Make external iframe render work

custom/conf/app.example.ini

@@ -2540,13 +2540,19 @@ LEVEL = Info
 ;; * sanitized: Sanitize the content and render it inside current page, default to only allow a few HTML tags and attributes. Customized sanitizer rules can be defined in [markup.sanitizer.*] .
 ;; * no-sanitizer: Disable the sanitizer and render the content inside current page. It's **insecure** and may lead to XSS attack if the content contains malicious code.
 ;; * iframe: Render the content in a separate standalone page and embed it into current page by iframe. The iframe is in sandbox mode with same-origin disabled, and the JS code are safely isolated from parent page.
-;RENDER_CONTENT_MODE=sanitized
-;;
+;RENDER_CONTENT_MODE = sanitized
+;; The sandbox applied to the iframe and Content-Security-Policy header when RENDER_CONTENT_MODE is `iframe`.
+;; It defaults to a safe set of "allow-*" restrictions (space separated).
+;; You can also set it by your requirements or use "disabled" to disable the sandbox completely.
+;; When set it, make sure there is no security risk:
+;; * PDF-only content: generally safe to use "disabled", and it needs to be "disabled" because PDF only renders with no sandbox.
+;; * HTML content with JS: if the "RENDER_COMMAND" can guarantee there is no XSS, then it is safe, otherwise, you need to fine tune the "allow-*" restrictions.
+;RENDER_CONTENT_SANDBOX =
 ;; Whether post-process the rendered HTML content, including:
 ;; resolve relative links and image sources, recognizing issue/commit references, escaping invisible characters,
 ;; mentioning users, rendering permlink code blocks, replacing emoji shorthands, etc.
 ;; By default, this is true when RENDER_CONTENT_MODE is `sanitized`, otherwise false.
-;NEED_POST_PROCESS=false
+;NEED_POST_PROCESS = false
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

modules/httplib/serve.go

@@ -126,6 +126,7 @@ func setServeHeadersByFile(r *http.Request, w http.ResponseWriter, mineBuf []byt
 		// no sandbox attribute for pdf as it breaks rendering in at least safari. this
 		// should generally be safe as scripts inside PDF can not escape the PDF document
 		// see https://bugs.chromium.org/p/chromium/issues/detail?id=413851 for more discussion
+		// HINT: PDF-RENDER-SANDBOX: PDF won't render in sandboxed context
 		w.Header().Set("Content-Security-Policy", "default-src 'none'; style-src 'unsafe-inline'")
 	}
 

modules/markup/external/external.go

@@ -58,14 +58,11 @@ func (p *Renderer) SanitizerRules() []setting.MarkupSanitizerRule {
 	return p.MarkupSanitizerRules
 }
 
-// SanitizerDisabled disabled sanitize if return true
-func (p *Renderer) SanitizerDisabled() bool {
-	return p.RenderContentMode == setting.RenderContentModeNoSanitizer || p.RenderContentMode == setting.RenderContentModeIframe
-}
-
-// DisplayInIFrame represents whether render the content with an iframe
-func (p *Renderer) DisplayInIFrame() bool {
-	return p.RenderContentMode == setting.RenderContentModeIframe
+func (p *Renderer) GetExternalRendererOptions() (ret markup.ExternalRendererOptions) {
+	ret.SanitizerDisabled = p.RenderContentMode == setting.RenderContentModeNoSanitizer || p.RenderContentMode == setting.RenderContentModeIframe
+	ret.DisplayInIframe = p.RenderContentMode == setting.RenderContentModeIframe
+	ret.ContentSandbox = p.RenderContentSandbox
+	return ret
 }
 
 func envMark(envName string) string {

modules/markup/internal/finalprocessor.go

@@ -5,11 +5,13 @@ package internal
 
 import (
 	"bytes"
+	"html/template"
 	"io"
 )
 
 type finalProcessor struct {
 	renderInternal *RenderInternal
+	extraHeadHTML  template.HTML
 
