[PATCH 1/2] KVM: arm64: Disable interrupts while walking userspace PTs

[David Matlack]

On Mon, Mar 13, 2023 at 10:21 AM Sean Christopherson <seanjc at google.com> wrote:
>
> On Mon, Mar 13, 2023, David Matlack wrote:
> > On Mon, Mar 13, 2023 at 8:53 AM Sean Christopherson <seanjc at google.com> wrote:
> > >
> > > +David
> > >
> > > On Mon, Mar 13, 2023, Marc Zyngier wrote:
> > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > > > index 7113587222ff..d7b8b25942df 100644
> > > > --- a/arch/arm64/kvm/mmu.c
> > > > +++ b/arch/arm64/kvm/mmu.c
> > > > @@ -666,14 +666,23 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
> > > >                                  CONFIG_PGTABLE_LEVELS),
> > > >               .mm_ops         = &kvm_user_mm_ops,
> > > >       };
> > > > +     unsigned long flags;
> > > >       kvm_pte_t pte = 0;      /* Keep GCC quiet... */
> > > >       u32 level = ~0;
> > > >       int ret;
> > > >
> > > > +     /*
> > > > +      * Disable IRQs so that we hazard against a concurrent
> > > > +      * teardown of the userspace page tables (which relies on
> > > > +      * IPI-ing threads).
> > > > +      */
> > > > +     local_irq_save(flags);
> > > >       ret = kvm_pgtable_get_leaf(&pgt, addr, &pte, &level);
> > > > -     VM_BUG_ON(ret);
> > > > -     VM_BUG_ON(level >= KVM_PGTABLE_MAX_LEVELS);
> > > > -     VM_BUG_ON(!(pte & PTE_VALID));
> > > > +     local_irq_restore(flags);
> > > > +
> > > > +     /* Oops, the userspace PTs are gone... */
> > > > +     if (ret || level >= KVM_PGTABLE_MAX_LEVELS || !(pte & PTE_VALID))
> > > > +             return -EFAULT;
> > >
> > > I don't think this should return -EFAULT all the way out to userspace.  Unless
> > > arm64 differs from x86 in terms of how the userspace page tables are managed, not
> > > having a valid translation _right now_ doesn't mean that one can't be created in
> > > the future, e.g. by way of a subsequent hva_to_pfn().
> > >
> > > FWIW, the approach x86 takes is to install a 4KiB (smallest granuale) translation,
> >
> > If I'm reading the ARM code correctly, returning -EFAULT here will
> > have that effect. get_user_mapping_size() is only called by
> > transparent_hugepage_adjust() which returns PAGE_SIZE if
> > get_user_mapping_size() returns anything less than PMD_SIZE.
>
> No, this patch adds
>
> +               int sz = get_user_mapping_size(kvm, hva);
> +
> +               if (sz < 0)
> +                       return sz;
> +
> +               if (sz < PMD_SIZE)
> +                       return PAGE_SIZE;
> +

Gah, I just looked at the trimmed patch in the reply. Thanks for
pointing that out.