ipn/ipnserver: add TS_PERMIT_CERT_UID envknob to give webservers cert access

So you can run Caddy etc as a non-root user and let it have access to
get certs.

Updates caddyserver/caddy#4541

Change-Id: Iecc5922274530e2b00ba107d4b536580f374109b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -32,6 +32,7 @@
 	"inet.af/netaddr"
 	"inet.af/peercred"
 	"tailscale.com/control/controlclient"
+	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/localapi"
@@ -445,6 +446,26 @@ func (s *Server) localAPIPermissions(ci connIdentity) (read, write bool) {
 	return false, false
 }
 
+// connCanFetchCerts reports whether ci is allowed to fetch HTTPS
+// certs from this server when it wouldn't otherwise be able to.
+//
+// That is, this reports whether ci should grant additional
+// capabilities over what the conn would otherwise be able to do.
+//
+// For now this only returns true on Unix machines when
+// TS_PERMIT_CERT_UID is set the to the userid of the peer
+// connection. It's intended to give your non-root webserver access
+// (www-data, caddy, nginx, etc) to certs.
+func (s *Server) connCanFetchCerts(ci connIdentity) bool {
+	if ci.IsUnixSock && ci.Creds != nil {
+		connUID, ok := ci.Creds.UserID()
+		if ok && connUID == envknob.String("TS_PERMIT_CERT_UID") {
+			return true
+		}
+	}
+	return false
+}
+
 // registerDisconnectSub adds ch as a subscribe to connection disconnect
 // events. If add is false, the subscriber is removed.
 func (s *Server) registerDisconnectSub(ch chan<- struct{}, add bool) {
@@ -1075,6 +1096,7 @@ func (psc *protoSwitchConn) Close() error {
 func (s *Server) localhostHandler(ci connIdentity) http.Handler {
 	lah := localapi.NewHandler(s.b, s.logf, s.backendLogID)
 	lah.PermitRead, lah.PermitWrite = s.localAPIPermissions(ci)
+	lah.PermitCert = s.connCanFetchCerts(ci)
 
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if strings.HasPrefix(r.URL.Path, "/localapi/") {

ipn/localapi/cert.go

@@ -66,7 +66,7 @@ func (h *Handler) certDir() (string, error) {
 var acmeDebug = envknob.Bool("TS_DEBUG_ACME")
 
 func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
-	if !h.PermitWrite {
+	if !h.PermitWrite && !h.PermitCert {
 		http.Error(w, "cert access denied", http.StatusForbidden)
 		return
 	}

ipn/localapi/localapi.go

@@ -52,8 +52,15 @@ type Handler struct {
 	PermitRead bool
 
 	// PermitWrite is whether mutating HTTP handlers are allowed.
+	// If PermitWrite is true, everything is allowed.
+	// It effectively means that the user is root or the admin
+	// (operator user).
 	PermitWrite bool
 
+	// PermitCert is whether the client is additionally granted
+	// cert fetching access.
+	PermitCert bool
+
 	b            *ipnlocal.LocalBackend
 	logf         logger.Logf
 	backendLogID string