 	output io.Writer
 	buf    bytes.Buffer
@@ -25,6 +27,32 @@ func (p *finalProcessor) Close() error {
 	// because "postProcess" already does so. In the future we could optimize the code to process data on the fly.
 	buf := p.buf.Bytes()
 	buf = bytes.ReplaceAll(buf, []byte(` data-attr-class="`+p.renderInternal.secureIDPrefix), []byte(` class="`))
-	_, err := p.output.Write(buf)
+
+	tmp := bytes.TrimSpace(buf)
+	isLikelyHTML := len(tmp) != 0 && tmp[0] == '<' && tmp[len(tmp)-1] == '>' && bytes.Index(tmp, []byte(`</`)) > 0
+	if !isLikelyHTML {
+		// not HTML, write back directly
+		_, err := p.output.Write(buf)
+		return err
+	}
+
+	// add our extra head HTML into output
+	headBytes := []byte("<head>")
+	posHead := bytes.Index(buf, headBytes)
+	var part1, part2 []byte
+	if posHead >= 0 {
+		part1, part2 = buf[:posHead+len(headBytes)], buf[posHead+len(headBytes):]
+	} else {
+		part1, part2 = nil, buf
+	}
+	if len(part1) > 0 {
+		if _, err := p.output.Write(part1); err != nil {
+			return err
+		}
+	}
+	if _, err := io.WriteString(p.output, string(p.extraHeadHTML)); err != nil {
+		return err
+	}
+	_, err := p.output.Write(part2)
 	return err
 }

modules/markup/internal/internal_test.go

@@ -12,7 +12,7 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func TestRenderInternal(t *testing.T) {
+func TestRenderInternalAttrs(t *testing.T) {
 	cases := []struct {
 		input, protected, recovered string
 	}{
@@ -30,7 +30,7 @@ func TestRenderInternal(t *testing.T) {
 	for _, c := range cases {
 		var r RenderInternal
 		out := &bytes.Buffer{}
-		in := r.init("sec", out)
+		in := r.init("sec", out, "")
 		protected := r.ProtectSafeAttrs(template.HTML(c.input))
 		assert.EqualValues(t, c.protected, protected)
 		_, _ = io.WriteString(in, string(protected))
@@ -41,7 +41,7 @@ func TestRenderInternal(t *testing.T) {
 	var r1, r2 RenderInternal
 	protected := r1.ProtectSafeAttrs(`<div class="test"></div>`)
 	assert.EqualValues(t, `<div class="test"></div>`, protected, "non-initialized RenderInternal should not protect any attributes")
-	_ = r1.init("sec", nil)
+	_ = r1.init("sec", nil, "")
 	protected = r1.ProtectSafeAttrs(`<div class="test"></div>`)
 	assert.EqualValues(t, `<div data-attr-class="sec:test"></div>`, protected)
 	assert.Equal(t, "data-attr-class", r1.SafeAttr("class"))
@@ -54,8 +54,37 @@ func TestRenderInternal(t *testing.T) {
 	assert.Empty(t, recovered)
 
 	out2 := &bytes.Buffer{}
-	in2 := r2.init("sec-other", out2)
+	in2 := r2.init("sec-other", out2, "")
 	_, _ = io.WriteString(in2, string(protected))
 	_ = in2.Close()
 	assert.Equal(t, `<div data-attr-class="sec:test"></div>`, out2.String(), "different secureID should not recover the value")
 }
+
+func TestRenderInternalExtraHead(t *testing.T) {
+	t.Run("HeadExists", func(t *testing.T) {
+		out := &bytes.Buffer{}
+		var r RenderInternal
+		in := r.init("sec", out, `<MY-TAG>`)
+		_, _ = io.WriteString(in, `<head>any</head>`)
+		_ = in.Close()
+		assert.Equal(t, `<head><MY-TAG>any</head>`, out.String())
+	})
+
+	t.Run("HeadNotExists", func(t *testing.T) {
+		out := &bytes.Buffer{}
+		var r RenderInternal
+		in := r.init("sec", out, `<MY-TAG>`)
+		_, _ = io.WriteString(in, `<div></div>`)
+		_ = in.Close()
+		assert.Equal(t, `<MY-TAG><div></div>`, out.String())
+	})
+
+	t.Run("NotHTML", func(t *testing.T) {
+		out := &bytes.Buffer{}
+		var r RenderInternal
+		in := r.init("sec", out, `<MY-TAG>`)
+		_, _ = io.WriteString(in, `<any>`)
+		_ = in.Close()
+		assert.Equal(t, `<any>`, out.String())
+	})
+}

modules/markup/internal/renderinternal.go

@@ -29,19 +29,19 @@ type RenderInternal struct {
 	secureIDPrefix string
 }
 
-func (r *RenderInternal) Init(output io.Writer) io.WriteCloser {
+func (r *RenderInternal) Init(output io.Writer, extraHeadHTML template.HTML) io.WriteCloser {
 	buf := make([]byte, 12)
 	_, err := rand.Read(buf)
 	if err != nil {
 		panic("unable to generate secure id")
 	}
-	return r.init(base64.URLEncoding.EncodeToString(buf), output)
+	return r.init(base64.URLEncoding.EncodeToString(buf), output, extraHeadHTML)
 }
 
