encoding/gob: cover missed cases when checking ignore depth

This change makes sure that we are properly checking the ignored field
recursion depth in decIgnoreOpFor consistently. This prevents stack
exhaustion when attempting to decode a message that contains an
extremely deeply nested struct which is ignored.

Thanks to Md Sakib Anwar of The Ohio State University (anwar.40@osu.edu)
for reporting this issue.

Fixes #69139
Fixes CVE-2024-34156

Change-Id: Iacce06be95a5892b3064f1c40fcba2e2567862d6
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/1440
Reviewed-by: Russ Cox <rsc@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/611239
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

src/encoding/gob/decode.go

@@ -911,8 +911,11 @@ func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProg
 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, depth int) *decOp {
-	if depth > maxIgnoreNestingDepth {
+func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp) *decOp {
+	// Track how deep we've recursed trying to skip nested ignored fields.
+	dec.ignoreDepth++
+	defer func() { dec.ignoreDepth-- }()
+	if dec.ignoreDepth > maxIgnoreNestingDepth {
 		error_(errors.New("invalid nesting depth"))
 	}
 	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
@@ -938,7 +941,7 @@ 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, depth+1)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
 				state.dec.ignoreArray(state, *elemOp, wire.ArrayT.Len)
 			}
@@ -946,15 +949,15 @@ 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, depth+1)
-			elemOp := dec.decIgnoreOpFor(elemId, inProgress, depth+1)
+			keyOp := dec.decIgnoreOpFor(keyId, inProgress)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
 			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, depth+1)
+			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
 			op = func(i *decInstr, state *decoderState, value reflect.Value) {
 				state.dec.ignoreSlice(state, *elemOp)
 			}
@@ -1115,7 +1118,7 @@ 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), 0)
+	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp))
 	ovfl := overflow(dec.typeString(remoteId))
 	engine.instr[0] = decInstr{*op, 0, nil, ovfl}
 	engine.numInstr = 1
@@ -1160,7 +1163,7 @@ 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), 0)
+			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp))
 			engine.instr[fieldnum] = decInstr{*op, fieldnum, nil, ovfl}
 			continue
 		}

src/encoding/gob/decoder.go

@@ -35,6 +35,8 @@ type Decoder struct {
 	freeList     *decoderState                           // list of free decoderStates; avoids reallocation
 	countBuf     []byte                                  // used for decoding integers while parsing messages
 	err          error
+	// ignoreDepth tracks the depth of recursively parsed ignored fields
+	ignoreDepth int
 }
 
 // NewDecoder returns a new decoder that reads from the [io.Reader].

src/encoding/gob/gobencdec_test.go

@@ -806,6 +806,8 @@ func TestIgnoreDepthLimit(t *testing.T) {
 	defer func() { maxIgnoreNestingDepth = oldNestingDepth }()
 	b := new(bytes.Buffer)
 	enc := NewEncoder(b)
+
+	// Nested slice
 	typ := reflect.TypeFor[int]()
 	nested := reflect.ArrayOf(1, typ)
 	for i := 0; i < 100; i++ {
@@ -819,4 +821,16 @@ func TestIgnoreDepthLimit(t *testing.T) {
 	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)
 	}
+
+	// Nested struct
+	nested = reflect.StructOf([]reflect.StructField{{Name: "F", Type: typ}})
+	for i := 0; i < 100; i++ {
+		nested = reflect.StructOf([]reflect.StructField{{Name: "F", Type: nested}})
+	}
+	badStruct = reflect.New(reflect.StructOf([]reflect.StructField{{Name: "F", Type: nested}}))
+	enc.Encode(badStruct.Interface())
+	dec = NewDecoder(b)
+	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)
+	}
 }