fix: tighten up checks against writable folders for SandboxPolicy (#2338)

I was looking at the implementation of `Session::get_writable_roots()`,
which did not seem right, as it was a copy of writable roots, which is
not guaranteed to be in sync with the `sandbox_policy` field.

I looked at who was calling `get_writable_roots()` and its only call
site was `apply_patch()` in `codex-rs/core/src/apply_patch.rs`, which
took the roots and forwarded them to `assess_patch_safety()` in
`safety.rs`. I updated `assess_patch_safety()` to take `sandbox_policy:
&SandboxPolicy` instead of `writable_roots: &[PathBuf]` (and replaced
`Session::get_writable_roots()` with `Session::get_sandbox_policy()`).

Within `safety.rs`, it was fairly easy to update
`is_write_patch_constrained_to_writable_paths()` to work with
`SandboxPolicy`, and in particular, it is far more accurate because, for
better or worse, `SandboxPolicy::get_writable_roots_with_cwd()` _returns
an empty vec_ for `SandboxPolicy::DangerFullAccess`, suggesting that
_nothing_ is writable when in reality _everything_ is writable. With
this PR, `is_write_patch_constrained_to_writable_paths()` now does the
right thing for each variant of `SandboxPolicy`.

I thought this would be the end of the story, but it turned out that
`test_writable_roots_constraint()` in `safety.rs` needed to be updated,
as well. In particular, the test was writing to
`std::env::current_dir()` instead of a `TempDir`, which I suspect was a
holdover from earlier when `SandboxPolicy::WorkspaceWrite` would always
make `TMPDIR` writable on macOS, which made it hard to write tests to
verify `SandboxPolicy` in `TMPDIR`. Fortunately, we now have
`exclude_tmpdir_env_var` as an option on
`SandboxPolicy::WorkspaceWrite`, so I was able to update the test to
preserve the existing behavior, but to no longer write to
`std::env::current_dir()`.







---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2338).
* #2345
* #2329
* #2343
* #2340
* __->__ #2338

codex-rs/core/src/apply_patch.rs

@@ -8,7 +8,6 @@ use crate::safety::assess_patch_safety;
 use codex_apply_patch::ApplyPatchAction;
 use codex_apply_patch::ApplyPatchFileChange;
 use std::collections::HashMap;
-use std::path::Path;
 use std::path::PathBuf;
 
 pub const CODEX_APPLY_PATCH_ARG1: &str = "--codex-run-as-apply-patch";
@@ -45,12 +44,10 @@ pub(crate) async fn apply_patch(
     call_id: &str,
     action: ApplyPatchAction,
 ) -> InternalApplyPatchInvocation {
-    let writable_roots_snapshot = sess.get_writable_roots().to_vec();
-
     match assess_patch_safety(
         &action,
         sess.get_approval_policy(),
-        &writable_roots_snapshot,
+        sess.get_sandbox_policy(),
         sess.get_cwd(),
     ) {
         SafetyCheck::AutoApprove { .. } => {
@@ -124,30 +121,3 @@ pub(crate) fn convert_apply_patch_to_protocol(
     }
     result
 }
-
-pub(crate) fn get_writable_roots(cwd: &Path) -> Vec<PathBuf> {
-    let mut writable_roots = Vec::new();
-    if cfg!(target_os = "macos") {
-        // On macOS, $TMPDIR is private to the user.
-        writable_roots.push(std::env::temp_dir());
-
-        // Allow pyenv to update its shims directory. Without this, any tool
-        // that happens to be managed by `pyenv` will fail with an error like:
-        //
-        //   pyenv: cannot rehash: $HOME/.pyenv/shims isn't writable
-        //
-        // which is emitted every time `pyenv` tries to run `rehash` (for
-        // example, after installing a new Python package that drops an entry
-        // point). Although the sandbox is intentionally read‑only by default,
-        // writing to the user's local `pyenv` directory is safe because it
-        // is already user‑writable and scoped to the current user account.
-        if let Ok(home_dir) = std::env::var("HOME") {
-            let pyenv_dir = PathBuf::from(home_dir).join(".pyenv");
-            writable_roots.push(pyenv_dir);
-        }
-    }
-
-    writable_roots.push(cwd.to_path_buf());
-
-    writable_roots
-}

