linux-kernel - Re: [PATCH 2/2] KVM: nVMX: postpone VMCS changes on MSR_IA32

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Date:	Fri, 12 Aug 2016 11:44:04 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
	Wincy Van <fanwenyi0529@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Bandan Das <bsd@...hat.com>
Subject: Re: [PATCH 2/2] KVM: nVMX: postpone VMCS changes on
 MSR_IA32_APICBASE write

2016-08-12 14:07+0800, Wanpeng Li:
> 2016-08-09 2:16 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> If vmcs12 does not intercept APIC_BASE writes, then KVM will handle the
>> write with vmcs02 as the current VMCS.
>> This will incorrectly apply modifications intended for vmcs01 to vmcs02
>> and L2 can use it to gain access to L0's x2APIC registers by disabling
>> virtualized x2APIC while using msr bitmap that assumes enabled.
>>
>> Postpone execution of vmx_set_virtual_x2apic_mode until vmcs01 is the
>> current VMCS.  An alternative solution would temporarily make vmcs01 the
>> current VMCS, but it requires more care.
> 
> There is a scenario both L1 and L2 are running on x2apic mode, L1
> don't own the APIC_BASE writes, then L2 is intended to disable x2apic
> mode, however, your logic will also disable x2apic mode for L1.

You mean a case where L1 does intercept APIC_BASE?

That case is not affected, because it should cause a nested VM exit, so
vmx_set_virtual_x2apic_mode() won't be called in the first place.