Re: [PATCH v2 7/7] KVM: WARN if is_unsync_root() is called on a root without a shadow page

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 28, 2022, Ben Gardon wrote:
> On Fri, Feb 25, 2022 at 10:23 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > WARN and bail if is_unsync_root() is passed a root for which there is no
> > shadow page, i.e. is passed the physical address of one of the special
> > roots, which do not have an associated shadow page.  The current usage
> > squeaks by without bug reports because neither kvm_mmu_sync_roots() nor
> > kvm_mmu_sync_prev_roots() calls the helper with pae_root or pml4_root,
> > and 5-level AMD CPUs are not generally available, i.e. no one can coerce
> > KVM into calling is_unsync_root() on pml5_root.
> >
> > Note, this doesn't fix the mess with 5-level nNPT, it just (hopefully)
> > prevents KVM from crashing.
> >
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 825996408465..3e7c8ad5bed9 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3634,6 +3634,14 @@ static bool is_unsync_root(hpa_t root)
> >          */
> >         smp_rmb();
> >         sp = to_shadow_page(root);
> > +
> > +       /*
> > +        * PAE roots (somewhat arbitrarily) aren't backed by shadow pages, the
> > +        * PDPTEs for a given PAE root need to be synchronized individually.
> > +        */
> > +       if (WARN_ON_ONCE(!sp))
> > +               return false;
> > +
> 
> I was trying to figure out if this should be returning true or false,
> but neither really seems correct. Since we never expect this to fire,
> perhaps it doesn't matter and it's easier to just return false so the
> callers don't need to be changed.

Yep, neither is correct.

> If this did fire in a production scenario, I'd want it to terminate the VM
> too.

Me too, but practically speaking this should never get anywhere near production.
IMO, it's not worth plumbing in @kvm just to be able to do KVM_BUG_ON.