codex-rs/core/src/codex.rs

@@ -31,12 +31,11 @@ use tracing::warn;
 use uuid::Uuid;
 
 use crate::ModelProviderInfo;
+use crate::apply_patch;
 use crate::apply_patch::ApplyPatchExec;
 use crate::apply_patch::CODEX_APPLY_PATCH_ARG1;
 use crate::apply_patch::InternalApplyPatchInvocation;
 use crate::apply_patch::convert_apply_patch_to_protocol;
-use crate::apply_patch::get_writable_roots;
-use crate::apply_patch::{self};
 use crate::client::ModelClient;
 use crate::client_common::Prompt;
 use crate::client_common::ResponseEvent;
@@ -231,7 +230,6 @@ pub(crate) struct Session {
     approval_policy: AskForApproval,
     sandbox_policy: SandboxPolicy,
     shell_environment_policy: ShellEnvironmentPolicy,
-    writable_roots: Vec<PathBuf>,
     disable_response_storage: bool,
     tools_config: ToolsConfig,
 
@@ -410,8 +408,6 @@ impl Session {
             state.history.record_items(&restored_items);
         }
 
-        let writable_roots = get_writable_roots(&cwd);
-
         // Handle MCP manager result and record any startup failures.
         let (mcp_connection_manager, failed_clients) = match mcp_res {
             Ok((mgr, failures)) => (mgr, failures),
@@ -465,7 +461,6 @@ impl Session {
             sandbox_policy,
             shell_environment_policy: config.shell_environment_policy.clone(),
             cwd,
-            writable_roots,
             mcp_connection_manager,
             notify,
             state: Mutex::new(state),
@@ -509,14 +504,14 @@ impl Session {
         Ok(sess)
     }
 
-    pub(crate) fn get_writable_roots(&self) -> &[PathBuf] {
-        &self.writable_roots
-    }
-
     pub(crate) fn get_approval_policy(&self) -> AskForApproval {
         self.approval_policy
     }
 
+    pub(crate) fn get_sandbox_policy(&self) -> &SandboxPolicy {
+        &self.sandbox_policy
+    }
+
     pub(crate) fn get_cwd(&self) -> &Path {
         &self.cwd
     }

codex-rs/core/src/protocol.rs

@@ -156,10 +156,31 @@ pub enum SandboxPolicy {
 /// not modified by the agent.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct WritableRoot {
+    /// Absolute path, by construction.
     pub root: PathBuf,
+
+    /// Also absolute paths, by construction.
     pub read_only_subpaths: Vec<PathBuf>,
 }
 
+impl WritableRoot {
+    pub(crate) fn is_path_writable(&self, path: &Path) -> bool {
+        // Check if the path is under the root.
+        if !path.starts_with(&self.root) {
+            return false;
+        }
+
+        // Check if the path is under any of the read-only subpaths.
+        for subpath in &self.read_only_subpaths {
+            if path.starts_with(subpath) {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
 impl FromStr for SandboxPolicy {
     type Err = serde_json::Error;
 

codex-rs/core/src/safety.rs

@@ -21,7 +21,7 @@ pub enum SafetyCheck {
 pub fn assess_patch_safety(
     action: &ApplyPatchAction,
     policy: AskForApproval,
-    writable_roots: &[PathBuf],
+    sandbox_policy: &SandboxPolicy,
     cwd: &Path,
 ) -> SafetyCheck {
     if action.is_empty() {
@@ -45,7 +45,7 @@ pub fn assess_patch_safety(
     // is possible that paths in the patch are hard links to files outside the
     // writable roots, so we should still run `apply_patch` in a sandbox in that
     // case.
-    if is_write_patch_constrained_to_writable_paths(action, writable_roots, cwd)
+    if is_write_patch_constrained_to_writable_paths(action, sandbox_policy, cwd)
         || policy == AskForApproval::OnFailure
     {
         // Only auto‑approve when we can actually enforce a sandbox. Otherwise
@@ -171,13 +171,19 @@ pub fn get_platform_sandbox() -> Option<SandboxType> {
 