-func (r *RenderInternal) init(secID string, output io.Writer) io.WriteCloser {
+func (r *RenderInternal) init(secID string, output io.Writer, extraHeadHTML template.HTML) io.WriteCloser {
 	r.secureID = secID
 	r.secureIDPrefix = r.secureID + ":"
-	return &finalProcessor{renderInternal: r, output: output}
+	return &finalProcessor{renderInternal: r, output: output, extraHeadHTML: extraHeadHTML}
 }
 
 func (r *RenderInternal) RecoverProtectedValue(v string) (string, bool) {

modules/markup/render.go

@@ -6,12 +6,14 @@ package markup
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"io"
 	"net/url"
 	"strconv"
 	"strings"
 	"time"
 
+	"code.gitea.io/gitea/modules/htmlutil"
 	"code.gitea.io/gitea/modules/markup/internal"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
@@ -163,24 +165,20 @@ func RenderString(ctx *RenderContext, content string) (string, error) {
 	return buf.String(), nil
 }
 
-func renderIFrame(ctx *RenderContext, output io.Writer) error {
-	// set height="0" ahead, otherwise the scrollHeight would be max(150, realHeight)
-	// at the moment, only "allow-scripts" is allowed for sandbox mode.
-	// "allow-same-origin" should never be used, it leads to XSS attack, and it makes the JS in iframe can access parent window's config and CSRF token
-	// TODO: when using dark theme, if the rendered content doesn't have proper style, the default text color is black, which is not easy to read
-	_, err := io.WriteString(output, fmt.Sprintf(`
-<iframe src="%s/%s/%s/render/%s/%s"
-name="giteaExternalRender"
-onload="this.height=giteaExternalRender.document.documentElement.scrollHeight"
-width="100%%" height="0" scrolling="no" frameborder="0" style="overflow: hidden"
-sandbox="allow-scripts"
-></iframe>`,
-		setting.AppSubURL,
+func renderIFrame(ctx *RenderContext, sandbox string, output io.Writer) error {
+	src := fmt.Sprintf("%s/%s/%s/render/%s/%s", setting.AppSubURL,
 		url.PathEscape(ctx.RenderOptions.Metas["user"]),
 		url.PathEscape(ctx.RenderOptions.Metas["repo"]),
-		ctx.RenderOptions.Metas["RefTypeNameSubURL"],
-		url.PathEscape(ctx.RenderOptions.RelativePath),
-	))
+		util.PathEscapeSegments(ctx.RenderOptions.Metas["RefTypeNameSubURL"]),
+		util.PathEscapeSegments(ctx.RenderOptions.RelativePath),
+	)
+
+	var sandboxAttrValue template.HTML
+	if sandbox != "" {
+		sandboxAttrValue = htmlutil.HTMLFormat(`sandbox="%s"`, sandbox)
+	}
+	iframe := htmlutil.HTMLFormat(`<iframe data-src="%s" class="external-render-iframe" %s></iframe>`, src, sandboxAttrValue)
+	_, err := io.WriteString(output, string(iframe))
 	return err
 }
 
@@ -192,22 +190,34 @@ func pipes() (io.ReadCloser, io.WriteCloser, func()) {
 	}
 }
 
+func getExternalRendererOptions(renderer Renderer) (ret ExternalRendererOptions, _ bool) {
+	if externalRender, ok := renderer.(ExternalRenderer); ok {
+		return externalRender.GetExternalRendererOptions(), true
+	}
+	return ret, false
+}
+
 func RenderWithRenderer(ctx *RenderContext, renderer Renderer, input io.Reader, output io.Writer) error {
-	if externalRender, ok := renderer.(ExternalRenderer); ok && externalRender.DisplayInIFrame() {
+	var extraHeadHTML template.HTML
+	if extOpts, ok := getExternalRendererOptions(renderer); ok && extOpts.DisplayInIframe {
 		if !ctx.RenderOptions.InStandalonePage {
 			// for an external "DisplayInIFrame" render, it could only output its content in a standalone page
 			// otherwise, a <iframe> should be outputted to embed the external rendered page
-			return renderIFrame(ctx, output)
+			return renderIFrame(ctx, extOpts.ContentSandbox, output)
 		}
-		// else: this is a standalone page, fallthrough to the real rendering
+		// else: this is a standalone page, fallthrough to the real rendering, and add extra JS/CSS
+		extraStyleHref := setting.AppSubURL + "/assets/css/external-render-iframe.css"
+		extraScriptSrc := setting.AppSubURL + "/assets/js/external-render-iframe.js"
+		// "<script>" must go before "<link>", to make Golang's http.DetectContentType() can still recognize the content as "text/html"
+		extraHeadHTML = htmlutil.HTMLFormat(`<script src="%s"></script><link rel="stylesheet" href="%s">`, extraScriptSrc, extraStyleHref)
 	}
 
