Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Reto Buerki <reet@codelabs.ch>
Subject: Re: [PATCH 0/2] KVM: nVMX: Bug fix for consuming stale vmcs02.GUEST_CR3
In-Reply-To: <20190926214302.21990-1-sean.j.christopherson@intel.com>
References: <20190926214302.21990-1-sean.j.christopherson@intel.com>
Date:   Fri, 27 Sep 2019 14:12:15 +0200
Message-ID: <87o8z65468.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> Reto Buerki reported a failure in a nested VMM when running with HLT
> interception disabled in L1.  When putting L2 into HLT, KVM never actually
> enters L2 and instead cancels the nested run and pretends that VM-Enter to
> L2 completed and then exited on HLT (which KVM intercepted).  Because KVM
> never actually runs L2, KVM skips the pending MMU update for L2 and so
> leaves a stale value in vmcs02.GUEST_CR3.  If the next wake event for L2
> triggers a nested VM-Exit, KVM will refresh vmcs12->guest_cr3 from
> vmcs02.GUEST_CR3 and consume the stale value.
>
> Fix the issue by unconditionally writing vmcs02.GUEST_CR3 during nested
> VM-Enter instead of deferring the update to vmx_set_cr3(), and skip the
> update of GUEST_CR3 in vmx_set_cr3() when running L2.  I.e. make the
> nested code fully responsible for vmcs02.GUEST_CR3.
>
> I really wanted to go with a different fix of handling this as a one-off
> case in the HLT flow (in nested_vmx_run()), and then following that up
> with a cleanup of VMX's CR3 handling, e.g. to do proper dirty tracking
> instead of having the nested code do manual VMREADs and VMWRITEs.  I even
> went so far as to hide vcpu->arch.cr3 (put CR3 in vcpu->arch.regs), but
> things went south when I started working through the dirty tracking logic.
>
> Because EPT can be enabled *without* unrestricted guest, enabling EPT
> doesn't always mean GUEST_CR3 really is the guest CR3 (unlike SVM's NPT).
> And because the unrestricted guest handling of GUEST_CR3 is dependent on
> whether the guest has paging enabled, VMX can't even do a clean handoff
> based on unrestricted guest.  In a nutshell, dynamically handling the
> transitions of GUEST_CR3 ownership in VMX is a nightmare, so fixing this
> purely within the context of nested VMX turned out to be the cleanest fix.
>
> Sean Christopherson (2):
>   KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter
>   KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date
>

Series:
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly