Re: [PATCH] futex: don't leak robust_list pointer on exec race

Thomas Gleixner
2025-06-13T13:08:49Z

On Wed, Jun 11 2025 at 19:33, Pranav Tyagi wrote:
> On Mon, Jun 9, 2025 at 3:15 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> Does the revised version below address the concerns more effectively
> or does it still need a bit more seasoning?
>
> "Currently, sys_get_robust_list() and compat_get_robust_list() perform a
> ptrace_may_access() check to verify if the calling task is allowed to
> query another task’s robust_list pointer. However, this check is racy
> against a concurrent exec() in the target process.
>
> During exec(), a task's credentials and memory mappings can change, and
> the task may transition from a non-privileged binary to a privileged one
> (e.g., a setuid binary). If get_robust_list() performs ptrace_may_access()
> before this transition, it may wrongly allow access to sensitive
> information after the target becomes privileged.
>
> To address this, a read lock is taken on signal->exec_update_lock prior
> to invoking ptrace_may_access() and accessing the robust_list. This
> ensures that the target task's exec state remains stable during the
> check, allowing for consistent and synchronized validation of
> credentials."

That's way better, but it still does not explain what the consequences
of the racy access are.
>>
>> You really did not have a better idea than copying all of that logic
>> into the compat code?
>
> As I’m still learning, I wasn’t quite sure how to avoid
> duplication there. Would factoring out the common logic into a helper function
> be the right direction? I’d be grateful for your suggestion.

Exactly.

Thanks,

tglx