wgengine/router: look up absolute path to netsh.exe on Windows

This is in response to logs from a customer that show that we're unable
to run netsh due to the following error:

    router: firewall: adding Tailscale-Process rule to allow UDP for "C:\\Program Files\\Tailscale\\tailscaled.exe" ...
    router: firewall: error adding Tailscale-Process rule: exec: "netsh": cannot run executable found relative to current directory:

There's approximately no reason to ever dynamically look up the path of
a system utility like netsh.exe, so instead let's first look for it
in the System32 directory and only if that fails fall back to the
previous behaviour.

Updates #10804

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I68cfeb4cab091c79ccff3187d35f50359a690573

wgengine/router/router_windows.go

@@ -12,6 +12,7 @@
 	"net/netip"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"slices"
 	"strings"
 	"sync"
@@ -155,6 +156,13 @@ type firewallTweaker struct {
 	// stop makes fwProc exit when closed.
 	fwProcWriter  io.WriteCloser
 	fwProcEncoder *json.Encoder
+
+	// The path to the 'netsh.exe' binary, populated during the first call
+	// to runFirewall.
+	//
+	// not protected by mu; netshPath is only mutated inside netshPathOnce
+	netshPathOnce sync.Once
+	netshPath     string
 }
 
 func (ft *firewallTweaker) clear() { ft.set(nil, nil, nil) }
@@ -185,10 +193,43 @@ func (ft *firewallTweaker) set(cidrs []string, routes, localRoutes []netip.Prefi
 	go ft.doAsyncSet()
 }
 
+// getNetshPath returns the path that should be used to execute netsh.
+//
+// We've seen a report from a customer that we're triggering the "cannot run
+// executable found relative to current directory" protection that was added to
+// prevent running possibly attacker-controlled binaries. To mitigate this,
+// first try looking up the path to netsh.exe in the System32 directory
+// explicitly, and then fall back to the prior behaviour of passing "netsh" to
+// os/exec.Command.
+func (ft *firewallTweaker) getNetshPath() string {
+	ft.netshPathOnce.Do(func() {
+		// The default value is the old approach: just run "netsh" and
+		// let os/exec resolve that into a full path.
+		ft.netshPath = "netsh"
+
+		path, err := windows.KnownFolderPath(windows.FOLDERID_System, 0)
+		if err != nil {
+			ft.logf("getNetshPath: error getting FOLDERID_System: %v", err)
+			return
+		}
+
+		expath := filepath.Join(path, "netsh.exe")
+		if _, err := os.Stat(expath); err == nil {
+			ft.netshPath = expath
+			return
+		} else if !os.IsNotExist(err) {
+			ft.logf("getNetshPath: error checking for existence of %q: %v", expath, err)
+		}
+
+		// Keep default
+	})
+	return ft.netshPath
+}
+
 func (ft *firewallTweaker) runFirewall(args ...string) (time.Duration, error) {
 	t0 := time.Now()
 	args = append([]string{"advfirewall", "firewall"}, args...)
-	cmd := exec.Command("netsh", args...)
+	cmd := exec.Command(ft.getNetshPath(), args...)
 	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
 	b, err := cmd.CombinedOutput()
 	if err != nil {

wgengine/router/router_windows_test.go

@@ -0,0 +1,19 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package router
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestGetNetshPath(t *testing.T) {
+	ft := &firewallTweaker{
+		logf: t.Logf,
+	}
+	path := ft.getNetshPath()
+	if !filepath.IsAbs(path) {
+		t.Errorf("expected absolute path for netsh.exe: %q", path)
+	}
+}