encoding/asn1: check bounds when parsing tag and length

This was found while fuzzing another program, triggering a panic in
x509.ParseECPrivateKey.

Fixes #11154

Change-Id: Ief35ead38adf14caec4d37b9eacf8a92e67cd1e6
Reviewed-on: https://go-review.googlesource.com/10712Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

src/encoding/asn1/asn1.go

@@ -20,6 +20,7 @@ package asn1
 // everything by any means.
 
 import (
+	"errors"
 	"fmt"
 	"math/big"
 	"reflect"
@@ -389,6 +390,12 @@ type RawContent []byte
 // don't distinguish between ordered and unordered objects in this code.
 func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset int, err error) {
 	offset = initOffset
+	// parseTagAndLength should not be called without at least a single
+	// byte to read. Thus this check is for robustness:
+	if offset >= len(bytes) {
+		err = errors.New("asn1: internal error in parseTagAndLength")
+		return
+	}
 	b := bytes[offset]
 	offset++
 	ret.class = int(b >> 6)
@@ -611,6 +618,10 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 		if params.application {
 			expectedClass = classApplication
 		}
+		if offset == len(bytes) {
+			err = StructuralError{"explicit tag has no child"}
+			return
+		}
 		if t.class == expectedClass && t.tag == *params.tag && (t.length == 0 || t.isCompound) {
 			if t.length > 0 {
 				t, offset, err = parseTagAndLength(bytes, offset)

src/encoding/asn1/asn1_test.go

@@ -867,3 +867,22 @@ func TestImplicitTaggedTime(t *testing.T) {
 		t.Errorf("Wrong result. Got %v, want %v", result.Time, expected)
 	}
 }
+
+type truncatedExplicitTagTest struct {
+	Test int `asn1:"explicit,tag:0"`
+}
+
+func TestTruncatedExplicitTag(t *testing.T) {
+	// This crashed Unmarshal in the past. See #11154.
+	der := []byte{
+		0x30, // SEQUENCE
+		0x02, // two bytes long
+		0xa0, // context-specific, tag 0
+		0x30, // 48 bytes long
+	}
+
+	var result truncatedExplicitTagTest
+	if _, err := Unmarshal(der, &result); err == nil {
+		t.Error("Unmarshal returned without error")
+	}
+}