Re: [PATCH] KVM: x86/mmu: add lockdep check before lookup_address_in_mm()

Mingwei Zhang
2022-04-27T01:24:34Z

On Tue, Apr 26, 2022 at 6:16 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Apr 26, 2022, Mingwei Zhang wrote:
> > > I completely agree that lookup_address() and friends are unnecessarily fragile,
> > > but I think that attempting to harden them to fix this KVM bug will open a can
> > > of worms and end up delaying getting KVM fixed.
> >
> > So basically, we need to:
> > - choose perf_get_page_size() instead of using any of the
> > lookup_address*() in mm.
> > - add a wrapper layer to adapt: 1) irq disabling/enabling and 2) size
> > -> level translation.
> >
> > Agree?
>
> Drat, I didn't see that it returns the page size, not the level. That's a bit
> unfortunate. It definitely makes me less averse to fixing lookup_address_in_pgd()
>
> Hrm. I guess since we know there's at least one broken user, and in theory
> fixing lookup_address_in_pgd() should do no harm to users that don't need protection,
> it makes sense to just fix lookup_address_in_pgd() and see if the x86 maintainers
> push back.

Yeah, fixing lookup_address_in_pgd() should be cleaner(), since the
page fault usage case does not need irq save/restore. But the other
one needs it. So, we can easily fix the function with READ_ONCE and
lockless staff. But wrapping the function with irq save/restore from
the KVM side.

ok, I think I can proceed and upload one version for that.

Thanks.
-Mingwei