fix: tighten up check for /usr/bin/sandbox-exec (#710)

* In both TypeScript and Rust, we now invoke `/usr/bin/sandbox-exec`
explicitly rather than whatever `sandbox-exec` happens to be on the
`PATH`.
* Changed `isSandboxExecAvailable` to use `access()` rather than
`command -v` so that:
  *  We only do the check once over the lifetime of the Codex process.
  * The check is specific to `/usr/bin/sandbox-exec`.
* We now do a syscall rather than incur the overhead of spawning a
process, dealing with timeouts, etc.

I think there is still room for improvement here where we should move
the `isSandboxExecAvailable` check earlier in the CLI, ideally right
after we do arg parsing to verify that we can provide the Seatbelt
sandbox if that is what the user has requested.

codex-cli/src/utils/agent/sandbox/macos-seatbelt.ts

@@ -12,6 +12,14 @@ function getCommonRoots() {
   ];
 }
 
+/**
+ * When working with `sandbox-exec`, only consider `sandbox-exec` in `/usr/bin`
+ * to defend against an attacker trying to inject a malicious version on the
+ * PATH. If /usr/bin/sandbox-exec has been tampered with, then the attacker
+ * already has root access.
+ */
+export const PATH_TO_SEATBELT_EXECUTABLE = "/usr/bin/sandbox-exec";
+
 export function execWithSeatbelt(
   cmd: Array<string>,
   opts: SpawnOptions,
@@ -57,7 +65,7 @@ export function execWithSeatbelt(
   );
 
   const fullCommand = [
-    "sandbox-exec",
+    PATH_TO_SEATBELT_EXECUTABLE,
     "-p",
     fullPolicy,
     ...policyTemplateParams,