 fn is_write_patch_constrained_to_writable_paths(
     action: &ApplyPatchAction,
-    writable_roots: &[PathBuf],
+    sandbox_policy: &SandboxPolicy,
     cwd: &Path,
 ) -> bool {
     // Early‑exit if there are no declared writable roots.
-    if writable_roots.is_empty() {
+    let writable_roots = match sandbox_policy {
-        return false;
+        SandboxPolicy::ReadOnly => {
-    }
+            return false;
+        }
+        SandboxPolicy::DangerFullAccess => {
+            return true;
+        }
+        SandboxPolicy::WorkspaceWrite { .. } => sandbox_policy.get_writable_roots_with_cwd(cwd),
+    };
 
     // Normalize a path by removing `.` and resolving `..` without touching the
     // filesystem (works even if the file does not exist).
@@ -209,15 +215,9 @@ fn is_write_patch_constrained_to_writable_paths(
             None => return false,
         };
 
-        writable_roots.iter().any(|root| {
+        writable_roots
-            let root_abs = if root.is_absolute() {
+            .iter()
-                root.clone()
+            .any(|writable_root| writable_root.is_path_writable(&abs))
-            } else {
-                normalize(&cwd.join(root)).unwrap_or_else(|| cwd.join(root))
-            };
-
-            abs.starts_with(&root_abs)
-        })
     };
 
     for (path, change) in action.changes() {
@@ -246,38 +246,56 @@ fn is_write_patch_constrained_to_writable_paths(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use tempfile::TempDir;
 
     #[test]
     fn test_writable_roots_constraint() {
-        let cwd = std::env::current_dir().unwrap();
+        // Use a temporary directory as our workspace to avoid touching
+        // the real current working directory.
+        let tmp = TempDir::new().unwrap();
+        let cwd = tmp.path().to_path_buf();
         let parent = cwd.parent().unwrap().to_path_buf();
 
-        // Helper to build a single‑entry map representing a patch that adds a
+        // Helper to build a single‑entry patch that adds a file at `p`.
-        // file at `p`.
         let make_add_change = |p: PathBuf| ApplyPatchAction::new_add_for_test(&p, "".to_string());
 
         let add_inside = make_add_change(cwd.join("inner.txt"));
         let add_outside = make_add_change(parent.join("outside.txt"));
 
+        // Policy limited to the workspace only; exclude system temp roots so
+        // only `cwd` is writable by default.
+        let policy_workspace_only = SandboxPolicy::WorkspaceWrite {
+            writable_roots: vec![],
+            network_access: false,
+            exclude_tmpdir_env_var: true,
+            exclude_slash_tmp: true,
+        };
+
         assert!(is_write_patch_constrained_to_writable_paths(
             &add_inside,
-            &[PathBuf::from(".")],
+            &policy_workspace_only,
             &cwd,
         ));
 
-        let add_outside_2 = make_add_change(parent.join("outside.txt"));
         assert!(!is_write_patch_constrained_to_writable_paths(
-            &add_outside_2,
+            &add_outside,
-            &[PathBuf::from(".")],
+            &policy_workspace_only,
             &cwd,
         ));
 
-        // With parent dir added as writable root, it should pass.
+        // With the parent dir explicitly added as a writable root, the
+        // outside write should be permitted.
+        let policy_with_parent = SandboxPolicy::WorkspaceWrite {
+            writable_roots: vec![parent.clone()],
+            network_access: false,
+            exclude_tmpdir_env_var: true,
+            exclude_slash_tmp: true,
+        };
         assert!(is_write_patch_constrained_to_writable_paths(
             &add_outside,
-            &[PathBuf::from("..")],
+            &policy_with_parent,
             &cwd,
-        ))
+        ));
     }
 
     #[test]