fix: insecure fallback directory and secure file creation (#111)

* 🔒 Fix insecure fallback directory and secure file creation

The daemon's fallback runtime directory `get_runtime_dir()` was hardcoded to `/run/pwsp`, creating a risk of shared, insecure access in multi-user systems.
This commit secures the fallback logic by:
1. Creating a user-specific temporary directory (`/tmp/pwsp-$UID`).
2. Ensuring directory creation happens atomically with `0o700` permissions using `std::fs::DirBuilder`.
3. Validating the fallback directory strictly (checking UID, 0o700 permissions, and symlink status) if it already exists to mitigate symlink attacks.
4. Using `libc::geteuid()` for robust cross-platform UID extraction.
5. Fixing `is_daemon_running` and locking logic to use `fs::OpenOptions` instead of `fs::File::create` to prevent accidental file truncation on active lock files.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* 🔒 Fix insecure fallback directory and secure file creation

The daemon's fallback runtime directory `get_runtime_dir()` was hardcoded to `/run/pwsp`, creating a risk of shared, insecure access in multi-user systems.
This commit secures the fallback logic by:
1. Creating a user-specific temporary directory (`/tmp/pwsp-$UID`).
2. Ensuring directory creation happens atomically with `0o700` permissions using `std::fs::DirBuilder`.
3. Validating the fallback directory strictly (checking UID, 0o700 permissions, and symlink status) if it already exists to mitigate symlink attacks.
4. Using safe `rustix::process::geteuid()` for robust cross-platform UID extraction, avoiding `unsafe` blocks.
5. Fixing `is_daemon_running` and locking logic to use `fs::OpenOptions` instead of `fs::File::create` to prevent accidental file truncation on active lock files.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* small refactor

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Cargo.lock

@@ -3177,6 +3177,7 @@ dependencies = [
  "rfd",
  "rodio",
  "rust-i18n",
+ "rustix 1.1.4",
  "serde",
  "serde_json",
  "sys-locale",

Cargo.toml

@@ -38,6 +38,7 @@ rfd = { version = "0.17.2", default-features = false, features = [
 opener = { version = "0.8.4", features = ["reveal"] }
 system-fonts = "0.1.0"
 anyhow = "1.0.102"
+rustix = { version = "1.1.4", features = ["process"] }
 
 rust-i18n = "4.0.0"
 sys-locale = "0.3.2"

src/bin/daemon.rs

@@ -39,7 +39,11 @@ async fn main() -> Result<()> {
 
     let runtime_dir = get_runtime_dir();
 
-    let lock_file = fs::File::create(runtime_dir.join("daemon.lock"))?;
+    let lock_file = fs::OpenOptions::new()
+        .create(true)
+        .write(true)
+        .truncate(false)
+        .open(runtime_dir.join("daemon.lock"))?;
     lock_file.lock()?;
 
     let socket_path = runtime_dir.join("daemon.sock");

src/utils/daemon.rs

@@ -5,9 +5,9 @@ use crate::types::{
 };
 
 use anyhow::Result;
-use std::os::unix::fs::PermissionsExt;
+use std::os::unix::fs::{DirBuilderExt, MetadataExt, PermissionsExt};
 use std::path::PathBuf;
-use std::{error::Error, fs};
+use std::{env, error::Error, fs};
 use tokio::{
     io::{AsyncReadExt, AsyncWriteExt},
     net::UnixStream,
@@ -37,22 +37,52 @@ pub fn get_daemon_config() -> DaemonConfig {
     })
 }
 
+fn get_current_uid() -> u32 {
+    rustix::process::geteuid().as_raw()
+}
+
 pub fn get_runtime_dir() -> PathBuf {
-    dirs::runtime_dir().unwrap_or(PathBuf::from("/run/pwsp"))
+    dirs::runtime_dir().unwrap_or_else(|| {
+        let uid = get_current_uid();
+        env::temp_dir().join(format!("pwsp-{}", uid))
+    })
 }
 
 pub fn create_runtime_dir() -> Result<()> {
     let runtime_dir = get_runtime_dir();
-    if !runtime_dir.exists() {
-        fs::create_dir_all(&runtime_dir)?;
+
+    if runtime_dir.exists() {
+        let meta = fs::symlink_metadata(&runtime_dir)?;
+        if meta.is_symlink() {
+            return Err(anyhow::anyhow!("Runtime directory is a symlink"));
+        }
+        let uid = get_current_uid();
+        if meta.uid() != uid {
+            return Err(anyhow::anyhow!(
+                "Runtime directory is owned by another user"
+            ));
+        }
+        if meta.permissions().mode() & 0o777 != 0o700 {
+            return Err(anyhow::anyhow!(
+                "Runtime directory has incorrect permissions"
+            ));
+        }
+    } else {
+        fs::DirBuilder::new()
+            .recursive(true)
+            .mode(0o700)
+            .create(&runtime_dir)?;
     }
-    fs::set_permissions(&runtime_dir, fs::Permissions::from_mode(0o700))?;
 
     Ok(())
 }
 
 pub fn is_daemon_running() -> Result<bool> {
-    let lock_file = fs::File::create(get_runtime_dir().join("daemon.lock"))?;
+    let lock_file = fs::OpenOptions::new()
+        .create(true)
+        .write(true)
+        .truncate(false)
+        .open(get_runtime_dir().join("daemon.lock"))?;
     match lock_file.try_lock() {
         Ok(_) => Ok(false),
         Err(_) => Ok(true),