wgengine/netstack: make userspace ping work when tailscaled has CAP_NET_RAW

Updates #3710

Change-Id: Ief56c7ac20f5f09a2f940a1906b9efbf1b0d6932
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -41,6 +41,7 @@ import (
 	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
+	"tailscale.com/version/distro"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/magicsock"
@@ -394,8 +395,14 @@ func (ns *Impl) shouldProcessInbound(p *packet.Parsed, t *tstun.Wrapper) bool {
 	return false
 }
 
+// setAmbientCapsRaw is non-nil on Linux for Synology, to run ping with
+// CAP_NET_RAW from tailscaled's binary.
+var setAmbientCapsRaw func(*exec.Cmd)
+
 var userPingSem = syncs.NewSemaphore(20) // 20 child ping processes at once
 
+var isSynology = runtime.GOOS == "linux" && distro.Get() == distro.Synology
+
 // userPing tried to ping dstIP and if it succeeds, injects pingResPkt
 // into the tundev.
 //
@@ -426,11 +433,21 @@ func (ns *Impl) userPing(dstIP netaddr.IP, pingResPkt []byte) {
 		}
 		err = exec.Command(ping, "-c", "1", "-w", "3", dstIP.String()).Run()
 	default:
-		err = exec.Command("ping", "-c", "1", "-W", "3", dstIP.String()).Run()
+		ping := "ping"
+		if isSynology {
+			ping = "/bin/ping"
+		}
+		cmd := exec.Command(ping, "-c", "1", "-W", "3", dstIP.String())
+		if isSynology && os.Getuid() != 0 {
+			// On DSM7 we run as non-root and need to pass
+			// CAP_NET_RAW if our binary has it.
+			setAmbientCapsRaw(cmd)
+		}
+		err = cmd.Run()
 	}
 	d := time.Since(t0)
 	if err != nil {
-		ns.logf("exec ping of %v failed in %v", dstIP, d)
+		ns.logf("exec ping of %v failed in %v: %v", dstIP, d, err)
 		return
 	}
 	if debugNetstack {

wgengine/netstack/netstack_linux.go

@@ -0,0 +1,20 @@
+// 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.
+
+package netstack
+
+import (
+	"os/exec"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	setAmbientCapsRaw = func(cmd *exec.Cmd) {
+		cmd.SysProcAttr = &syscall.SysProcAttr{
+			AmbientCaps: []uintptr{unix.CAP_NET_RAW},
+		}
+	}
+}