Harden Seatbelt writable root path binding (#39706)

## Why Resolving attacker-mutable path components while preparing a Seatbelt profile can let a writable root be rebound to a different location before the sandbox is applied. File roots also need to remain confined to the file itself rather than granting access to descendants after replacement. ## What changed - Preserve mutable components of writable-root paths until Seatbelt binds them, while still normalizing trusted top-level aliases such as `/tmp`. - Use literal grants for existing file and device roots, and subpath grants for directories and missing roots. - Exclude both logical and resolved forms of protected subpaths so symlinked metadata directories remain read-only. ## Testing Add coverage for ancestor rebinding, file-root symlink and directory replacement, missing directory roots, top-level aliases, and symlinked metadata carveouts. GitOrigin-RevId: 63c00e44dd30766e873b8acdad09d6657657fad9
2026-10-18 17:18:52 +00:00 · 2026-08-20 12:08:35 +00:00
parent 37a9da9901
commit 02de49f718
3 changed files with 540 additions and 47 deletions
@@ -238,6 +238,21 @@ pub struct FileSystemSandboxPolicy {
    pub entries: Vec<FileSystemSandboxEntry>,
 }

+#[derive(Clone, Copy)]
+enum WritableRootPathResolution {
+    Effective,
+    PreserveMutableComponents,
+}
+
+impl WritableRootPathResolution {
+    fn resolve(self, path: AbsolutePathBuf) -> AbsolutePathBuf {
+        match self {
+            Self::Effective => normalize_effective_absolute_path(path),
+            Self::PreserveMutableComponents => normalize_trusted_top_level_alias(path),
+        }
+    }
+}
+
 /// Serialized filesystem policy used at legacy string-based seams.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
 #[schemars(rename = "FileSystemSandboxPolicy")]
@@ -1149,6 +1164,30 @@ impl FileSystemSandboxPolicy {
    /// Returns the writable roots together with read-only carveouts resolved
    /// against the provided cwd.
    pub fn get_writable_roots_with_cwd(&self, cwd: &Path) -> Vec<WritableRoot> {
+        self.get_writable_roots_with_cwd_impl(cwd, WritableRootPathResolution::Effective)
+    }
+
+    /// Returns writable roots without following attacker-mutable path components.
+    ///
+    /// Trusted top-level aliases such as `/tmp -> /private/tmp` are still
+    /// normalized so roots and carveouts are compared in the same namespace.
+    /// Deeper components remain exactly as configured until the platform
+    /// sandbox binds them.
+    pub fn get_writable_roots_with_cwd_preserving_mutable_paths(
+        &self,
+        cwd: &Path,
+    ) -> Vec<WritableRoot> {
+        self.get_writable_roots_with_cwd_impl(
+            cwd,
+            WritableRootPathResolution::PreserveMutableComponents,
+        )
+    }
+
+    fn get_writable_roots_with_cwd_impl(
+        &self,
+        cwd: &Path,
+        path_resolution: WritableRootPathResolution,
+    ) -> Vec<WritableRoot> {
        if self.has_full_disk_write_access() {
            return Vec::new();
        }
@@ -1162,8 +1201,12 @@ impl FileSystemSandboxPolicy {
            .collect();

