linux-kernel - Re: [PATCH 1/12] KVM: X86: Fix when shadow_root_level=5 && guest root

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Date:   Fri, 10 Dec 2021 17:34:35 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 1/12] KVM: X86: Fix when shadow_root_level=5 && guest
 root_level<4



On 2021/12/9 09:16, Sean Christopherson wrote:
> On Wed, Nov 24, 2021, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>
>> If the is an L1 with nNPT in 32bit, the shadow walk starts with
>> pae_root.
>>
>> Fixes: a717a780fc4e ("KVM: x86/mmu: Support shadowing NPT when 5-level paging is enabled in host)
> 
> Have you actually run with 5-level nNPT?  I don't have access to hardware, at least
> not that I know of :-)

The code is just obvious incorrect for shadow_root_level=5 && guest root_level<4.

> 
> I'm staring at kvm_mmu_sync_roots() and don't see how it can possibly work for
> 5-level nNPT with a 4-level NPT guest.
> 

It doesn't use pml5_root for 5-level nNPT with a 4-level NPT guest, so
kvm_mmu_sync_roots() can work in a silence way with an "unexpected" root shadow
page.  It has problems for 5-level nNPT with a 4-level NPT guest.

See:
https://lore.kernel.org/lkml/20211210092508.7185-1-jiangshanlai@gmail.com/

especially patch4.

Your this reply motivated me to complete the changelog of a patchset and send
it, thanks!

Although the patchset is immature, it would be better than losing it.