keyPEM := pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: keyDER})
+	tmpl := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject:      pkix.Name{CommonName: "test"},
+	}
+	certDER, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, key.Public(), key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	certPEM := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: certDER})
+
+	t.Run("x509keypairleaf=0", func(t *testing.T) {
+		t.Setenv("GODEBUG", "x509keypairleaf=0")
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf != nil {
+			t.Fatal("Leaf should not be populated")
+		}
+	})
+	t.Run("x509keypairleaf=1", func(t *testing.T) {
+		t.Setenv("GODEBUG", "x509keypairleaf=1")
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf == nil {
+			t.Fatal("Leaf should be populated")
+		}
+	})
+	t.Run("GODEBUG unset", func(t *testing.T) {
+		cert, err := X509KeyPair(certPEM, keyPEM)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if cert.Leaf == nil {
+			t.Fatal("Leaf should be populated")
+		}
+	})
+}