        dedup_absolute_paths(
-            writable_entries.clone(),
-            /*normalize_effective_paths*/ true,
+            writable_entries
+                .iter()
+                .cloned()
+                .map(|root| path_resolution.resolve(root))
+                .collect(),
+            /*normalize_effective_paths*/ false,
        )
        .into_iter()
        .map(|root| {
@@ -1177,13 +1220,13 @@ impl FileSystemSandboxPolicy {
            let preserve_raw_carveout_paths = root.as_path().parent().is_some();
            let raw_writable_roots: Vec<&AbsolutePathBuf> = writable_entries
                .iter()
-                .filter(|path| normalize_effective_absolute_path((*path).clone()) == root)
+                .filter(|path| path_resolution.resolve((*path).clone()) == root)
                .collect();
            let protected_metadata_names =
                protected_metadata_names_for_writable_root(self, &root, &raw_writable_roots, cwd);
            let protect_missing_dot_codex = AbsolutePathBuf::from_absolute_path(cwd)
                .ok()
-                .is_some_and(|cwd| normalize_effective_absolute_path(cwd) == root);
+                .is_some_and(|cwd| path_resolution.resolve(cwd) == root);
            let mut read_only_subpaths: Vec<AbsolutePathBuf> =
                default_read_only_subpaths_for_writable_root(&root, protect_missing_dot_codex)
                    .into_iter()
@@ -1202,7 +1245,7 @@ impl FileSystemSandboxPolicy {
                    .filter(|entry| !entry.access.can_write())
                    .filter(|entry| !self.can_write_path_with_cwd(entry.path.as_path(), cwd))
                    .filter_map(|entry| {
-                        let effective_path = normalize_effective_absolute_path(entry.path.clone());
+                        let effective_path = path_resolution.resolve(entry.path.clone());
                        // Preserve the literal in-root path whenever the
                        // carveout itself lives under this writable root, even
                        // if following symlinks would resolve back to the root
@@ -1754,6 +1797,27 @@ fn normalize_effective_absolute_path(path: AbsolutePathBuf) -> AbsolutePathBuf {
    path
 }

+fn normalize_trusted_top_level_alias(path: AbsolutePathBuf) -> AbsolutePathBuf {
+    let Some(top_level) = path.as_path().ancestors().find(|ancestor| {
+        ancestor.parent().is_some() && ancestor.parent().and_then(Path::parent).is_none()
+    }) else {
+        return path;
+    };
+    let Ok(metadata) = std::fs::symlink_metadata(top_level) else {
+        return path;
+    };
+    if !metadata.file_type().is_symlink() {
+        return path;
+    }
+    let Ok(canonical_top_level) = top_level.canonicalize() else {
+        return path;
+    };
+    let Ok(suffix) = path.as_path().strip_prefix(top_level) else {
+        return path;
+    };
+    AbsolutePathBuf::from_absolute_path(canonical_top_level.join(suffix)).unwrap_or(path)
+}
+
 pub(crate) fn default_read_only_subpaths_for_writable_root(
    writable_root: &AbsolutePathBuf,
    protect_missing_dot_codex: bool,
@@ -2296,6 +2360,94 @@ mod tests {
        );
    }

