Offer Trust this version in the Windows block popup (#81)

* Offer Trust this version in the Windows block popup

* Show the Strix logo in the Windows binaries and block popup (#89)

Co-authored-by: Ian Graham Martinez <[email protected]>

---------

Co-authored-by: Ian Graham Martinez <[email protected]>

core/Cargo.lock

@@ -711,6 +711,20 @@ version = "1.18.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "252afb9ae5eaa683babdc6a068b3f5726eb19e05070c731f9b2a23a7c3e8ed34"
 
+[[package]]
+name = "embed-resource"
+version = "3.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6168487b5e7eb2673e6efe1200b1848016e436a6ca0b8031bf04bd9d493a249f"
+dependencies = [
+ "cc",
+ "memchr",
+ "rustc_version",
+ "toml",
+ "vswhom",
+ "winreg",
+]
+
 [[package]]
 name = "encoding_rs"
 version = "0.8.42"
@@ -3243,6 +3257,15 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_spanned"
+version = "1.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4e7523beb55eece201a2356bee0bbca0d1ab466c14c07703b2e0ee6d42cb0c2c"
+dependencies = [
+ "serde_core",
+]
+
 [[package]]
 name = "sha1"
 version = "0.11.0"
@@ -3375,6 +3398,7 @@ dependencies = [
  "clap",
  "core-foundation 0.10.1",
  "directories",
+ "embed-resource",
  "muda",
  "percent-encoding",
  "png",
@@ -3671,10 +3695,25 @@ dependencies = [
 ]
 
 [[package]]
-name = "toml_datetime"
-version = "1.1.1+spec-1.1.0"
+name = "toml"
+version = "1.1.8+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3165f65f62e28e0115a00b2ebdd37eb6f3b641855f9d636d3cd4103767159ad7"
+checksum = "20489e00e4d8741d6be680764cc12e270655e375a20d1011e844a9c3379e678d"
+dependencies = [
+ "indexmap",
+ "serde_core",
+ "serde_spanned",
+ "toml_datetime",
+ "toml_parser",
+ "toml_writer",
+ "winnow",
+]
+
+[[package]]
+name = "toml_datetime"
+version = "1.1.2+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2b86d767906c6c42421dcba507eb9d203e779497710a47782a224bb871653053"
 dependencies = [
  "serde_core",
 ]
@@ -3693,13 +3732,19 @@ dependencies = [
 
 [[package]]
 name = "toml_parser"
-version = "1.1.3+spec-1.1.0"
+version = "1.1.5+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1d38ac1cf9b95face32296c0a3ede1fdc270627c9d9c02a7274dd6d960dc4d56"
+checksum = "baa693a8032d7e1cada7d0041e96126df243179ff061456783ac7f12bda4744c"
 dependencies = [
  "winnow",
 ]
 
+[[package]]
+name = "toml_writer"
+version = "1.1.3+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "06bdbd8cfc056b8d2e2e85f29b56a3bdbecb527cef81eb39e3e7b98af4652770"
+
 [[package]]
 name = "tower-layer"
 version = "0.3.3"
@@ -3909,6 +3954,26 @@ version = "0.9.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b928f33d975fc6ad9f86c8f283853ad26bdd5b10b7f1542aa2fa15e2289105a"
 
+[[package]]
+name = "vswhom"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "be979b7f07507105799e854203b470ff7c78a1639e330a58f183b5fea574608b"
+dependencies = [
+ "libc",
+ "vswhom-sys",
+]
+
+[[package]]
+name = "vswhom-sys"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fb067e4cbd1ff067d1df46c9194b5de0e98efd2810bbc95c5d5e5f25a3231150"
+dependencies = [
+ "cc",
+ "libc",
+]
+
 [[package]]
 name = "walkdir"
 version = "2.5.0"
@@ -4284,6 +4349,16 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "winreg"
+version = "0.56.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7d6f32a0ff4a9f6f01231eb2059cc85479330739333e0e58cadf03b6af2cca10"
+dependencies = [
+ "cfg-if",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "wit-bindgen"
 version = "0.57.1"

