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

[Ingo Molnar <mingo@kernel.org>]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Wed, 21 Nov 2018, Tim Chen wrote:
> 
> > 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.
> 
> We had that discussion before with SSBD:
> 
> seccomp_set_mode_filter()
>    seccomp_attach_filter()
>       seccomp_sync_threads()
>          for_each_thread(t)
> 	    if (t == current)
>               continue;
> 	    seccomp_assign_mode(t)
> 	      arch_seccomp_spec_mitigate(t);
> 
> seccomp_assign_mode(current...)
>   arch_seccomp_spec_mitigate();
> 
> > 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.
> 
> We also had that discussion with SSBD and decided that we won't chase
> threads and send IPIs around. Yes, it's not perfect, but not the end of the
> world either. For PRCTL it's a non issue.

Fair enough and agreed - but please add a comment for all this, as it's a 
non-trivial and rare call context and a non-trivial implementation 
trade-off as a result.

Thanks,

	Ingo