linux-kernel - Re: [PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler

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Date:   Wed, 22 Nov 2017 17:56:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <LIRAN.ALON@...CLE.COM>,
        Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] KVM: VMX: Fix vmx->nested freeing when no SMI handler

On 22/11/2017 10:43, Liran Alon wrote:
>>>
>>> I think we should also set "vmx->nested.smm.vmxon = false;"
>>> after "vmx->nested.vmxon = false;" to correctlyhandle the case
>>> VMXOFF is executed from SMI handler. Otherwise, when SMI handler
>>> executes RSM, we will reach vmx_pre_leave_smm() which will set
>>> again "vmx->nested.vmxon = true;" which I think shouldn't
>>> happen.
>>
>> I didn't see a real scenario for this.
>
> Actually I later saw that handle_vmoff() calls
> nested_vmx_check_permission() which indeed won't allow to continue
> executing if running from SMI because vmx->nested.vmxon=false; and
> therefore this will raise a #UD. So you are right. :)

Still, not clearing the flag is wrong.  free_nested is also called by
vmx_leave_nested when the host writes 0 to MSR_IA32_FEATURE_CONTROL with
KVM_SET_MSRS.

Paolo