core/Cargo.toml

@@ -129,7 +129,7 @@ schannel = "0.1.29"
 tokio = { version = "1.53.1", features = ["net"] }
 tracing-appender = "0.2.5"
 tray-icon = { version = "0.26.1", default-features = false }
-windows = { version = "0.62", features = ["Win32_Foundation", "Win32_Security", "Win32_Security_Authorization", "Win32_Security_Cryptography", "Win32_Storage_FileSystem", "Win32_System_Com", "Win32_System_Console", "Win32_System_Environment", "Win32_System_JobObjects", "Win32_System_Pipes", "Win32_System_Threading", "Win32_UI_Shell", "Win32_UI_WindowsAndMessaging"] }
+windows = { version = "0.62", features = ["Win32_Foundation", "Win32_Security", "Win32_Security_Authorization", "Win32_Security_Cryptography", "Win32_Storage_FileSystem", "Win32_System_Com", "Win32_System_Console", "Win32_System_Environment", "Win32_System_JobObjects", "Win32_System_LibraryLoader", "Win32_System_Pipes", "Win32_System_Registry", "Win32_System_Threading", "Win32_UI_Controls", "Win32_UI_Shell", "Win32_UI_WindowsAndMessaging"] }
 windows-registry = "0.6.1"
 windows-service = "0.8.0"
 
@@ -139,3 +139,7 @@ tokio = { version = "1.53.1", features = ["macros", "rt-multi-thread"] }
 [profile.dist]
 inherits = "release"
 lto = "thin"
+
+# Only on Windows hosts, where the tray and service are built.
+[target.'cfg(windows)'.build-dependencies]
+embed-resource = "3.0.12"

core/build.rs

@@ -0,0 +1,16 @@
+//! The tray's task dialogs exist only in version 6 of the common controls, which takes a manifest.
+//! Both machine-wide binaries also get the Strix icon.
+fn main() {
+    println!("cargo:rerun-if-changed=src/bin/firewall-tray.manifest");
+    let msvc = std::env::var("CARGO_CFG_TARGET_ENV").as_deref() == Ok("msvc");
+    if msvc && std::env::var_os("CARGO_FEATURE_MACHINE_WIDE").is_some() {
+        let manifest = concat!(env!("CARGO_MANIFEST_DIR"), "/src/bin/firewall-tray.manifest");
+        println!("cargo:rustc-link-arg-bin=firewall-tray=/MANIFEST:EMBED");
+        println!("cargo:rustc-link-arg-bin=firewall-tray=/MANIFESTINPUT:{manifest}");
+        #[cfg(windows)]
+        embed_resource::compile_for("src/bin/firewall.rc", ["firewall-tray", "firewall-service"], embed_resource::NONE)
+            // Fails the build, rather than ship without the icon.
+            .manifest_required()
+            .unwrap();
+    }
+}

core/src/bin/firewall-tray.manifest

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+  <dependency>
+    <dependentAssembly>
+      <assemblyIdentity type="win32" name="Microsoft.Windows.Common-Controls" version="6.0.0.0"
+        processorArchitecture="*" publicKeyToken="6595b64144ccf1df" language="*"/>
+    </dependentAssembly>
+  </dependency>
+</assembly>

core/src/bin/firewall.rc

@@ -0,0 +1,2 @@
+// The icon Explorer, UAC and the tray's popups show.
+1 ICON "strix.ico"

core/src/os/windows/pipe.rs

@@ -1,24 +1,32 @@
-//! How the tray asks the service what it blocked, as the macOS app messages its extension.
+//! How the tray asks the service what it blocked, and allows a new package, as the macOS app
+//! messages its extension.
 
-use super::security_descriptor;
-use crate::blocks;
+use super::{config_file, security_descriptor};
+use crate::{blocks, Firewall};
 use rama::error::BoxError;
+use std::os::windows::io::AsRawHandle;
+use std::sync::Arc;
 use std::time::Duration;
 use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncWriteExt, BufReader};
 use tokio::net::windows::named_pipe::{NamedPipeServer, ServerOptions};
-use windows::Win32::Security::SECURITY_ATTRIBUTES;
+use windows::core::{Owned, PWSTR};
+use windows::Win32::Foundation::HANDLE;
+use windows::Win32::Security::*;
+use windows::Win32::System::Pipes::{GetNamedPipeClientProcessId, ImpersonateNamedPipeClient};
+use windows::Win32::System::Threading::*;
 
 pub(super) const PIPE: &str = r"\\.\pipe\StrixFirewall";
 /// Signed-in users can connect, as the tray runs as them.
 const PIPE_ACL: &str = "D:P(A;;GA;;;SY)(A;;GA;;;BA)(A;;GRGW;;;IU)";
 
