encoding/protodelim: error out when size exceeds math.MaxInt

This results in an error (instead of a runtime panic) when users
explicitly specify MaxSize=-1 and then process a too-large message.

Fixes golang/protobuf#1706

For protocolbuffers/protobuf#25069

Change-Id: I8faee6a177bb8614b6ea7eff25b8cab770a7d9c7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/734760
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

Author: stapelberg

encoding/protodelim/protodelim.go

@@ -10,6 +10,7 @@ import (
 	"encoding/binary"
 	"fmt"
 	"io"
+	"math"
 
 	"google.golang.org/protobuf/encoding/protowire"
 	"google.golang.org/protobuf/internal/errors"
@@ -117,7 +118,13 @@ func (o UnmarshalOptions) UnmarshalFrom(r Reader, m proto.Message) error {
 	if maxSize == 0 {
 		maxSize = defaultMaxSize
 	}
-	if maxSize != -1 && size > uint64(maxSize) {
+	if maxSize == -1 {
+		// No limit specified: Just check that size fits into an integer,
+		// otherwise the make([]byte, size) call below will panic.
+		if size > math.MaxInt {
+			return errors.Wrap(&SizeTooLargeError{Size: size, MaxSize: math.MaxInt}, "")
+		}
+	} else if size > uint64(maxSize) {
 		return errors.Wrap(&SizeTooLargeError{Size: size, MaxSize: uint64(maxSize)}, "")
 	}
 

encoding/protodelim/protodelim_test.go

@@ -10,6 +10,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"io"
+	"math"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -159,6 +160,26 @@ func TestMaxSize(t *testing.T) {
 	}
 }
 
+func TestMaxSizeOverflow(t *testing.T) {
+	buf := &bytes.Buffer{}
+	sb := protowire.AppendVarint(nil, math.MaxInt+1)
+	if _, err := buf.Write(sb); err != nil {
+		t.Fatalf("buf.Write(%v) = _, %v", sb, err)
+	}
+
+	out := &test3.TestAllTypes{}
+	err := protodelim.UnmarshalOptions{MaxSize: -1}.UnmarshalFrom(bufio.NewReader(buf), out)
+
+	var errSize *protodelim.SizeTooLargeError
+	if !errors.As(err, &errSize) {
+		t.Errorf("protodelim.UnmarshalOptions{MaxSize: 1}.UnmarshalFrom(_, _) = %v (%T), want %T", err, err, errSize)
+	}
+	got, want := errSize, &protodelim.SizeTooLargeError{Size: math.MaxInt + 1, MaxSize: math.MaxInt}
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("protodelim.UnmarshalOptions{MaxSize: 1}.UnmarshalFrom(_, _): diff -want +got = %s", diff)
+	}
+}
+
 func TestUnmarshalFrom_UnexpectedEOF(t *testing.T) {
 	buf := &bytes.Buffer{}