feat: secure Windows client spool state

internal/client/spool/private_windows.go

@@ -0,0 +1,76 @@
+//go:build windows
+
+package spool
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+const privateStateSDDL = "D:P(A;;FA;;;SY)(A;;FA;;;BA)"
+
+// Windows service state is owned by LocalSystem. The DACL is protected against
+// parent inheritance and grants full access only to SYSTEM and Administrators;
+// unprivileged execution contexts access it solely through the service.
+func ensurePrivateFile(path string) error {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR, 0o600)
+	if err == nil {
+		err = file.Close()
+	} else if !errors.Is(err, os.ErrExist) {
+		return err
+	}
+	if err != nil {
+		return err
+	}
+	info, err := os.Lstat(path)
+	if err != nil {
+		return err
+	}
+	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 || hasReparsePoint(path) {
+		return fmt.Errorf("%w: %s is not a regular private file", ErrUnsafeDataDirectory, path)
+	}
+	return applyPrivateStateACL(path)
+}
+
+func ensurePrivateDirectory(path string) error {
+	info, err := os.Lstat(path)
+	if os.IsNotExist(err) {
+		if err := os.MkdirAll(path, 0o700); err != nil {
+			return err
+		}
+		info, err = os.Lstat(path)
+	}
+	if err != nil {
+		return err
+	}
+	if !info.IsDir() || info.Mode()&os.ModeSymlink != 0 || hasReparsePoint(path) {
+		return fmt.Errorf("%w: %s is not a real private directory", ErrUnsafeDataDirectory, path)
+	}
+	return applyPrivateStateACL(path)
+}
+
+func securePrivateFile(path string) error { return applyPrivateStateACL(path) }
+
+func hasReparsePoint(path string) bool {
+	value, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return true
+	}
+	attributes, err := windows.GetFileAttributes(value)
+	return err != nil || attributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0
+}
+
+func applyPrivateStateACL(path string) error {
+	descriptor, err := windows.SecurityDescriptorFromString(privateStateSDDL)
+	if err != nil {
+		return err
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil {
+		return err
+	}
+	return windows.SetNamedSecurityInfo(path, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION, nil, nil, dacl, nil)
+}