-pub(super) async fn serve() -> Result<(), BoxError> {
+pub(super) async fn serve(firewall: Arc<Firewall>) -> Result<(), BoxError> {
     let mut server = create(true)?;
     loop {
         server.connect().await?;
         let client = std::mem::replace(&mut server, create(false)?);
+        let firewall = Arc::clone(&firewall);
         tokio::spawn(async move {
-            if let Err(error) = answer(client).await {
+            if let Err(error) = answer(client, &firewall).await {
                 tracing::debug!(%error, "could not answer the tray");
             }
         });
@@ -40,19 +48,55 @@ fn create(first: bool) -> Result<NamedPipeServer, BoxError> {
 }
 
 /// A line each way.
-async fn answer(client: NamedPipeServer) -> Result<(), BoxError> {
+async fn answer(client: NamedPipeServer, firewall: &Firewall) -> Result<(), BoxError> {
     let mut client = BufReader::new(client);
     let mut request = String::new();
     (&mut client)
         .take(64 * 1024)
         .read_line(&mut request)
         .await?;
-    let reply = match request.trim_end() {
-        "blocks" => serde_json::to_string(&blocks::take())?,
-        other => format!("error: unknown request {other}"),
+    let request = request.trim_end();
+    // Only the app can message the macOS extension; here any program could connect.
+    let reply = if !client_is_tray(client.get_ref())? {
+        "error: only the tray can ask".to_owned()
+    } else if request.starts_with("allow ") && !client_is_admin(client.get_ref())? {
+        "error: trusting a package takes an administrator".to_owned()
+    } else {
+        blocks::answer(firewall, &config_file(), request)
     };
     client.write_all(format!("{reply}\n").as_bytes()).await?;
     // Closing before the tray hangs up could discard the reply.
     let _ = tokio::time::timeout(Duration::from_secs(5), client.read(&mut [0])).await;
     Ok(())
 }
+
+fn client_is_tray(client: &NamedPipeServer) -> Result<bool, BoxError> {
+    let mut id = 0;
+    unsafe { GetNamedPipeClientProcessId(HANDLE(client.as_raw_handle()), &mut id)? };
+    let process = unsafe { Owned::new(OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, false, id)?) };
+    let mut path = [0u16; 1024];
+    let mut len = path.len() as u32;
+    let flags = PROCESS_NAME_WIN32;
+    unsafe { QueryFullProcessImageNameW(*process, flags, PWSTR(path.as_mut_ptr()), &mut len)? };
+    let path = String::from_utf16(&path[..len as usize])?;
+    let tray = std::env::current_exe()?.with_file_name("firewall-tray.exe");
+    // Windows paths ignore case.
+    Ok(path.to_lowercase() == tray.to_string_lossy().to_lowercase())
+}
+
+/// Elevated, as only an elevated token has the Administrators group enabled.
+fn client_is_admin(client: &NamedPipeServer) -> windows::core::Result<bool> {
+    let mut sid = [0u32; SECURITY_MAX_SID_SIZE as usize / 4];
+    let admins = PSID(sid.as_mut_ptr().cast());
+    let mut size = SECURITY_MAX_SID_SIZE;
+    unsafe { CreateWellKnownSid(WinBuiltinAdministratorsSid, None, Some(admins), &mut size)? };
+    unsafe { ImpersonateNamedPipeClient(HANDLE(client.as_raw_handle()))? };
+    let mut member = false.into();
+    let checked = unsafe { CheckTokenMembership(None, admins, &mut member) };
+    // Carrying on as the client would be worse than stopping.
+    if unsafe { RevertToSelf() }.is_err() {
+        std::process::abort();
+    }
+    checked?;
+    Ok(member.as_bool())
+}

core/src/os/windows/service.rs

@@ -155,8 +155,9 @@ async fn start_screening(status: &ServiceStatusHandle) -> Result<Vec<&'static st
     let crl = HttpServer::auto(exec.clone()).service(crl);
     tokio::spawn(listen_on(CRL_ADDRESS, 80, &exec)?.serve(crl));
 
