fix: audit fixes — CString zeroize, FP account check, PAM timeout, blur downscale (v0.6.5)

Address findings from second triple audit (quality, performance, security):

- Wrap PAM CString password in Zeroizing<CString> to wipe on drop (S-H1)
- Add check_account() for pam_acct_mgmt after fingerprint unlock,
  with resume_async() to restart FP on transient failure (S-M1)
- 30s PAM timeout with generation counter to prevent stale result
  interference from parallel auth attempts (S-M3)
- Downscale wallpaper to max 1920px before GPU blur, reducing work
  by ~4x on 4K wallpapers (P-M1)
- exit(1) instead of return on no-monitor after lock.lock() (Q-2.1)

src/lockscreen.rs

@@ -7,7 +7,7 @@ use glib::clone;
 use graphene_rs as graphene;
 use gtk4::prelude::*;
 use gtk4::{self as gtk, gio};
-use std::cell::RefCell;
+use std::cell::{Cell, RefCell};
 use std::path::Path;
 use std::rc::Rc;
 
@@ -32,6 +32,7 @@ pub struct LockscreenHandles {
 
 const AVATAR_SIZE: i32 = 128;
 const FAILLOCK_MAX_ATTEMPTS: u32 = 3;
+const PAM_TIMEOUT_SECS: u64 = 30;
 
 /// Shared mutable state for the lockscreen.
 struct LockscreenState {
@@ -221,13 +222,19 @@ pub fn create_lockscreen_window(
 
     overlay.add_overlay(&power_box);
 
-    // Password entry "activate" handler
+    // Password entry "activate" handler.
+    // A generation counter tracks which auth attempt is current. When the user
+    // submits a new password, the generation increments — stale PAM results from
+    // prior attempts are ignored (except success: a correct password always unlocks).
     let username = user.username.clone();
+    let auth_generation = Rc::new(Cell::new(0u32));
     password_entry.connect_activate(clone!(
         #[strong]
         state,
         #[strong]
         unlock_callback,
+        #[strong]
+        auth_generation,
         #[weak]
         error_label,
         #[weak]
@@ -242,6 +249,35 @@ pub fn create_lockscreen_window(
             let username = username.clone();
             let unlock_cb = unlock_callback.clone();
 
+            // Invalidate stale timeouts/results from prior attempts
+            let auth_gen = auth_generation.get().wrapping_add(1);
+            auth_generation.set(auth_gen);
+            let gen_timeout = auth_generation.clone();
+            let gen_result = auth_generation.clone();
+
+            // If PAM hangs (e.g. broken LDAP module), the timeout re-enables the UI
+            glib::timeout_add_local_once(
+                std::time::Duration::from_secs(PAM_TIMEOUT_SECS),
+                clone!(
+                    #[weak]
+                    error_label,
+                    #[weak]
+                    password_entry,
+                    move || {
+                        if gen_timeout.get() != auth_gen {
+                            return;
+                        }
+                        log::error!("PAM authentication timed out after {PAM_TIMEOUT_SECS}s");
+                        let strings = load_strings(None);
+                        password_entry.set_text("");
+                        password_entry.set_sensitive(true);
+                        password_entry.grab_focus();
+                        error_label.set_text(strings.auth_timeout);
+                        error_label.set_visible(true);
+                    }
+                ),
+            );
+
             glib::spawn_future_local(clone!(
                 #[strong]
                 state,
@@ -255,6 +291,20 @@ pub fn create_lockscreen_window(
                         auth::authenticate(&user, &password)
                     }).await;
 
+                    // Stale result from a superseded attempt — only unlock on success
+                    // (a correct password should always unlock, regardless of timing)
+                    if gen_result.get() != auth_gen {
+                        if matches!(result, Ok(true)) {
+                            let s = state.borrow();
+                            if let Some(ref fp_rc) = s.fp_listener_rc {
+                                fp_rc.borrow_mut().stop();
+                            }
+                            drop(s);
+                            unlock_cb();
+                        }
+                        return;
+                    }
+
                     match result {
                         Ok(true) => {
                             let s = state.borrow();
@@ -373,10 +423,12 @@ pub fn start_fingerprint(
     let unlock_cb_fp = handles.unlock_callback.clone();
 
