linux-kernel - Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow

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Date:   Wed, 19 Jul 2023 16:16:26 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Xu <peterx@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow_pfn

On Wed, Jul 19, 2023 at 3:35 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
>
> On Tue, Jul 04, 2023 at 04:50:50PM +0900, David Stevens wrote:
> > @@ -4451,7 +4461,8 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> >
> >  out_unlock:
> >       write_unlock(&vcpu->kvm->mmu_lock);
> > -     kvm_release_pfn_clean(fault->pfn);
> > +     if (fault->is_refcounted_page)
> > +             kvm_set_page_accessed(pfn_to_page(fault->pfn));
> For a refcounted page, as now KVM puts its ref early in kvm_faultin_pfn(),
> should this kvm_set_page_accessed() be placed before unlocking mmu_lock?
>
> Otherwise, if the user unmaps a region (which triggers kvm_unmap_gfn_range()
> with mmu_lock holding for write), and release the page, and if the two
> steps happen after checking page_count() in kvm_set_page_accessed() and
> before mark_page_accessed(), the latter function may mark accessed to a page
> that is released or does not belong to current process.
>
> Is it true?

Yes, good catch. During some testing last week, I actually found this
bug thanks to the WARN_ON the first patch in this series added to
kvm_is_ad_tracked_page. I'll fix it in the next revision, after Sean
gets a chance to comment on the series.

Thanks,
David