safesocket, ipn/ipnserver: look up peer creds on Darwin

And open up socket permissions like Linux, now that we know who connections are from.

WIP DO NOT SUBMIT

Updates #1347
Fixes #1348

go.mod

@@ -42,3 +42,5 @@ require (
 	inet.af/netaddr v0.0.0-20210105212526-648fbc18a69d
 	rsc.io/goversion v1.2.0
 )
+
+replace golang.org/x/sys => /Users/bradfitz/src/golang.org/x/sys

ipn/ipnserver/conn_darwin.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin,!redo
+
+package ipnserver
+
+import (
+	"net"
+
+	"golang.org/x/sys/unix"
+	"tailscale.com/types/logger"
+)
+
+const (
+	xLOCAL_PEERCRED  = 0x1
+	xLOCAL_PEEREPID  = 0x3
+	xLOCAL_PEEREUUID = 0x5
+	xLOCAL_PEERPID   = 0x2
+	xLOCAL_PEERUUID  = 0x4
+)
+
+func isReadonlyConn(c net.Conn, logf logger.Logf) (ro bool) {
+	ro = true // conservative default for naked returns below
+	uc, ok := c.(*net.UnixConn)
+	if !ok {
+		logf("unexpected connection type %T", c)
+		return
+	}
+	raw, err := uc.SyscallConn()
+	if err != nil {
+		logf("SyscallConn: %v", err)
+		return
+	}
+
+	var cred unix.Xucred
+	cerr := raw.Control(func(fd uintptr) {
+		err = unix.GetsockoptXucred(int(fd), unix.SOL_LOCAL, unix.LOCAL_PEERCRED, &cred)
+	})
+	if cerr != nil {
+		logf("raw.Control: %v", err)
+		return
+	}
+	if err != nil {
+		logf("raw.GetsockoptXucred: %v", err)
+		return
+	}
+	logf("XXX got creds %+v", cred)
+	if cred.Uid == 0 {
+		// root is not read-only.
+		return false
+	}
+	logf("non-root connection from %v (read-only)", cred.Uid)
+	return true
+}

ipn/ipnserver/conn_no_ucred.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux
+// +build !linux,!darwin darwin,redo
 
 package ipnserver
 

safesocket/unixsocket.go

@@ -103,8 +103,9 @@ func tailscaledRunningUnderLaunchd() bool {
 // socketPermissionsForOS returns the permissions to use for the
 // tailscaled.sock.
 func socketPermissionsForOS() os.FileMode {
-	if runtime.GOOS == "linux" {
-		// On Linux, the ipn/ipnserver package looks at the Unix peer creds
+	switch runtime.GOOS {
+	case "linux", "darwin":
+		// On Linux and Farwin, the ipn/ipnserver package looks at the Unix peer creds
 		// and only permits read-only actions from non-root users, so we want
 		// this opened up wider.
 		//