Re: [PATCH 1/7] KVM: x86: Retry page fault if MMU reload is pending and root has no sp

Sean Christopherson
2021-12-15T18:53:22Z

On Fri, Dec 10, 2021, Sean Christopherson wrote:
> On Fri, Dec 10, 2021, Paolo Bonzini wrote:
> > On 12/10/21 17:01, Sean Christopherson wrote:
> > > > KVM_REQ_MMU_RELOAD is raised after kvm->arch.mmu_valid_gen is fixed (of
> > > > course, otherwise the other CPU might just not see any obsoleted page
> > > > from the legacy MMU), therefore any check on KVM_REQ_MMU_RELOAD is just
> > > > advisory.
> > >
> > > I disagree. IMO, KVM should not be installing SPTEs into obsolete shadow pages,
> > > which is what continuing on allows. I don't _think_ it's problematic, but I do
> > > think it's wrong.
> > >
> > > [...] Eh, for all intents and purposes, KVM_REQ_MMU_RELOAD very much says
> > > special roots are obsolete. The root will be unloaded, i.e. will no
> > > longer be used, i.e. is obsolete.
> >
> > I understand that---but it takes some unspoken details to understand that.
>
> Eh, it takes just as many unspoken details to understand why it's safe-ish to
> install SPTEs into an obsolete shadow page.
>
> > In particular that both kvm_reload_remote_mmus and is_page_fault_stale are
> > called under mmu_lock write-lock, and that there's no unlock between
> > updating mmu_valid_gen and calling kvm_reload_remote_mmus.
> >
> > (This also suggests, for the other six patches, keeping
> > kvm_reload_remote_mmus and just moving it to arch/x86/kvm/mmu/mmu.c, with an
> > assertion that the MMU lock is held for write).
> >
> > But since we have a way forward for having no special roots to worry about,
> > it seems an unnecessary overload for 1) a patch that will last one or two
> > releasees at most
>
> Yeah, I don't disagree, which is why I'm not totally opposed to punting this and
> naturally fixing it by allocating shadow pages for the special roots. But this
> code needs to be modified by Jiangshan's series either way, so it's not like we're
> saving anything meaningful.
>
> > 2) a case that has been handled in the inefficient way forever.
>
> I don't care about inefficiency, I'm worried about correctness. It's extremely
> unlikely this fixes a true bug in the legacy MMU, but there's also no real
> downside to adding the check.
>
> Anyways, either way is fine.

Ping, in case this dropped off your radar. Regardless of how we fix this goof,
it needs to get fixed in 5.16.