encoding/asn1: support 31 bit identifiers with OID

The current implementation uses a max of 28 bits when decoding an
ObjectIdentifier.  This change makes it so that an int64 is used to
accumulate up to 35 bits.  If the resulting data would not overflow
an int32, it is used as an int.  Thus up to 31 bits may be used to
represent each subidentifier of an ObjectIdentifier.

Fixes #19933

Change-Id: I95d74b64b24cdb1339ff13421055bce61c80243c
Reviewed-on: https://go-review.googlesource.com/40436Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

src/encoding/asn1/asn1.go

@@ -22,6 +22,7 @@ package asn1
 import (
 	"errors"
 	"fmt"
+	"math"
 	"math/big"
 	"reflect"
 	"strconv"
@@ -293,16 +294,24 @@ type Flag bool
 // given byte slice. It returns the value and the new offset.
 func parseBase128Int(bytes []byte, initOffset int) (ret, offset int, err error) {
 	offset = initOffset
+	var ret64 int64
 	for shifted := 0; offset < len(bytes); shifted++ {
-		if shifted == 4 {
+		// 5 * 7 bits per byte == 35 bits of data
+		// Thus the representation is either non-minimal or too large for an int32
+		if shifted == 5 {
 			err = StructuralError{"base 128 integer too large"}
 			return
 		}
-		ret <<= 7
+		ret64 <<= 7
 		b := bytes[offset]
-		ret |= int(b & 0x7f)
+		ret64 |= int64(b & 0x7f)
 		offset++
 		if b&0x80 == 0 {
+			ret = int(ret64)
+			// Ensure that the returned value fits in an int on all platforms
+			if ret64 > math.MaxInt32 {
+				err = StructuralError{"base 128 integer too large"}
+			}
 			return
 		}
 	}

src/encoding/asn1/asn1_test.go

@@ -7,6 +7,7 @@ package asn1
 import (
 	"bytes"
 	"fmt"
+	"math"
 	"math/big"
 	"reflect"
 	"strings"
@@ -386,6 +387,8 @@ var tagAndLengthData = []tagAndLengthTest{
 	{[]byte{0xa0, 0x81, 0x7f}, false, tagAndLength{}},
 	// Tag numbers which would overflow int32 are rejected. (The value below is 2^31.)
 	{[]byte{0x1f, 0x88, 0x80, 0x80, 0x80, 0x00, 0x00}, false, tagAndLength{}},
+	// Tag numbers that fit in an int32 are valid. (The value below is 2^31 - 1.)
+	{[]byte{0x1f, 0x87, 0xFF, 0xFF, 0xFF, 0x7F, 0x00}, true, tagAndLength{tag: math.MaxInt32}},
 	// Long tag number form may not be used for tags that fit in short form.
 	{[]byte{0x1f, 0x1e, 0x00}, false, tagAndLength{}},
 }