Fix fs util toctou

.changeset/resolute-toctou.md

@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Resolve TOCTOU race condition in `fs_util::atomic_write` and `atomic_write_async` to securely enforce 0600 file permissions upon file creation, preventing intermediate local read access to secrets.

Cargo.toml

@@ -42,6 +42,7 @@ reqwest = { version = "0.12", features = ["json", "stream", "rustls-tls-native-r
 rand = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+tempfile = "3"
 sha2 = "0.10"
 thiserror = "2"
 tokio = { version = "1", features = ["full"] }
@@ -66,4 +67,3 @@ lto = "thin"
 
 [dev-dependencies]
 serial_test = "3.4.0"
-tempfile = "3"

src/fs_util.rs

@@ -30,35 +30,28 @@ use std::path::Path;
 /// Returns an `io::Error` if the temporary file cannot be written or if the
 /// rename fails.
 pub fn atomic_write(path: &Path, data: &[u8]) -> io::Result<()> {
-    // Derive a sibling tmp path, e.g. `/home/user/.config/gws/credentials.enc.tmp`
-    let file_name = path
-        .file_name()
-        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, "path has no file name"))?;
-    let tmp_name = format!("{}.tmp", file_name.to_string_lossy());
-    let tmp_path = path
-        .parent()
-        .map(|p| p.join(&tmp_name))
-        .unwrap_or_else(|| std::path::PathBuf::from(&tmp_name));
+    let parent = path.parent().unwrap_or_else(|| std::path::Path::new(""));
+    let mut tmp_file = tempfile::Builder::new()
+        .prefix(".tmp")
+        .make_in(parent)?;
 
-    std::fs::write(&tmp_path, data)?;
-    std::fs::rename(&tmp_path, path)?;
+    {
+        use std::io::Write;
+        tmp_file.write_all(data)?;
+        tmp_file.as_file_mut().sync_all()?;
+    }
+
+    tmp_file.persist(path).map_err(|e| e.error)?;
     Ok(())
 }
 
 /// Async variant of [`atomic_write`] for use with tokio.
 pub async fn atomic_write_async(path: &Path, data: &[u8]) -> io::Result<()> {
-    let file_name = path
-        .file_name()
-        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidInput, "path has no file name"))?;
-    let tmp_name = format!("{}.tmp", file_name.to_string_lossy());
-    let tmp_path = path
-        .parent()
-        .map(|p| p.join(&tmp_name))
-        .unwrap_or_else(|| std::path::PathBuf::from(&tmp_name));
-
-    tokio::fs::write(&tmp_path, data).await?;
-    tokio::fs::rename(&tmp_path, path).await?;
-    Ok(())
+    let path = path.to_path_buf();
+    let data = data.to_vec();
+    tokio::task::spawn_blocking(move || atomic_write(&path, &data))
+        .await
+        .map_err(|e| io::Error::new(io::ErrorKind::Other, e))?
 }
 
 #[cfg(test)]