encoding/xml: use iterative Skip, rather than recursive

Prevents exhausting the stack limit in _incredibly_ deeply nested
structures.

Fixes #53614
Fixes CVE-2022-28131

Change-Id: I47db4595ce10cecc29fbd06afce7b299868599e6
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1419912
Reviewed-by: Julie Qiu <julieqiu@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/417062
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/encoding/xml/read.go

@@ -747,12 +747,12 @@ Loop:
 }
 
 // Skip reads tokens until it has consumed the end element
-// matching the most recent start element already consumed.
-// It recurs if it encounters a start element, so it can be used to
-// skip nested structures.
+// matching the most recent start element already consumed,
+// skipping nested structures.
 // It returns nil if it finds an end element matching the start
 // element; otherwise it returns an error describing the problem.
 func (d *Decoder) Skip() error {
+	var depth int64
 	for {
 		tok, err := d.Token()
 		if err != nil {
@@ -760,11 +760,12 @@ func (d *Decoder) Skip() error {
 		}
 		switch tok.(type) {
 		case StartElement:
-			if err := d.Skip(); err != nil {
-				return err
-			}
+			depth++
 		case EndElement:
-			return nil
+			if depth == 0 {
+				return nil
+			}
+			depth--
 		}
 	}
 }

src/encoding/xml/read_test.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"io"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -1109,3 +1110,19 @@ func TestCVE202228131(t *testing.T) {
 		t.Fatalf("Unmarshal unexpected error: got %q, want %q", err, errExeceededMaxUnmarshalDepth)
 	}
 }
+
+func TestCVE202230633(t *testing.T) {
+	if runtime.GOARCH == "wasm" {
+		t.Skip("causes memory exhaustion on js/wasm")
+	}
+	defer func() {
+		p := recover()
+		if p != nil {
+			t.Fatal("Unmarshal panicked")
+		}
+	}()
+	var example struct {
+		Things []string
+	}
+	Unmarshal(bytes.Repeat([]byte("<a>"), 17_000_000), &example)
+}