+    #[cfg(target_os = "macos")]
+    #[test]
+    fn preserving_mutable_paths_normalizes_top_level_aliases_consistently() {
+        let root = TempDir::new_in("/tmp").expect("tempdir under /tmp");
+        let logical_root =
+            AbsolutePathBuf::from_absolute_path(root.path()).expect("absolute logical root");
+        let canonical_root = AbsolutePathBuf::from_absolute_path(
+            root.path().canonicalize().expect("canonicalize root"),
+        )
+        .expect("absolute canonical root");
+        let protected = canonical_root.join("protected");
+        fs::create_dir(&protected).expect("create protected path");
+        let policy = FileSystemSandboxPolicy::restricted(vec![
+            FileSystemSandboxEntry::new(logical_root.into(), FileSystemAccessMode::Write),
+            FileSystemSandboxEntry::new(protected.clone().into(), FileSystemAccessMode::Read),
+        ]);
+
+        let roots = policy.get_writable_roots_with_cwd_preserving_mutable_paths(root.path());
+
+        assert_eq!(roots.len(), 1);
+        assert_eq!(roots[0].root, canonical_root);
+        assert!(roots[0].read_only_subpaths.contains(&protected));
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn preserving_writable_roots_cannot_be_rebound_during_projection() {
+        use std::sync::Arc;
+        use std::sync::atomic::AtomicBool;
+        use std::sync::atomic::AtomicUsize;
+        use std::sync::atomic::Ordering;
+        use std::thread;
+
+        let tmp = TempDir::new().expect("tempdir");
+        let active_ancestor = tmp.path().join("active");
+        let parked_ancestor = tmp.path().join("parked");
+        let outside_ancestor = tmp.path().join("outside");
+        let writable_root = active_ancestor.join("workspace");
+        let outside_root = outside_ancestor.join("workspace");
+        fs::create_dir_all(&writable_root).expect("create writable root");
+        fs::create_dir_all(&outside_root).expect("create outside root");
+        let writable_root =
+            AbsolutePathBuf::from_absolute_path(writable_root).expect("absolute writable root");
+        let outside_root =
+            AbsolutePathBuf::from_absolute_path(outside_root).expect("absolute outside root");
+        let expected_writable_root = normalize_trusted_top_level_alias(writable_root.clone());
+        let expected_outside_root = normalize_trusted_top_level_alias(outside_root);
+        let policy = FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry::new(
+            writable_root.into(),
+            FileSystemAccessMode::Write,
+        )]);
+
+        let stop = Arc::new(AtomicBool::new(false));
+        let swaps = Arc::new(AtomicUsize::new(0));
+        let racer_stop = Arc::clone(&stop);
+        let racer_swaps = Arc::clone(&swaps);
+        let racer = thread::spawn(move || {
+            while !racer_stop.load(Ordering::Relaxed) {
+                if fs::rename(&active_ancestor, &parked_ancestor).is_err() {
+                    thread::yield_now();
+                    continue;
+                }
+                if symlink_dir(&outside_ancestor, &active_ancestor).is_ok() {
+                    racer_swaps.fetch_add(1, Ordering::Relaxed);
+                    thread::yield_now();
+                    let _ = fs::remove_file(&active_ancestor);
+                }
+                fs::rename(&parked_ancestor, &active_ancestor).expect("restore writable ancestor");
+            }
+        });
+
+        let mut rebound_root = None;
+        for _ in 0..2_000 {
+            let roots = policy.get_writable_roots_with_cwd_preserving_mutable_paths(tmp.path());
+            if roots.len() != 1 || roots[0].root != expected_writable_root {
+                rebound_root = roots.first().map(|root| root.root.clone());
+                break;
+            }
+            assert_ne!(roots[0].root, expected_outside_root);
+            thread::yield_now();
+        }
+        stop.store(true, Ordering::Relaxed);
+        racer.join().expect("join path racer");
+
+        assert!(swaps.load(Ordering::Relaxed) > 0, "racer did not run");
+        assert_eq!(rebound_root, None);
+    }
+
    #[test]
    fn legacy_workspace_write_projection_preserves_symbolic_project_root() {
        let policy = SandboxPolicy::WorkspaceWrite {
@@ -9,7 +9,6 @@ use codex_protocol::permissions::PROTECTED_METADATA_PATH_NAMES;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::WritableRoot;
 use codex_utils_absolute_path::AbsolutePathBuf;
-use codex_utils_absolute_path::canonicalize_preserving_symlinks;
 use std::collections::BTreeMap;
 use std::collections::BTreeSet;
 use std::collections::HashMap;
@@ -381,6 +380,18 @@ enum SeatbeltAccessKind {
    Write,
 }

+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum SeatbeltPathMatch {
+    Literal,
+    Subpath,
+}
+
+#[derive(Debug)]
+enum NormalizedWritableRoot {
+    Subpath(AbsolutePathBuf),
+    Literal(AbsolutePathBuf),
+}
+
 fn nested_symlink_component(path: &Path) -> Option<&Path> {
    // Keep top-level macOS aliases such as `/tmp -> /private/tmp` compatible,
    // but reject symlinks in user-controlled path components.
@@ -392,9 +403,43 @@ fn nested_symlink_component(path: &Path) -> Option<&Path> {
    })
 }

