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Date:   Fri, 15 Oct 2021 18:29:07 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Xiaoyao Li <xiaoyao.li@intel.com>
Cc:     Chenyi Qiang <chenyi.qiang@intel.com>, pbonzini@redhat.com,
        vkuznets@redhat.com, wanpengli@tencent.com, jmattson@google.com,
        joro@8bytes.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit
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On Mon, Sep 13, 2021, Xiaoyao Li wrote:
> On 9/10/2021 2:59 AM, Sean Christopherson wrote:
> > Yes, and no longer being able to run the vCPU is precisely the problem.  The
> > condition(s) matters because if there's a possibility, however small, that enabling
> > NOTIFY_WINDOW can kill a well-behaved guest then it absolutely cannot be enabled by
> > default.
> 
> For now, no condition will set it. For future, I believe it will be set only
> for some fatal case. However, we cannot guarantee no silicon bug to break a
> well-behaved the guest. Maybe let's make it opt-in?

Ya, I think an off-by-default module param makes sense.

> > > Either KVM_BUG_ON() or a specific EXIT to userspace should be OK?
> > 
> > Not if the VM_CONTEXT_INVALID happens while L2 is running.  If software can trigger
> > VM_CONTEXT_INVALID at will, then killing the VM would open up the door to a
> > malicious L2 killing L1 (which would be rather ironic since this is an anti-DoS
> > feature).  IIUC, VM_CONTEXT_INVALID only means the current VMCS is garbage, thus
> > an occurence while L2 is active means that vmcs02 is junk, but L1's state in vmcs01,
> > vmcs12, etc... is still valid.
> > 
> 
> Maybe we can kill the L2 when VM_CONTEXT_INVALID happens in L2.

Ya, synthesizing a nested EXIT_REASON_TRIPLE_FAULT and sanitizing vmcs02/vmcs12 is
the least awful solution I can think of.  I could have sworn I suggested as much,
but apparently that thought never made it from my brain to the keyboard.