Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Mar 08, 2024 at 07:23:58AM -0800, Dave Hansen wrote:
> On 3/7/24 17:34, Andy Lutomirski wrote:
> >> Fix this by making sure we write a new CR3 if LAM is not
> >> up-to-date. No problems were observed in practice, this was found
> >> by code inspection.
> > I think it should be fixed with a much bigger hammer: explicit IPIs.
> > Just don't ever let it get out of date, like install_ldt().
> I guess it matters whether the thing that matters is having a persistent
> inconsistency or a temporary one.  IPIs will definitely turn a permanent
> one into a temporary one.
> 
> But this is all easier to reason about if we can get rid of even the
> temporary inconsistency.
> 
> Wouldn't this be even simpler than IPIs?
> 
> static inline unsigned long set_tlbstate_lam_mode(struct mm_struct *mm)
> {
> 	unsigned long lam = READ_ONCE(mm->context.lam_cr3_mask);
> 
> +	/* LAM is for userspace only.  Ignore it for kernel threads: */
> +	if (tsk->flags & PF_KTHREAD)
> +		return 0;
> 
> 	this_cpu_write(cpu_tlbstate.lam, lam >> X86_CR3_LAM_U57_BIT);
>  	this_cpu_write(tlbstate_untag_mask, mm->context.untag_mask);
> 	return lam;
> }

Hmm I don't see how this fixes the problem addressed by this patch. I
think this fixes the problem addressed by patch 1, where CR3 and
cpu_tlbstate.lam may get out of sync if LAM enablement races with
switch_mm_irqs_off().

However, this patch is fixing a deeper problem (an actual bug).
Precisely this situation:

CPU 1					CPU 2
/* kthread */
kthread_use_mm()
					/* user thread */
					prctl_enable_tagged_addr()
					/* LAM enabled */
					context_switch() /* to CPU 1 */
switch_mm_irqs_off()
/* user thread */
---> LAM is disabled here <---


When switch_mm_irqs_off() runs on CPU 1 to switch from the kthread to
the user thread, because the mm is not actually changing, we may not
write CR3. In this case, the user thread runs on CPU 1 with LAM
disabled, right?

The IPI would fix this problem because prctl_enable_tagged_addr() will
make sure that CPU 1 enables LAM before it returns to userspace.
Alternatively, this patch fixes the problem by making sure we write CR3
in switch_mm_irqs_off() if LAM is out-of-date.

I don't see how skipping set_tlbstate_lam_mode() for kthreads fixes this
problem. Do you mind elaborating?