+fn normalize_top_level_alias_for_sandbox(
+    path: AbsolutePathBuf,
+) -> Result<AbsolutePathBuf, SeatbeltPreparationError> {
+    let Some(top_level) = path.as_path().ancestors().find(|ancestor| {
+        ancestor.parent().is_some() && ancestor.parent().and_then(Path::parent).is_none()
+    }) else {
+        return Ok(path);
+    };
+    if !std::fs::symlink_metadata(top_level).is_ok_and(|metadata| metadata.file_type().is_symlink())
+    {
+        return Ok(path);
+    }
+
+    let canonical_top_level = top_level.canonicalize().map_err(|err| {
+        SeatbeltPreparationError::FileSystem(format!(
+            "failed to normalize top-level alias {} for Seatbelt: {err}",
+            top_level.display()
+        ))
+    })?;
+    let suffix = path.as_path().strip_prefix(top_level).map_err(|err| {
+        SeatbeltPreparationError::FileSystem(format!(
+            "failed to preserve path {} after normalizing {}: {err}",
+            path.display(),
+            top_level.display()
+        ))
+    })?;
+    AbsolutePathBuf::from_absolute_path(canonical_top_level.join(suffix)).map_err(|err| {
+        SeatbeltPreparationError::FileSystem(format!(
+            "failed to normalize top-level alias for path {}: {err}",
+            path.display()
+        ))
+    })
+}
+
 fn normalize_writable_root_for_sandbox(
    root: AbsolutePathBuf,
-) -> Result<AbsolutePathBuf, SeatbeltPreparationError> {
+) -> Result<NormalizedWritableRoot, SeatbeltPreparationError> {
    if let Some(symlink) = nested_symlink_component(root.as_path()) {
        return Err(SeatbeltPreparationError::FileSystem(format!(
            "writable root {} contains symlink component {}; symlinked writable roots are not supported",
@@ -403,18 +448,28 @@ fn normalize_writable_root_for_sandbox(
        )));
    }

-    let normalized = canonicalize_preserving_symlinks(root.as_path()).map_err(|err| {
-        SeatbeltPreparationError::FileSystem(format!(
-            "failed to normalize writable root {} for Seatbelt: {err}",
-            root.display()
-        ))
-    })?;
-    AbsolutePathBuf::from_absolute_path(normalized).map_err(|err| {
-        SeatbeltPreparationError::FileSystem(format!(
-            "failed to normalize writable root {} for Seatbelt: {err}",
-            root.display()
-        ))
-    })
+    // Resolve only top-level system aliases such as `/tmp -> /private/tmp`.
+    // Deeper components can be mutated by an already-running sandboxed process,
+    // so following them here would turn a path check into a new authority grant.
+    let normalized = normalize_top_level_alias_for_sandbox(root)?;
+
+    let metadata = match std::fs::symlink_metadata(normalized.as_path()) {
+        Ok(metadata) => metadata,
+        Err(err) if err.kind() == std::io::ErrorKind::NotFound => {
+            return Ok(NormalizedWritableRoot::Subpath(normalized));
+        }
+        Err(err) => {
+            return Err(SeatbeltPreparationError::FileSystem(format!(
+                "failed to inspect Seatbelt writable root {}: {err}",
+                normalized.display()
+            )));
+        }
+    };
+    if metadata.is_dir() {
+        return Ok(NormalizedWritableRoot::Subpath(normalized));
+    }
+
+    Ok(NormalizedWritableRoot::Literal(normalized))
 }

 fn build_seatbelt_access_policy(
@@ -430,11 +485,18 @@ fn build_seatbelt_access_policy(
    };

    for (index, access_root) in roots.into_iter().enumerate() {
-        let root = match access_kind {
+        let (root, path_match) = match access_kind {
            SeatbeltAccessKind::Read => {
-                normalize_path_for_sandbox(access_root.root.as_path()).unwrap_or(access_root.root)
+                let root = normalize_path_for_sandbox(access_root.root.as_path())
+                    .unwrap_or(access_root.root);
+                (root, SeatbeltPathMatch::Subpath)
+            }
+            SeatbeltAccessKind::Write => {
+                match normalize_writable_root_for_sandbox(access_root.root)? {
+                    NormalizedWritableRoot::Subpath(root) => (root, SeatbeltPathMatch::Subpath),
+                    NormalizedWritableRoot::Literal(root) => (root, SeatbeltPathMatch::Literal),
+                }
            }
-            SeatbeltAccessKind::Write => normalize_writable_root_for_sandbox(access_root.root)?,
        };
        let root_param = format!("{param_prefix}_{index}");
        params.push((root_param.clone(), root.clone().into_path_buf()));
@@ -445,31 +507,52 @@ fn build_seatbelt_access_policy(
                "(deny file-write-unlink (require-all (literal (param \"{root_param}\")) (vnode-type DIRECTORY)))"
            ));
        }
+        let root_filter = match path_match {
+            SeatbeltPathMatch::Literal => format!("(literal (param \"{root_param}\"))"),
+            SeatbeltPathMatch::Subpath => format!("(subpath (param \"{root_param}\"))"),
+        };

