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

Mingwei Zhang
2022-04-27T02:56:59Z

On Tue, Apr 26, 2022 at 6:30 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Apr 26, 2022, Mingwei Zhang wrote:
> > 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.
>
> I think it makes sense to do the save/restore in lookup_address_in_pgd(). The
> Those helpers are exported, so odds are good there are broken users that will
> benefit from fixing all paths.

no, lookup_address_in_pgd() is probably just broken for KVM. In other
call sites, some may already disable IRQ, so doing that again inside
lookup_address_in_pgd() will be bad.

I am looking at here:
https://elixir.bootlin.com/linux/latest/source/arch/arm/kernel/traps.c#L304

so, the save/restore are done in oops_begin() and oops_end(), which is
wrapping show_fault_oops() that calls lookup_address_in_pgd().

So, I think we need to ensure the READ_ONCE.

hmm, regarding the lockless macros, Paolo is right, for x86 it makes
no difference. s390 seems to have a different implementation, but
kvm_mmu_max_mapping_level() as well as host_pfn_mapping_level are both
functions in x86 mmu.

Thanks.
-Mingwei