encoding/asn1: return error instead of dereferencing nil *big.Int in marshaling

Fixes #17461

Change-Id: I9954f6ae46c7e15560d7460841be8f2bc37233a9
Reviewed-on: https://go-review.googlesource.com/31121Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/asn1/asn1_test.go

@@ -132,7 +132,11 @@ func TestParseBigInt(t *testing.T) {
 			if ret.String() != test.base10 {
 				t.Errorf("#%d: bad result from %x, got %s want %s", i, test.in, ret.String(), test.base10)
 			}
-			e := makeBigInt(ret)
+			e, err := makeBigInt(ret)
+			if err != nil {
+				t.Errorf("%d: err=%q", i, err)
+				continue
+			}
 			result := make([]byte, e.Len())
 			e.Encode(result)
 			if !bytes.Equal(result, test.in) {

src/encoding/asn1/marshal.go

@@ -150,7 +150,11 @@ func appendBase128Int(dst []byte, n int64) []byte {
 	return dst
 }
 
-func makeBigInt(n *big.Int) encoder {
+func makeBigInt(n *big.Int) (encoder, error) {
+	if n == nil {
+		return nil, StructuralError{"empty integer"}
+	}
+
 	if n.Sign() < 0 {
 		// A negative number has to be converted to two's-complement
 		// form. So we'll invert and subtract 1. If the
@@ -163,20 +167,20 @@ func makeBigInt(n *big.Int) encoder {
 			bytes[i] ^= 0xff
 		}
 		if len(bytes) == 0 || bytes[0]&0x80 == 0 {
-			return multiEncoder([]encoder{byteFFEncoder, bytesEncoder(bytes)})
+			return multiEncoder([]encoder{byteFFEncoder, bytesEncoder(bytes)}), nil
 		}
-		return bytesEncoder(bytes)
+		return bytesEncoder(bytes), nil
 	} else if n.Sign() == 0 {
 		// Zero is written as a single 0 zero rather than no bytes.
-		return byte00Encoder
+		return byte00Encoder, nil
 	} else {
 		bytes := n.Bytes()
 		if len(bytes) > 0 && bytes[0]&0x80 != 0 {
 			// We'll have to pad this with 0x00 in order to stop it
 			// looking like a negative number.
-			return multiEncoder([]encoder{byte00Encoder, bytesEncoder(bytes)})
+			return multiEncoder([]encoder{byte00Encoder, bytesEncoder(bytes)}), nil
 		}
-		return bytesEncoder(bytes)
+		return bytesEncoder(bytes), nil
 	}
 }
 
@@ -409,7 +413,7 @@ func makeBody(value reflect.Value, params fieldParameters) (e encoder, err error
 	case objectIdentifierType:
 		return makeObjectIdentifier(value.Interface().(ObjectIdentifier))
 	case bigIntType:
-		return makeBigInt(value.Interface().(*big.Int)), nil
+		return makeBigInt(value.Interface().(*big.Int))
 	}
 
 	switch v := value; v.Kind() {

src/encoding/asn1/marshal_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"encoding/hex"
 	"math/big"
+	"strings"
 	"testing"
 	"time"
 )
@@ -167,6 +168,29 @@ func TestMarshal(t *testing.T) {
 	}
 }
 
+type marshalErrTest struct {
+	in  interface{}
+	err string
+}
+
+var marshalErrTests = []marshalErrTest{
+	{bigIntStruct{nil}, "empty integer"},
+}
+
+func TestMarshalError(t *testing.T) {
+	for i, test := range marshalErrTests {
+		_, err := Marshal(test.in)
+		if err == nil {
+			t.Errorf("#%d should fail, but success", i)
+			continue
+		}
+
+		if !strings.Contains(err.Error(), test.err) {
+			t.Errorf("#%d got: %v want %v", i, err, test.err)
+		}
+	}
+}
+
 func TestInvalidUTF8(t *testing.T) {
 	_, err := Marshal(string([]byte{0xff, 0xff}))
 	if err == nil {