[release-branch.go1.17] encoding/gob: add a depth limit for ignored fields

Enforce a nesting limit of 10,000 for ignored fields during decoding
of messages. This prevents the possibility of triggering stack
exhaustion.

Fixes #53709
Updates #53615
Fixes CVE-2022-30635

Change-Id: I05103d06dd5ca3945fcba3c1f5d3b5a645e8fb0f
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1484771
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
(cherry picked from commit 55e8f938d22bfec29cc9dc9671044c5a41d1ea9c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/417074
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/encoding/gob/decode.go

@@ -871,8 +871,13 @@ func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProg
 	return &op
 }
 
+var maxIgnoreNestingDepth = 10000
+
 // decIgnoreOpFor returns the decoding op for a field that has no destination.
-func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp) *decOp {
+func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp, depth int) *decOp {
+	if depth > maxIgnoreNestingDepth {
+		error_(errors.New("invalid nesting depth"))
+	}
 	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
 	// Return the pointer to the op we're already building.
 	if opPtr := inProgress[wireId]; opPtr != nil {
@@ -896,7 +901,7 @@ func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp)
 			errorf("bad data: undefined type %s", wireId.string())
 		case wire.ArrayT != nil:
 			elemId := wire.ArrayT.Elem
-			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress, depth+1)
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
 				state.dec.ignoreArray(state, *elemOp, wire.ArrayT.Len)
 			}
@@ -904,15 +909,15 @@ func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp)
 		case wire.MapT != nil:
 			keyId := dec.wireType[wireId].MapT.Key
 			elemId := dec.wireType[wireId].MapT.Elem
-			keyOp := dec.decIgnoreOpFor(keyId, inProgress)
+			keyOp := dec.decIgnoreOpFor(keyId, inProgress, depth+1)
-			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress, depth+1)
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
 				state.dec.ignoreMap(state, *keyOp, *elemOp)
 			}
 
 		case wire.SliceT != nil:
 			elemId := wire.SliceT.Elem
-			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress, depth+1)
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
 				state.dec.ignoreSlice(state, *elemOp)
 			}
@@ -1073,7 +1078,7 @@ func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *de
 func (dec *Decoder) compileIgnoreSingle(remoteId typeId) *decEngine {
 	engine := new(decEngine)
 	engine.instr = make([]decInstr, 1) // one item
-	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp))
+	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp), 0)
 	ovfl := overflow(dec.typeString(remoteId))
 	engine.instr[0] = decInstr{*op, 0, nil, ovfl}
 	engine.numInstr = 1
@@ -1118,7 +1123,7 @@ func (dec *Decoder) compileDec(remoteId typeId, ut *userTypeInfo) (engine *decEn
 		localField, present := srt.FieldByName(wireField.Name)
 		// TODO(r): anonymous names
 		if !present || !isExported(wireField.Name) {
-			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp))
+			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp), 0)
 			engine.instr[fieldnum] = decInstr{*op, fieldnum, nil, ovfl}
 			continue
 		}

src/encoding/gob/gobencdec_test.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -796,3 +797,26 @@ func TestNetIP(t *testing.T) {
 		t.Errorf("decoded to %v, want 1.2.3.4", ip.String())
 	}
 }
+
+func TestIngoreDepthLimit(t *testing.T) {
+	// We don't test the actual depth limit because it requires building an
+	// extremely large message, which takes quite a while.
+	oldNestingDepth := maxIgnoreNestingDepth
+	maxIgnoreNestingDepth = 100
+	defer func() { maxIgnoreNestingDepth = oldNestingDepth }()
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	typ := reflect.TypeOf(int(0))
+	nested := reflect.ArrayOf(1, typ)
+	for i := 0; i < 100; i++ {
+		nested = reflect.ArrayOf(1, nested)
+	}
+	badStruct := reflect.New(reflect.StructOf([]reflect.StructField{{Name: "F", Type: nested}}))
+	enc.Encode(badStruct.Interface())
+	dec := NewDecoder(b)
+	var output struct{ Hello int }
+	expectedErr := "invalid nesting depth"
+	if err := dec.Decode(&output); err == nil || err.Error() != expectedErr {
+		t.Errorf("Decode didn't fail with depth limit of 100: want %q, got %q", expectedErr, err)
+	}
+}