 	ctx.usedByRender = true
 	if ctx.RenderHelper != nil {
 		defer ctx.RenderHelper.CleanUp()
 	}
 
-	finalProcessor := ctx.RenderInternal.Init(output)
+	finalProcessor := ctx.RenderInternal.Init(output, extraHeadHTML)
 	defer finalProcessor.Close()
 
 	// input -> (pw1=pr1) -> renderer -> (pw2=pr2) -> SanitizeReader -> finalProcessor -> output
@@ -218,7 +228,7 @@ func RenderWithRenderer(ctx *RenderContext, renderer Renderer, input io.Reader,
 	eg, _ := errgroup.WithContext(ctx)
 	var pw2 io.WriteCloser = util.NopCloser{Writer: finalProcessor}
 
-	if r, ok := renderer.(ExternalRenderer); !ok || !r.SanitizerDisabled() {
+	if r, ok := renderer.(ExternalRenderer); !ok || !r.GetExternalRendererOptions().SanitizerDisabled {
 		var pr2 io.ReadCloser
 		var close2 func()
 		pr2, pw2, close2 = pipes()

modules/markup/renderer.go

@@ -25,13 +25,15 @@ type PostProcessRenderer interface {
 	NeedPostProcess() bool
 }
 
+type ExternalRendererOptions struct {
+	SanitizerDisabled bool
+	DisplayInIframe   bool
+	ContentSandbox    string
+}
+
 // ExternalRenderer defines an interface for external renderers
 type ExternalRenderer interface {
-	// SanitizerDisabled disabled sanitize if return true
-	SanitizerDisabled() bool
-
-	// DisplayInIFrame represents whether render the content with an iframe
-	DisplayInIFrame() bool
+	GetExternalRendererOptions() ExternalRendererOptions
 }
 
 // RendererContentDetector detects if the content can be rendered

modules/setting/markup.go

@@ -63,6 +63,7 @@ type MarkupRenderer struct {
 	NeedPostProcess      bool
 	MarkupSanitizerRules []MarkupSanitizerRule
 	RenderContentMode    string
+	RenderContentSandbox string
 }
 
 // MarkupSanitizerRule defines the policy for whitelisting attributes on
@@ -253,13 +254,22 @@ func newMarkupRenderer(name string, sec ConfigSection) {
 		renderContentMode = RenderContentModeSanitized
 	}
 
+	// ATTENTION! at the moment, only a safe set like "allow-scripts" are allowed for sandbox mode.
+	// "allow-same-origin" should never be used, it leads to XSS attack, and it makes the JS in iframe can access parent window's config and CSRF token
+	renderContentSandbox := sec.Key("RENDER_CONTENT_SANDBOX").MustString("allow-scripts allow-popups")
+	if renderContentSandbox == "disabled" {
+		renderContentSandbox = ""
+	}
+
 	ExternalMarkupRenderers = append(ExternalMarkupRenderers, &MarkupRenderer{
-		Enabled:           sec.Key("ENABLED").MustBool(false),
-		MarkupName:        name,
-		FileExtensions:    exts,
-		Command:           command,
-		IsInputFile:       sec.Key("IS_INPUT_FILE").MustBool(false),
-		RenderContentMode: renderContentMode,
+		Enabled:        sec.Key("ENABLED").MustBool(false),
+		MarkupName:     name,
+		FileExtensions: exts,
+		Command:        command,
+		IsInputFile:    sec.Key("IS_INPUT_FILE").MustBool(false),
+
+		RenderContentMode:    renderContentMode,
+		RenderContentSandbox: renderContentSandbox,
 
 		// if no sanitizer is needed, no post process is needed
 		NeedPostProcess: sec.Key("NEED_POST_PROCESS").MustBool(renderContentMode == RenderContentModeSanitized),