-    tokio::spawn(async {
-        if let Err(error) = pipe::serve().await {
+    let tray_firewall = Arc::clone(&firewall);
+    tokio::spawn(async move {
+        if let Err(error) = pipe::serve(tray_firewall).await {
             tracing::error!(%error, "the tray cannot reach the service");
         }
     });

core/src/os/windows/tray.rs

@@ -2,16 +2,19 @@
 
 use super::pipe::PIPE;
 use super::*;
-use muda::{Menu, MenuEvent, MenuItem, PredefinedMenuItem, Submenu};
+use muda::{IsMenuItem, Menu, MenuEvent, MenuItem, PredefinedMenuItem, Submenu};
 use serde::Deserialize;
 use std::io::{BufRead, BufReader, Cursor, Write};
 use std::os::windows::io::AsRawHandle;
+use std::sync::mpsc::{channel, Receiver, Sender};
 use std::time::Duration;
 use tray_icon::{Icon, MouseButtonState, TrayIcon, TrayIconBuilder, TrayIconEvent};
-use windows::core::{w, PCWSTR};
+use windows::core::{w, HRESULT, PCWSTR};
 use windows::Win32::Foundation::*;
+use windows::Win32::System::LibraryLoader::GetModuleHandleW;
 use windows::Win32::System::Pipes::GetNamedPipeServerProcessId;
-use windows::Win32::System::Threading::CreateMutexW;
+use windows::Win32::System::Threading::*;
+use windows::Win32::UI::Controls::*;
 use windows::Win32::UI::Shell::*;
 use windows::Win32::UI::WindowsAndMessaging::*;
 use windows_registry::CURRENT_USER;
@@ -22,6 +25,9 @@ const MENU_PAUSE: &str = "pause";
 const MENU_RESTART: &str = "restart";
 const MENU_LOG: &str = "log";
 const MENU_QUIT: &str = "quit";
+/// Also the service's requests, and `allow` the elevated `firewall-tray` command.
+const ALLOW: &str = "allow";
+const ALLOW_ONCE: &str = "allow-once";
 const LOGO: &[u8] = include_bytes!("tray.png");
 /// Where blocked packages are listed, below Open logs.
 const BLOCKED_AT: usize = 5;
@@ -29,6 +35,19 @@ const BLOCKED_KEPT: usize = 10;
 
 /// Runs until Quit, once per session.
 pub fn run() -> Result<(), BoxError> {
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    if let [verb, id, label @ ..] = &args[..] {
+        if verb == ALLOW {
+            // Run by `run_elevated`, so the service can tell an admin asked.
+            let trusted = ask_service(&format!("{ALLOW} {id}"))
+                .and_then(|reply| answered(&format!("Trusted {}", label.join(" ")), &reply));
+            if let Err(error) = trusted {
+                show_error(error);
+                std::process::exit(1);
+            }
+            return Ok(());
+        }
+    }
     // Held until we exit.
     let _mutex = unsafe { CreateMutexW(None, false, w!(r"Local\StrixFirewallTray"))? };
     if unsafe { GetLastError() } == ERROR_ALREADY_EXISTS {
@@ -62,9 +81,11 @@ pub fn run() -> Result<(), BoxError> {
         menu,
         blocked_separator,
         blocked: Vec::new(),
+        allowed: channel(),
     };
     tray.update();
-    unsafe { SetTimer(None, 0, 2000, None) };
+    // The service holds a new package's download only while it is asked this often.
+    unsafe { SetTimer(None, 0, 500, None) };
     let mut message = MSG::default();
     while unsafe { GetMessageW(&mut message, None, 0, 0) }.0 > 0 {
         unsafe { DispatchMessageW(&message) };
@@ -112,10 +133,13 @@ struct Tray {
     blocked_separator: PredefinedMenuItem,
     /// The most recent, as the menu lists them.
     blocked: Vec<BlockedPackage>,
+    /// Allowed from another thread, so no longer listed.
+    allowed: (Sender<BlockedPackage>, Receiver<BlockedPackage>),
 }
 
