From b5de49917c5f632e49c838222e0a12ceb79c609d Mon Sep 17 00:00:00 2001
From: Roberto Santalla <roobre@users.noreply.github.com>
Date: Mon, 21 Dec 2020 11:55:16 +0100
Subject: [PATCH] cmd/relaypoolsrv: Allow validation of relay join requests by
 certificate (fixes #7196) (#7217)

---
 cmd/strelaypoolsrv/main.go | 21 ++++++++++++++++++++-
 cmd/strelaysrv/main.go     |  2 +-
 cmd/strelaysrv/pool.go     | 22 ++++++++++++++++++++--
 3 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/cmd/strelaypoolsrv/main.go b/cmd/strelaypoolsrv/main.go
index 17b668e68..1e9b57b06 100644
--- a/cmd/strelaypoolsrv/main.go
+++ b/cmd/strelaypoolsrv/main.go
@@ -6,9 +6,11 @@ import (
 	"compress/gzip"
 	"context"
 	"crypto/tls"
+	"crypto/x509"
 	"encoding/json"
 	"flag"
 	"fmt"
+	"github.com/syncthing/syncthing/lib/protocol"
 	"io"
 	"io/ioutil"
 	"log"
@@ -208,6 +210,7 @@ func main() {
 		tlsCfg := &tls.Config{
 			Certificates: []tls.Certificate{cert},
 			MinVersion:   tls.VersionTLS10, // No SSLv3
+			ClientAuth:   tls.RequestClientCert,
 			CipherSuites: []uint16{
 				// No RC4
 				tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
@@ -338,6 +341,12 @@ func handleGetRequest(rw http.ResponseWriter, r *http.Request) {
 }
 
 func handlePostRequest(w http.ResponseWriter, r *http.Request) {
+	var relayCert *x509.Certificate
+	if r.TLS != nil && len(r.TLS.PeerCertificates) > 0 {
+		relayCert = r.TLS.PeerCertificates[0]
+		log.Printf("Got TLS cert from relay server")
+	}
+
 	var newRelay relay
 	err := json.NewDecoder(r.Body).Decode(&newRelay)
 	r.Body.Close()
@@ -359,6 +368,16 @@ func handlePostRequest(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if relayCert != nil {
+		advertisedId := uri.Query().Get("id")
+		idFromCert := protocol.NewDeviceID(relayCert.Raw).String()
+		if advertisedId != idFromCert {
+			log.Println("Warning: Relay server requested to join with an ID different from the join request, rejecting")
+			http.Error(w, "mismatched advertised id and join request cert", http.StatusBadRequest)
+			return
+		}
+	}
+
 	host, port, err := net.SplitHostPort(uri.Host)
 	if err != nil {
 		if debug {
@@ -379,7 +398,7 @@ func handlePostRequest(w http.ResponseWriter, r *http.Request) {
 	if ip == nil || ip.IsUnspecified() {
 		uri.Host = net.JoinHostPort(rhost, port)
 		newRelay.URL = uri.String()
-	} else if host != rhost {
+	} else if host != rhost && relayCert == nil {
 		if debug {
 			log.Println("IP address advertised does not match client IP address", r.RemoteAddr, uri)
 		}
diff --git a/cmd/strelaysrv/main.go b/cmd/strelaysrv/main.go
index 1c0f86d42..b568d0817 100644
--- a/cmd/strelaysrv/main.go
+++ b/cmd/strelaysrv/main.go
@@ -247,7 +247,7 @@ func main() {
 	for _, pool := range pools {
 		pool = strings.TrimSpace(pool)
 		if len(pool) > 0 {
-			go poolHandler(pool, uri, mapping)
+			go poolHandler(pool, uri, mapping, cert)
 		}
 	}
 
diff --git a/cmd/strelaysrv/pool.go b/cmd/strelaysrv/pool.go
index fd8ee83a5..9ddde16c5 100644
--- a/cmd/strelaysrv/pool.go
+++ b/cmd/strelaysrv/pool.go
@@ -4,6 +4,7 @@ package main
 
 import (
 	"bytes"
+	"crypto/tls"
 	"encoding/json"
 	"io/ioutil"
 	"log"
@@ -16,7 +17,7 @@ const (
 	httpStatusEnhanceYourCalm = 429
 )
 
-func poolHandler(pool string, uri *url.URL, mapping mapping) {
+func poolHandler(pool string, uri *url.URL, mapping mapping, ownCert tls.Certificate) {
 	if debug {
 		log.Println("Joining", pool)
 	}
@@ -31,7 +32,24 @@ func poolHandler(pool string, uri *url.URL, mapping mapping) {
 			uriCopy.String(),
 		})
 
-		resp, err := httpClient.Post(pool, "application/json", &b)
+		poolUrl, err := url.Parse(pool)
+		if err != nil {
+			log.Printf("Could not parse pool url '%s': %v", pool, err)
+		}
+
+		client := http.DefaultClient
+		if poolUrl.Scheme == "https" {
+			// Sent our certificate in join request
+			client = &http.Client{
+				Transport: &http.Transport{
+					TLSClientConfig: &tls.Config{
+						Certificates: []tls.Certificate{ownCert},
+					},
+				},
+			}
+		}
+
+		resp, err := client.Post(pool, "application/json", &b)
 		if err != nil {
 			log.Printf("Error joining pool %v: HTTP request: %v", pool, err)
 			time.Sleep(time.Minute)