        if access_root.excluded_subpaths.is_empty()
            && access_root.protected_metadata_names.is_empty()
        {
-            policy_components.push(format!("(subpath (param \"{root_param}\"))"));
+            policy_components.push(root_filter);
            continue;
        }

-        let mut require_parts = vec![format!("(subpath (param \"{root_param}\"))")];
+        let mut require_parts = vec![root_filter];
        for (excluded_index, excluded_subpath) in
            access_root.excluded_subpaths.into_iter().enumerate()
        {
-            let excluded_subpath =
-                normalize_path_for_sandbox(excluded_subpath.as_path()).unwrap_or(excluded_subpath);
            let excluded_param = format!("{param_prefix}_{index}_EXCLUDED_{excluded_index}");
-            params.push((excluded_param.clone(), excluded_subpath.into_path_buf()));
-            // Exclude both the exact protected path and anything beneath it.
-            // `subpath` alone leaves a gap for first-time creation of the
-            // protected directory itself, such as `mkdir .codex`.
-            require_parts.push(format!(
-                "(require-not (literal (param \"{excluded_param}\")))"
-            ));
-            require_parts.push(format!(
-                "(require-not (subpath (param \"{excluded_param}\")))"
-            ));
+            let excluded_subpaths = match access_kind {
+                SeatbeltAccessKind::Read => vec![(
+                    excluded_param.clone(),
+                    normalize_path_for_sandbox(excluded_subpath.as_path())
+                        .unwrap_or(excluded_subpath),
+                )],
+                SeatbeltAccessKind::Write => {
+                    let logical = normalize_top_level_alias_for_sandbox(excluded_subpath)?;
+                    let resolved = normalize_path_for_sandbox(logical.as_path())
+                        .filter(|resolved| resolved != &logical);
+                    let mut paths = vec![(excluded_param.clone(), logical)];
+                    if let Some(resolved) = resolved {
+                        paths.push((format!("{excluded_param}_RESOLVED"), resolved));
+                    }
+                    paths
+                }
+            };
+            for (excluded_param, excluded_subpath) in excluded_subpaths {
+                params.push((excluded_param.clone(), excluded_subpath.into_path_buf()));
+                // Exclude both the exact protected path and anything beneath it.
+                // `subpath` alone leaves a gap for first-time creation of the
+                // protected directory itself, such as `mkdir .codex`.
+                require_parts.push(format!(
+                    "(require-not (literal (param \"{excluded_param}\")))"
+                ));
+                require_parts.push(format!(
+                    "(require-not (subpath (param \"{excluded_param}\")))"
+                ));
+            }
        }
        for metadata_name in access_root.protected_metadata_names {
            let regex =
@@ -775,7 +858,8 @@ pub(crate) fn create_seatbelt_command_args_with_profile(

    let unreadable_roots =
        file_system_sandbox_policy.get_unreadable_roots_with_cwd(sandbox_policy_cwd);
-    let writable_roots = file_system_sandbox_policy.get_writable_roots_with_cwd(sandbox_policy_cwd);
+    let writable_roots = file_system_sandbox_policy
+        .get_writable_roots_with_cwd_preserving_mutable_paths(sandbox_policy_cwd);
    // Protect ancestors of read-only paths so renaming a writable directory
    // cannot move its descendants outside their policy carveouts.
    let mut protected_ancestors = BTreeSet::new();
@@ -65,7 +65,7 @@ fn seatbelt_policy_arg(args: &[String]) -> &str {
        .expect("seatbelt args should include policy text")
 }

