linux-kernel - Re: [PATCH 05/28] KVM: x86/mmu: Check for present SPTE when clearing dirty bit in TDP MMU

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Date:   Mon, 22 Nov 2021 11:57:19 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>
Subject: Re: [PATCH 05/28] KVM: x86/mmu: Check for present SPTE when clearing
 dirty bit in TDP MMU

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Explicitly check for preset SPTEs when clearing dirty bits in the TDP
> MMU.  This isn't strictly required for correctness, as setting the dirty
> bit in a defunct SPTE will not change the SPTE from !PRESENT to PRESENT.
> However, the guarded MMU_WARN_ON() in spte_ad_need_write_protect() would
> complain if anyone actually turned on KVM's MMU debugging.
>
> Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
> Cc: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 1db8496259ad..c575df121b19 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1246,6 +1246,9 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>                 if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
>                         continue;
>
> +               if (!is_shadow_present_pte(iter.old_spte))
> +                       continue;
> +
>                 if (spte_ad_need_write_protect(iter.old_spte)) {
>                         if (is_writable_pte(iter.old_spte))
>                                 new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>