hostinfo: change Windows implementation to directly query version information using API and registry

We replace the cmd.exe invocation with RtlGetNtVersionNumbers for the first
three fields. On Windows 10+, we query for the fourth field which is available
via the registry.

The fourth field is not really documented anywhere; Firefox has been querying
it successfully since Windows 10 was released, so we can be pretty confident in
its longevity at this point.

Fixes https://github.com/tailscale/tailscale/issues/1478

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

cmd/tailscale/depaware.txt

@@ -103,7 +103,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         golang.org/x/sys/cpu                                         from golang.org/x/crypto/blake2b+
   LD    golang.org/x/sys/unix                                        from tailscale.com/net/netns+
    W    golang.org/x/sys/windows                                     from golang.org/x/sys/windows/registry+
-   W    golang.org/x/sys/windows/registry                            from golang.zx2c4.com/wireguard/windows/tunnel/winipcfg
+   W    golang.org/x/sys/windows/registry                            from golang.zx2c4.com/wireguard/windows/tunnel/winipcfg+
         golang.org/x/text/secure/bidirule                            from golang.org/x/net/idna
         golang.org/x/text/transform                                  from golang.org/x/text/secure/bidirule+
         golang.org/x/text/unicode/bidi                               from golang.org/x/net/idna+

hostinfo/hostinfo_windows.go

@@ -5,10 +5,11 @@
 package hostinfo
 
 import (
-	"os/exec"
-	"strings"
+	"fmt"
 	"sync/atomic"
-	"syscall"
+
+	"golang.org/x/sys/windows"
+	"golang.org/x/sys/windows/registry"
 )
 
 func init() {
@@ -21,19 +22,37 @@ func osVersionWindows() string {
 	if s, ok := winVerCache.Load().(string); ok {
 		return s
 	}
-	cmd := exec.Command("cmd", "/c", "ver")
-	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
-	out, _ := cmd.Output() // "\nMicrosoft Windows [Version 10.0.19041.388]\n\n"
-	s := strings.TrimSpace(string(out))
-	s = strings.TrimPrefix(s, "Microsoft Windows [")
-	s = strings.TrimSuffix(s, "]")
-
-	// "Version 10.x.y.z", with "Version" localized. Keep only stuff after the space.
-	if sp := strings.Index(s, " "); sp != -1 {
-		s = s[sp+1:]
+	major, minor, build := windows.RtlGetNtVersionNumbers()
+	s := fmt.Sprintf("%d.%d.%d", major, minor, build)
+	// Windows 11 still uses 10 as its major number internally
+	if major == 10 {
+		if ubr, err := getUBR(); err == nil {
+			s += fmt.Sprintf(".%d", ubr)
+		}
 	}
 	if s != "" {
 		winVerCache.Store(s)
 	}
 	return s // "10.0.19041.388", ideally
 }
+
+// getUBR obtains a fourth version field, the "Update Build Revision",
+// from the registry. This field is only available beginning with Windows 10.
+func getUBR() (uint32, error) {
+	key, err := registry.OpenKey(registry.LOCAL_MACHINE,
+		`SOFTWARE\Microsoft\Windows NT\CurrentVersion`, registry.QUERY_VALUE|registry.WOW64_64KEY)
+	if err != nil {
+		return 0, err
+	}
+	defer key.Close()
+
+	val, valType, err := key.GetIntegerValue("UBR")
+	if err != nil {
+		return 0, err
+	}
+	if valType != registry.DWORD {
+		return 0, registry.ErrUnexpectedType
+	}
+
+	return uint32(val), nil
+}