-#[cfg(target_os = "macos")]
+#[cfg(unix)]
 fn restricted_write_policy(paths: &[&Path]) -> FileSystemSandboxPolicy {
    let mut entries = vec![FileSystemSandboxEntry::new(
        FileSystemPath::Special {
@@ -1553,6 +1553,51 @@ fn seatbelt_prevents_writable_root_replacement() {
    );
 }

+#[cfg(unix)]
+#[test]
+fn create_seatbelt_args_uses_literal_non_directory_writable_roots() {
+    let tmp = TempDir::new().expect("tempdir");
+    let target = tmp.path().join("target.txt");
+    fs::write(&target, "contents").expect("write target");
+    let policy = restricted_write_policy(&[target.as_path(), Path::new("/dev/null")]);
+    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec!["/usr/bin/true".to_string()],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: tmp.path(),
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("file writable root should be supported");
+    let policy_text = seatbelt_policy_arg(&args);
+
+    assert!(
+        policy_text.contains("(literal (param \"WRITABLE_ROOT_0\"))"),
+        "expected literal file grant in policy:\n{policy_text}"
+    );
+    assert!(
+        !policy_text.contains("(subpath (param \"WRITABLE_ROOT_0\"))"),
+        "file grant should not include descendants:\n{policy_text}"
+    );
+    assert!(
+        policy_text.contains("(literal (param \"WRITABLE_ROOT_1\"))"),
+        "expected literal device grant in policy:\n{policy_text}"
+    );
+    assert!(
+        !policy_text.contains("(subpath (param \"WRITABLE_ROOT_1\"))"),
+        "device grant should not include descendants:\n{policy_text}"
+    );
+    assert!(
+        policy_text.contains(
+            "(deny file-write-unlink (require-all (literal (param \"WRITABLE_ROOT_0\")) (vnode-type DIRECTORY)))"
+        ),
+        "file grant should protect the path if it becomes a directory:\n{policy_text}"
+    );
+}
+
 #[cfg(target_os = "macos")]
 #[test]
 fn seatbelt_allows_file_root_replacement_and_deletion() {
@@ -1595,6 +1640,159 @@ fn seatbelt_allows_file_root_replacement_and_deletion() {
    );
    assert!(!target.exists(), "target should have been deleted");
    assert!(!replacement.exists(), "replacement should have been moved");
+
+    create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec!["/usr/bin/true".to_string()],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: tmp.path(),
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("deleted file roots should remain usable by later commands");
+}
+
+#[cfg(target_os = "macos")]
+#[test]
+fn seatbelt_file_root_does_not_follow_replacement_symlink() {
+    use std::os::unix::fs::symlink;
+
+    let tmp = TempDir::new().expect("tempdir");
+    let writable_file = tmp.path().join("writable.txt");
+    let outside_file = tmp.path().join("outside.txt");
+    fs::write(&writable_file, "writable").expect("write writable file");
+    fs::write(&outside_file, "outside").expect("write outside file");
+    let policy = restricted_write_policy(&[writable_file.as_path()]);
+    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec![
+            "/bin/sh".to_string(),
+            "-c".to_string(),
+            "printf escaped > \"$1\"".to_string(),
+            "sh".to_string(),
+            writable_file.display().to_string(),
+        ],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: tmp.path(),
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("build seatbelt command");
+    fs::remove_file(&writable_file).expect("remove writable file");