 #[derive(Clone, PartialEq, Deserialize)]
 struct BlockedPackage {
+    id: u64,
     eco: String,
     name: String,
     version: Option<String>,
@@ -136,6 +160,11 @@ impl BlockedPackage {
     fn malicious(&self) -> bool {
         self.filter != "age"
     }
+
+    /// Malware is never allowed, and a new package only by version.
+    fn trustable(&self) -> bool {
+        !self.malicious() && self.version.is_some()
+    }
 }
 
 impl Tray {
@@ -161,6 +190,11 @@ impl Tray {
 
     /// The service hands each block over once, so whichever tray asks first shows it.
     fn collect_blocks(&mut self) {
+        let allowed: Vec<_> = self.allowed.1.try_iter().collect();
+        if !allowed.is_empty() {
+            self.blocked.retain(|blocked| !allowed.contains(blocked));
+            self.list_blocked();
+        }
         if self.state != Some(ServiceState::Running) {
             return;
         }
@@ -184,7 +218,14 @@ impl Tray {
         self.blocked.drain(..excess);
         self.list_blocked();
         // Off the UI thread, so the menu keeps working while they are open.
-        std::thread::spawn(move || new.iter().for_each(alert));
+        let allowed = self.allowed.0.clone();
+        std::thread::spawn(move || {
+            for package in new {
+                if let Some(trust) = alert(&package) {
+                    allow(package, trust, &allowed);
+                }
+            }
+        });
     }
 
     fn list_blocked(&self) {
@@ -204,14 +245,31 @@ impl Tray {
                 package.reason.clone()
             };
             let reason = MenuItem::new(reason, false, None);
+            let trustable = package.trustable();
+            let once = MenuItem::with_id(format!("{ALLOW_ONCE}:{i}"), "Allow Once", trustable, None);
+            let always = MenuItem::with_id(format!("{ALLOW}:{i}"), "Trust this version", trustable, None);
+            let items: &[&dyn IsMenuItem] = if package.malicious() {
+                &[&reason]
+            } else {
+                &[&reason, &once, &always]
+            };
             let label = format!("{verb} {}", package.label());
-            if let Ok(item) = Submenu::with_items(label, true, &[&reason]) {
+            if let Ok(item) = Submenu::with_items(label, true, items) {
                 let _ = self.menu.insert(&item, BLOCKED_AT + 1 + i);
             }
         }
     }
 
     fn on_menu(&self, id: &str) {
+        if let Some((verb @ (ALLOW | ALLOW_ONCE), i)) = id.split_once(':') {
+            let trust = verb == ALLOW;
+            let package = i.parse().ok().and_then(|i: usize| self.blocked.get(i));
+            if let Some(package) = package.cloned() {
+                let allowed = self.allowed.0.clone();
+                std::thread::spawn(move || allow(package, trust, &allowed));
+            }
+            return;
+        }
         let state = self.state;
         let screening = state == Some(ServiceState::Running);
         let action: fn() -> Result<(), BoxError> = match id {
@@ -237,19 +295,124 @@ impl Tray {
     }
 }
 
