Re: [patch 20/24] x86/speculation: Split out TIF update

[Tim Chen]

On Wed, Nov 21, 2018 at 09:14:50PM +0100, Thomas Gleixner wrote:
> +static void task_update_spec_tif(struct task_struct *tsk, int tifbit, bool on)
>  {
>       bool update;
>
> +     if (on)
> +             update = !test_and_set_tsk_thread_flag(tsk, tifbit);
> +     else
> +             update = test_and_clear_tsk_thread_flag(tsk, tifbit);
> +
> +     /*
> +      * If being set on non-current task, delay setting the CPU
> +	 * mitigation until it is scheduled next.
> +	 */
> +     if (tsk == current && update)
> +             speculation_ctrl_update_current();

I think all the call paths from prctl and seccomp coming here
has tsk == current.

But if task_update_spec_tif gets used in the future
where tsk is running on a remote CPU, this could lead to the MSR
getting out of sync with the running task's TIF flag. This will break
either performance or security.

Should we add a
        WARN_ON(smp_processor_id() != task_cpu(tsk));

in case the assumption breaks that task is
on local CPU, or document this assumption?

Thanks.

Tim