+    symlink(&outside_file, &writable_file).expect("replace writable file with symlink");
+
+    let output = Command::new(MACOS_PATH_TO_SEATBELT_EXECUTABLE)
+        .args(&args)
+        .current_dir(tmp.path())
+        .output()
+        .expect("execute seatbelt command");
+
+    assert!(
+        !output.status.success(),
+        "replacement symlink should not grant access to its target"
+    );
+    assert_eq!(
+        fs::read_to_string(&outside_file).expect("read outside file"),
+        "outside"
+    );
+}
+
+#[cfg(target_os = "macos")]
+#[test]
+fn seatbelt_protects_file_root_replaced_with_directory() {
+    let tmp = TempDir::new().expect("tempdir");
+    let writable_file = tmp.path().join("writable");
+    fs::write(&writable_file, "contents").expect("write writable file");
+    let policy = restricted_write_policy(&[writable_file.as_path()]);
+    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec![
+            "/bin/rmdir".to_string(),
+            writable_file.display().to_string(),
+        ],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: tmp.path(),
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("build seatbelt command");
+    fs::remove_file(&writable_file).expect("remove writable file");
+    fs::create_dir(&writable_file).expect("replace writable file with directory");
+
+    let output = Command::new(MACOS_PATH_TO_SEATBELT_EXECUTABLE)
+        .args(&args)
+        .current_dir(tmp.path())
+        .output()
+        .expect("execute seatbelt command");
+
+    assert!(
+        !output.status.success(),
+        "directory replacement should remain anchored"
+    );
+    assert!(
+        writable_file.is_dir(),
+        "replacement directory should remain in place"
+    );
+}
+
+#[cfg(target_os = "macos")]
+#[test]
+fn seatbelt_does_not_follow_rebound_writable_root_ancestor() {
+    use std::os::unix::fs::symlink;
+
+    let tmp = TempDir::new().expect("tempdir");
+    let ancestor = tmp.path().join("ancestor");
+    let original_ancestor = tmp.path().join("ancestor-original");
+    let writable_root = ancestor.join("workspace");
+    let outside_ancestor = tmp.path().join("outside");
+    let outside_root = outside_ancestor.join("workspace");
+    let logical_file = writable_root.join("escaped.txt");
+    let escaped_file = outside_root.join("escaped.txt");
+    fs::create_dir_all(&writable_root).expect("create writable root");
+    fs::create_dir_all(&outside_root).expect("create outside root");
+    let policy = restricted_write_policy(&[writable_root.as_path()]);
+    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec![
+            "/bin/sh".to_string(),
+            "-c".to_string(),
+            "printf escaped > \"$1\"".to_string(),
+            "sh".to_string(),
+            logical_file.display().to_string(),
+        ],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: tmp.path(),
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("build seatbelt command");
+
+    fs::rename(&ancestor, &original_ancestor).expect("move writable ancestor");
+    symlink(&outside_ancestor, &ancestor).expect("rebind writable ancestor");
+
+    let output = Command::new(MACOS_PATH_TO_SEATBELT_EXECUTABLE)
+        .args(&args)
+        .current_dir(tmp.path())
+        .output()
+        .expect("execute seatbelt command");
+
+    assert!(
+        !output.status.success(),
+        "rebound ancestor should not grant the outside root"
+    );
+    assert!(!escaped_file.exists(), "outside file should not be created");
 }

