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

Mingwei Zhang
2022-04-26T17:20:08Z

On Mon, Mar 28, 2022 at 11:15 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Mar 28, 2022, Mingwei Zhang wrote:
> > With that, I start to feel this is a bug. The issue is just so rare
> > that it has never triggered a problem.
> >
> > lookup_address_in_mm() walks the host page table as if it is a
> > sequence of _static_ memory chunks. This is clearly dangerous.
>
> Yeah, it's broken. The proper fix is do something like what perf uses, or maybe
> just genericize and reuse the code from commit 8af26be06272
> ("perf/core: Fix arch_perf_get_page_size()).

hmm, I am thinking about this. We clearly need an adaptor layer if we
choose to use this function, e.g., size -> layer change; using irq or
not. Alternatively, I am wondering if we can just modify
lookup_address_in_mm() to make the code compatible with "lockless"
walk?

On top of that, since kvm_mmu_max_mapping_level() is used in two
places: 1) ept violation and 2) disabling dirty logging. The former
does not require disable/enable irq since it is safe. So maybe add a
parameter in this function and plumb through towards
host_pfn_mapping_level()?
>
> > But right now, kvm_mmu_max_mapping_level() are used in other places
> > as well: kvm_mmu_zap_collapsible_spte(), which does not satisfy the
> > strict requirement of walking the host page table.
>
> The host pfn size is used only as a hueristic, so false postives/negatives are
> ok, the only race that needs to be avoided is dereferencing freed page table
> memory. lookup_address_in_pgd() is really broken because it doesn't even ensure
> a given PxE is READ_ONCE(). I suppose one could argue the caller is broken, but
> I doubt KVM is the only user that doesn't provide the necessary protections.

right. since lookup_address_in_pgd() is so broken. I am thinking about
just fix it in place instead of switching to a different function.