-fn alert(package: &BlockedPackage) {
+const ALLOW_ONCE_BUTTON: i32 = 100;
+const TRUST_BUTTON: i32 = 101;
+/// `firewall.rc`'s icon, embedded by `build.rs`.
+const STRIX_ICON: PCWSTR = PCWSTR(1 as _);
+
+/// `Some(true)` if the user chose to trust it, `Some(false)` to allow it once.
+/// A task dialog, as a message box can't have both buttons.
+fn alert(package: &BlockedPackage) -> Option<bool> {
     let text = format!("{} {}.", package.label(), package.reason);
-    let (title, text, style) = if package.malicious() {
+    let (title, text, icon) = if package.malicious() {
         let title = format!("Blocked malware: {}", package.label());
         let text = text
             + "\n\nIt was not installed. Don't install it from another source. If you didn't expect this "
             + "package, a dependency may be compromised; tell your security team.";
-        (title, text, MB_ICONERROR)
+        (title, text, TD_ERROR_ICON)
     } else {
-        ("Held back a new package".to_owned(), text, MB_ICONINFORMATION)
+        ("Held back a new package".to_owned(), text, STRIX_ICON)
     };
+    let button = |id, text| TASKDIALOG_BUTTON { nButtonID: id, pszButtonText: text };
+    let allow = [button(ALLOW_ONCE_BUTTON, w!("Allow Once")), button(TRUST_BUTTON, w!("Trust this version"))];
+    let buttons: &[_] = if package.trustable() { &allow } else { &[] };
     let (title, text) = (HSTRING::from(title), HSTRING::from(text));
-    unsafe { MessageBoxW(None, &text, &title, style | MB_SETFOREGROUND) };
+    let dialog = TASKDIALOGCONFIG {
+        cbSize: size_of::<TASKDIALOGCONFIG>() as u32,
+        // Where `STRIX_ICON` is loaded from; the `TD_` icons are Windows'.
+        hInstance: unsafe { GetModuleHandleW(None) }.ok()?.into(),
+        dwCommonButtons: TDCBF_CLOSE_BUTTON,
+        pszWindowTitle: w!("Strix Firewall"),
+        Anonymous1: TASKDIALOGCONFIG_0 { pszMainIcon: icon },
+        pszMainInstruction: PCWSTR(title.as_ptr()),
+        pszContent: PCWSTR(text.as_ptr()),
+        cButtons: buttons.len() as u32,
+        pButtons: buttons.as_ptr(),
+        pfCallback: Some(on_dialog),
+        ..Default::default()
+    };
+    let mut chosen = 0;
+    unsafe { TaskDialogIndirect(&dialog, Some(&raw mut chosen), None, None) }.ok()?;
+    match chosen {
+        ALLOW_ONCE_BUTTON => Some(false),
+        TRUST_BUTTON => Some(true),
+        _ => None,
+    }
+}
+
+/// Asks for the foreground, as `MB_SETFOREGROUND` did; Windows may still leave it behind.
+unsafe extern "system" fn on_dialog(
+    window: HWND,
+    event: TASKDIALOG_NOTIFICATIONS,
+    _: WPARAM,
+    _: LPARAM,
+    _: isize,
+) -> HRESULT {
+    if event == TDN_CREATED {
+        let _ = unsafe { SetForegroundWindow(window) };
+    }
+    S_OK
+}
+
+/// Trusting writes the config for every user, so it goes through a UAC prompt.
+/// The elevated process shows its own errors and late replies.
+fn allow(package: BlockedPackage, trust: bool, allowed: &Sender<BlockedPackage>) {
+    let allowed_it = if trust {
+        run_elevated(&[ALLOW, &package.id.to_string(), &package.label()])
+            .map(|code| code == Some(0))
+    } else {
+        let title = format!("Allowed {} for the next minute", package.label());
+        ask_service(&format!("{ALLOW_ONCE} {}", package.id))
+            .and_then(|reply| answered(&title, &reply))
+            .map(|()| true)
+    };
+    match allowed_it {
+        Ok(true) => {
+            let _ = allowed.send(package);
+        }
+        Ok(false) => {}
+        Err(error) => show_error(error),
+    }
+}
+
+/// Tells the user, under `title`, when the install had already stopped waiting.
+fn answered(title: &str, reply: &str) -> Result<(), BoxError> {
+    match reply {
+        "ok" => Ok(()),
+        "late" => {
+            let text = "The install had already stopped waiting for it. Run it again to finish.";
+            let (title, text) = (HSTRING::from(title), HSTRING::from(text));
+            unsafe { MessageBoxW(None, &text, &title, MB_ICONINFORMATION | MB_SETFOREGROUND) };
+            Ok(())
+        }
+        other => Err(other.into()),
+    }
+}
+
+/// This program as an admin, waiting for its exit code; `None` if the user said no.
+fn run_elevated(args: &[&str]) -> Result<Option<u32>, BoxError> {
+    let exe = HSTRING::from(std::env::current_exe()?.as_path());
+    let parameters = HSTRING::from(args.join(" "));
+    let mut info = SHELLEXECUTEINFOW {
+        cbSize: size_of::<SHELLEXECUTEINFOW>() as u32,
+        fMask: SEE_MASK_NOCLOSEPROCESS,
+        lpVerb: w!("runas"),
+        lpFile: PCWSTR(exe.as_ptr()),
+        lpParameters: PCWSTR(parameters.as_ptr()),
+        nShow: SW_HIDE.0,
+        ..Default::default()
+    };
+    if let Err(error) = unsafe { ShellExecuteExW(&mut info) } {
+        if error.code() == ERROR_CANCELLED.to_hresult() {
+            return Ok(None);
+        }
+        return Err(error.into());
+    }
+    let process = unsafe { Owned::new(info.hProcess) };
+    unsafe { WaitForSingleObject(*process, INFINITE) };
+    let mut code = 1;
+    unsafe { GetExitCodeProcess(*process, &mut code)? };
+    Ok(Some(code))
 }
 
 /// One request per connection, as the service answers.