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

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Date:	Mon, 15 Aug 2016 13:19:19 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Radim Krčmář <rkrcmar@...hat.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 19:39 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2016-08-12 18:14+0800, Wanpeng Li:
>> 2016-08-12 17:44 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>>> 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.
>>
>> I mean L1 doesn't intercept APIC_BASE.
>
> Then L2's write to APIC_BASE should only affect L1.
> L2 is buggy if it intended to disable its x2APIC with the write
> or L1 set up intercepts incorrectly for the indented L2.

Do you mean OS disable x2APIC during its running is buggy?

> In the non-nested case, if we didn't intercept APIC_BASE in KVM, then
> the guest wouldn't change either;  only the host would change, so I
> think it is correct to disable x2APIC mode in L1 only.

Agreed. :)

Regards,
Wanpeng Li