cmd/godoc: restrict requests to HTTPS and canonical host names

Fixes golang/go#12684

Change-Id: Ibb49e604bfd92c902c9fd2bf9506ded60b2a9d8d
Reviewed-on: https://go-review.googlesource.com/14814
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/appinit.go

@@ -20,9 +20,12 @@ import (
 	"golang.org/x/tools/godoc/vfs"
 	"golang.org/x/tools/godoc/vfs/mapfs"
 	"golang.org/x/tools/godoc/vfs/zipfs"
+
+	"appengine"
 )
 
 func init() {
+	enforceHosts = !appengine.IsDevAppServer()
 	playEnabled = true
 
 	log.Println("initializing godoc ...")

cmd/godoc/handlers.go

@@ -15,6 +15,7 @@ package main
 import (
 	"log"
 	"net/http"
+	"strings"
 	"text/template"
 
 	"golang.org/x/tools/godoc"
@@ -27,16 +28,51 @@ var (
 	fs   = vfs.NameSpace{}
 )
 
+var enforceHosts = false // set true in production on app engine
+
+// hostEnforcerHandler redirects requests to "http://foo.golang.org/bar"
+// to "https://golang.org/bar".
+// It permits requests to the host "godoc-test.golang.org" for testing.
+type hostEnforcerHandler struct {
+	h http.Handler
+}
+
+func (h hostEnforcerHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if !enforceHosts {
+		h.h.ServeHTTP(w, r)
+		return
+	}
+	if r.TLS == nil || !h.validHost(r.Host) {
+		r.URL.Scheme = "https"
+		if !h.validHost(r.Host) {
+			r.URL.Host = "golang.org"
+		}
+		http.Redirect(w, r, r.URL.String(), http.StatusFound)
+		return
+	}
+	h.h.ServeHTTP(w, r)
+}
+
+func (h hostEnforcerHandler) validHost(host string) bool {
+	switch strings.ToLower(host) {
+	case "golang.org", "godoc-test.golang.org":
+		return true
+	}
+	return false
+}
+
 func registerHandlers(pres *godoc.Presentation) {
 	if pres == nil {
 		panic("nil Presentation")
 	}
-	http.HandleFunc("/doc/codewalk/", codewalk)
-	http.Handle("/doc/play/", pres.FileServer())
-	http.Handle("/robots.txt", pres.FileServer())
-	http.Handle("/", pres)
-	http.Handle("/pkg/C/", redirect.Handler("/cmd/cgo/"))
-	redirect.Register(nil)
+	mux := http.NewServeMux()
+	mux.HandleFunc("/doc/codewalk/", codewalk)
+	mux.Handle("/doc/play/", pres.FileServer())
+	mux.Handle("/robots.txt", pres.FileServer())
+	mux.Handle("/", pres)
+	mux.Handle("/pkg/C/", redirect.Handler("/cmd/cgo/"))
+	redirect.Register(mux)
+	http.Handle("/", hostEnforcerHandler{mux})
 }
 
 func readTemplate(name string) *template.Template {