     let fp_rc_success = fp_rc.clone();
+    let fp_username = handles.username.clone();
     let on_success = move || {
         let label = fp_label_success.clone();
         let cb = unlock_cb_fp.clone();
         let fp = fp_rc_success.clone();
+        let username = fp_username.clone();
         glib::idle_add_local_once(move || {
             let strings = load_strings(None);
             label.set_text(strings.fingerprint_success);
@@ -384,7 +436,39 @@ pub fn start_fingerprint(
             // stop() is idempotent — cleanup_dbus() already ran inside on_verify_status,
             // but this mirrors the PAM success path for defense-in-depth.
             fp.borrow_mut().stop();
-            cb();
+
+            // Enforce PAM account policies (lockout, expiry) before unlocking.
+            // Fingerprint auth bypasses pam_authenticate, so we must explicitly
+            // check account restrictions via pam_acct_mgmt.
+            glib::spawn_future_local(async move {
+                let user = username.clone();
+                let result = gio::spawn_blocking(move || {
+                    auth::check_account(&user)
+                }).await;
+                match result {
+                    Ok(true) => cb(),
+                    _ => {
+                        log::error!("PAM account check failed after fingerprint auth");
+                        let strings = load_strings(None);
+                        label.set_text(strings.wrong_password);
+                        label.remove_css_class("success");
+                        label.add_css_class("failed");
+                        // Restart FP verification after delay — the failure may be
+                        // transient (e.g. PAM module timeout). If the account is truly
+                        // locked, check_account will fail again on next match.
+                        glib::timeout_add_local_once(
+                            std::time::Duration::from_secs(2),
+                            move || {
+                                label.set_text(load_strings(None).fingerprint_prompt);
+                                label.remove_css_class("failed");
+                                glib::spawn_future_local(async move {
+                                    FingerprintListener::resume_async(&fp, &username).await;
+                                });
+                            },
+                        );
+                    }
+                }
+            });
         });
     };
 
@@ -481,12 +565,20 @@ fn create_background_picture(
     background
 }
 
+/// Maximum texture dimension for blur input. Textures larger than this are
+/// downscaled before blurring — the blur destroys detail anyway, so there is
+/// no visible quality loss, but GPU work is reduced significantly.
+const MAX_BLUR_DIMENSION: f32 = 1920.0;
+
 /// Render a blurred texture using the widget's GPU renderer.
 /// Returns None if the renderer is not available.
 ///
 /// To avoid edge darkening (blur samples transparent pixels outside bounds),
 /// the texture is rendered with padding equal to 3x the blur sigma. The blur
 /// is applied to the padded area, then cropped back to the original size.
+///
+/// Large textures (> MAX_BLUR_DIMENSION) are downscaled before blurring to
+/// reduce GPU work. The sigma is scaled proportionally.
 fn render_blurred_texture(
     widget: &impl IsA<gtk::Widget>,
     texture: &gdk::Texture,
@@ -495,15 +587,27 @@ fn render_blurred_texture(
     let native = widget.native()?;
     let renderer = native.renderer()?;
 
-    let w = texture.width() as f32;
-    let h = texture.height() as f32;
+    let orig_w = texture.width() as f32;
+    let orig_h = texture.height() as f32;
+
+    // Downscale large textures to reduce GPU blur work
+    let max_dim = orig_w.max(orig_h);
+    let scale = if max_dim > MAX_BLUR_DIMENSION {
+        MAX_BLUR_DIMENSION / max_dim
+    } else {
+        1.0
+    };
+    let w = (orig_w * scale).round();
+    let h = (orig_h * scale).round();
+    let scaled_sigma = sigma * scale;
+
     // Padding must cover the blur kernel radius (typically ~3x sigma)
-    let pad = (sigma * 3.0).ceil();
+    let pad = (scaled_sigma * 3.0).ceil();
 
     let snapshot = gtk::Snapshot::new();
-    // Clip output to original texture size
+    // Clip output to scaled texture size
     snapshot.push_clip(&graphene::Rect::new(pad, pad, w, h));
-    snapshot.push_blur(sigma as f64);
+    snapshot.push_blur(scaled_sigma as f64);
     // Render texture with padding on all sides (edges repeat via oversized bounds)
     snapshot.append_texture(texture, &graphene::Rect::new(0.0, 0.0, w + 2.0 * pad, h + 2.0 * pad));
     snapshot.pop(); // blur