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
