Limit the Linux /proc preflight filesystem view (#34409)

## What changed

- Run the bubblewrap `/proc` mount probe with the minimal read-only filesystem policy instead of the command's filesystem policy and working directory.
- Preserve the requested network namespace mode during the probe.
- Add coverage that the probe mounts a temporary root and `/proc` without binding the full filesystem.

GitOrigin-RevId: e3bc4352f510980aeebe81dedf3f9211947982ed

codex-rs/linux-sandbox/src/linux_run_main.rs

@@ -27,7 +27,11 @@ use crate::proxy_routing::activate_proxy_routes_in_netns;
 use crate::proxy_routing::prepare_host_proxy_route_spec;
 use codex_protocol::error::Result as CodexResult;
 use codex_protocol::models::PermissionProfile;
+use codex_protocol::protocol::FileSystemAccessMode;
+use codex_protocol::protocol::FileSystemPath;
+use codex_protocol::protocol::FileSystemSandboxEntry;
 use codex_protocol::protocol::FileSystemSandboxPolicy;
+use codex_protocol::protocol::FileSystemSpecialPath;
 use codex_protocol::protocol::NetworkSandboxPolicy;
 use codex_sandboxing::landlock::CODEX_LINUX_SANDBOX_ARG0;
 
@@ -328,13 +332,8 @@ fn run_bwrap_with_proc_fallback(
     let command_cwd = command_cwd.unwrap_or(sandbox_policy_cwd);
 
     if mount_proc
-        && !preflight_proc_mount_support(
-            sandbox_policy_cwd,
-            command_cwd,
-            file_system_sandbox_policy,
-            network_mode,
-        )
-        .unwrap_or_else(|err| exit_with_bwrap_build_error(err))
+        && !preflight_proc_mount_support(network_mode)
+            .unwrap_or_else(|err| exit_with_bwrap_build_error(err))
     {
         // Keep the retry silent so sandbox-internal diagnostics do not leak into the
         // child process stderr stream.
@@ -441,34 +440,28 @@ fn current_process_argv0() -> String {
     }
 }
 
-fn preflight_proc_mount_support(
-    sandbox_policy_cwd: &Path,
-    command_cwd: &Path,
-    file_system_sandbox_policy: &FileSystemSandboxPolicy,
-    network_mode: BwrapNetworkMode,
-) -> CodexResult<bool> {
-    let preflight_argv = build_preflight_bwrap_argv(
-        sandbox_policy_cwd,
-        command_cwd,
-        file_system_sandbox_policy,
-        network_mode,
-    )?;
+fn preflight_proc_mount_support(network_mode: BwrapNetworkMode) -> CodexResult<bool> {
+    let preflight_argv = build_preflight_bwrap_argv(network_mode)?;
     let stderr = run_bwrap_in_child_capture_stderr(preflight_argv);
     Ok(!is_proc_mount_failure(stderr.as_str()))
 }
 
 fn build_preflight_bwrap_argv(
-    sandbox_policy_cwd: &Path,
-    command_cwd: &Path,
-    file_system_sandbox_policy: &FileSystemSandboxPolicy,
     network_mode: BwrapNetworkMode,
 ) -> CodexResult<crate::bwrap::BwrapArgs> {
+    let file_system_sandbox_policy =
+        FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry {
+            path: FileSystemPath::Special {
+                value: FileSystemSpecialPath::Minimal,
+            },
+            access: FileSystemAccessMode::Read,
+        }]);
     let preflight_command = vec![resolve_true_command()];
     build_bwrap_argv(
         preflight_command,
-        file_system_sandbox_policy,
-        sandbox_policy_cwd,
-        command_cwd,
+        &file_system_sandbox_policy,
+        Path::new("/"),
+        Path::new("/"),
         BwrapOptions {
             mount_proc: true,
             network_mode,

codex-rs/linux-sandbox/src/linux_run_main_tests.rs

@@ -258,20 +258,32 @@ fn root_write_read_only_carveout_requires_direct_runtime_enforcement() {
 }
 
 #[test]
-fn managed_proxy_preflight_argv_is_wrapped_for_full_access_policy() {
+fn managed_proxy_preflight_argv_unshares_network() {
     let mode = bwrap_network_mode(
         NetworkSandboxPolicy::Enabled,
         /*allow_network_for_proxy*/ true,
     );
-    let argv = build_preflight_bwrap_argv(
-        Path::new("/"),
-        Path::new("/"),
-        &FileSystemSandboxPolicy::unrestricted(),
-        mode,
-    )
-    .expect("build preflight argv")
-    .args;
+    let argv = build_preflight_bwrap_argv(mode)
+        .expect("build preflight argv")
+        .args;
     assert!(argv.iter().any(|arg| arg == "--"));
+    assert!(argv.iter().any(|arg| arg == "--unshare-net"));
+}
+
+#[test]
+fn proc_mount_preflight_does_not_bind_the_full_filesystem() {
+    let argv = build_preflight_bwrap_argv(BwrapNetworkMode::FullAccess)
+        .expect("build preflight argv")
+        .args;
+
+    assert!(argv.windows(2).any(|window| window == ["--tmpfs", "/"]));
+    assert!(argv.windows(2).any(|window| window == ["--proc", "/proc"]));
+    assert!(
+        !argv
+            .windows(3)
+            .any(|window| window == ["--ro-bind", "/", "/"])
+    );
+    assert!(!argv.windows(3).any(|window| window == ["--bind", "/", "/"]));
 }
 
 #[test]