 #[cfg(target_os = "macos")]
@@ -1603,13 +1801,15 @@ fn seatbelt_prevents_writable_directory_root_rename() {
    let tmp = TempDir::new().expect("tempdir");
    let source = tmp.path().join("source");
    let destination = tmp.path().join("destination");
+    let renamed = destination.join("renamed");
    fs::create_dir(&source).expect("create source");
+    fs::create_dir(&destination).expect("create destination");
    let policy = restricted_write_policy(&[source.as_path(), destination.as_path()]);
    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
        command: vec![
            "/bin/mv".to_string(),
            source.display().to_string(),
-            destination.display().to_string(),
+            renamed.display().to_string(),
        ],
        file_system_sandbox_policy: &policy,
        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
@@ -1634,8 +1834,8 @@ fn seatbelt_prevents_writable_directory_root_rename() {
    );
    assert!(source.is_dir(), "source directory should remain in place");
    assert!(
-        !destination.exists(),
-        "destination should not have been created"
+        !renamed.exists(),
+        "renamed directory should not have been created"
    );
 }

@@ -1644,14 +1844,17 @@ fn seatbelt_prevents_writable_directory_root_rename() {
 fn seatbelt_protects_writable_root_created_as_directory() {
    let tmp = TempDir::new().expect("tempdir");
    let writable_root = tmp.path().join("writable-root");
-    let policy = restricted_write_policy(&[writable_root.as_path()]);
+    let progress = tmp.path().join("progress.txt");
+    fs::write(&progress, "pending").expect("write progress file");
+    let policy = restricted_write_policy(&[writable_root.as_path(), progress.as_path()]);
    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
        command: vec![
            "/bin/sh".to_string(),
            "-c".to_string(),
-            "mkdir \"$1\" && rmdir \"$1\"".to_string(),
+            "mkdir \"$1\" && touch \"$1/file\" && printf ok > \"$2\" && rm \"$1/file\" && rmdir \"$1\"".to_string(),
            "sh".to_string(),
            writable_root.display().to_string(),
+            progress.display().to_string(),
        ],
        file_system_sandbox_policy: &policy,
        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
@@ -1678,6 +1881,60 @@ fn seatbelt_protects_writable_root_created_as_directory() {
        writable_root.is_dir(),
        "newly created directory root should remain protected"
    );
+    assert_eq!(
+        fs::read_to_string(&progress).expect("read progress file"),
+        "ok",
+        "missing writable root should allow descendant writes"
+    );
+}
+
+#[cfg(target_os = "macos")]
+#[test]
+fn seatbelt_protects_resolved_target_of_symlinked_metadata_directory() {
+    use std::os::unix::fs::symlink;
+
+    let tmp = TempDir::new().expect("tempdir");
+    let writable_root = tmp.path().join("workspace");
+    let actual_config = writable_root.join("actual-config");
+    let dot_codex = writable_root.join(".codex");
+    let config_toml = actual_config.join("config.toml");
+    fs::create_dir_all(&actual_config).expect("create actual config directory");
+    fs::write(&config_toml, "original").expect("write config");
+    symlink(&actual_config, &dot_codex).expect("create .codex symlink");
+    let policy = restricted_write_policy(&[writable_root.as_path()]);
+    let args = create_seatbelt_command_args(CreateSeatbeltCommandArgsParams {
+        command: vec![
+            "/bin/sh".to_string(),
+            "-c".to_string(),
+            "printf escaped > \"$1\"".to_string(),
+            "sh".to_string(),
+            config_toml.display().to_string(),
+        ],
+        file_system_sandbox_policy: &policy,
+        network_sandbox_policy: NetworkSandboxPolicy::Restricted,
+        sandbox_policy_cwd: &writable_root,
+        enforce_managed_network: false,
+        managed_network: None,
+        environment_id: None,
+        network: None,
+        extra_allow_unix_sockets: &[],
+    })
+    .expect("build seatbelt command");
+
+    let output = Command::new(MACOS_PATH_TO_SEATBELT_EXECUTABLE)
+        .args(&args)
+        .current_dir(&writable_root)
+        .output()
+        .expect("execute seatbelt command");
+
+    assert!(
+        !output.status.success(),
+        "resolved .codex target should remain read-only"
+    );
+    assert_eq!(
+        fs::read_to_string(&config_toml).expect("read config"),
+        "original"
+    );
 }

 #[test]
@@ -1927,13 +2184,13 @@ fn create_seatbelt_args_for_cwd_as_git_repo() {
        args.contains(&expected_dot_codex),
        "missing {expected_dot_codex}: {args:#?}"
    );
-    let unexpected_dot_agents = format!(
-        "-DWRITABLE_ROOT_0_EXCLUDED_1={}",
+    let expected_dot_agents = format!(
+        "-DWRITABLE_ROOT_0_EXCLUDED_2={}",
        dot_agents_canonical.to_string_lossy()
    );
    assert!(
-        !args.contains(&unexpected_dot_agents),
-        "missing .agents should be handled by regex rather than materialized as a path param: {args:#?}"
+        args.contains(&expected_dot_agents),
+        "missing {expected_dot_agents}: {args:#?}"
    );
    let expected_slash_tmp = format!("-DWRITABLE_ROOT_1={}", slash_tmp.to_string_lossy());
    assert!(