Re: [PATCH v2 5/8] KVM: nVMX: Fix guest CR3 read-back on VM-exit

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-08-06 17:53, Gleb Natapov wrote:
> On Tue, Aug 06, 2013 at 05:48:54PM +0200, Jan Kiszka wrote:
>> On 2013-08-06 17:04, Zhang, Yang Z wrote:
>>> Gleb Natapov wrote on 2013-08-06:
>>>> On Tue, Aug 06, 2013 at 02:12:51PM +0000, Zhang, Yang Z wrote:
>>>>> Gleb Natapov wrote on 2013-08-06:
>>>>>> On Tue, Aug 06, 2013 at 11:44:41AM +0000, Zhang, Yang Z wrote:
>>>>>>> Gleb Natapov wrote on 2013-08-06:
>>>>>>>> On Tue, Aug 06, 2013 at 10:39:59AM +0200, Jan Kiszka wrote:
>>>>>>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>>>>>>
>>>>>>>>> If nested EPT is enabled, the L2 guest may change CR3 without any
>>>>>>>>> exits. We therefore have to read the current value from the VMCS
>>>>>>>>> when switching to L1. However, if paging wasn't enabled, L0 tracks
>>>>>>>>> L2's CR3, and GUEST_CR3 rather contains the real-mode identity map.
>>>>>>>>> So we need to retrieve CR3 from the architectural state after
>>>>>>>>> conditionally updating it - and this is what kvm_read_cr3 does.
>>>>>>>>>
>>>>>>>> I have a headache from trying to think about it already, but
>>>>>>>> shouldn't
>>>>>>>> L1 be the one who setups identity map for L2? I traced what
>>>>>>>> vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) return here and do not
>>>>>>>> see
>>>>>>> Here is my understanding:
>>>>>>> In vmx_set_cr3(), if enabled ept, it will check whether target
>>>>>>> vcpu is enabling
>>>>>> paging. When L2 running in real mode, then target vcpu is not
>>>>>> enabling paging and it will use L0's identity map for L2. If you
>>>>>> read GUEST_CR3 from VMCS, then you may get the L2's identity map
>>>>>> not
>>>> L1's.
>>>>>>>
>>>>>> Yes, but why it makes sense to use L0 identity map for L2? I didn't
>>>>>> see different vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) values because
>>>>>> L0 and L1 use the same identity map address. When I changed identity
>>>>>> address L1 configures vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) are
>>>>>> indeed different, but the real CR3 L2 uses points to L0 identity map.
>>>>>> If I zero L1 identity map page L2 still works.
>>>>>>
>>>>> If L2 in real mode, then L2PA == L1PA. So L0's identity map also works
>>>>> if L2 is in real mode.
>>>>>
>>>> That not the point. It may work accidentally for kvm on kvm, but what
>>>> if other hypervisor plays different tricks and builds different ident map for its guest?
>>> Yes, if other hypervisor doesn't build the 1:1 mapping for its guest, it will fail to work. But I cannot imagine what kind of hypervisor will do this and what the purpose is.
>>> Anyway, current logic is definitely wrong. It should use L1's identity map instead L0's.
>>
>> So something like this is rather needed?
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 44494ed..60a3644 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -3375,8 +3375,10 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>>  	if (enable_ept) {
>>  		eptp = construct_eptp(cr3);
>>  		vmcs_write64(EPT_POINTER, eptp);
>> -		guest_cr3 = is_paging(vcpu) ? kvm_read_cr3(vcpu) :
>> -			vcpu->kvm->arch.ept_identity_map_addr;
>> +		if (is_paging(vcpu) || is_guest_mode(vcpu))
>> +			guest_cr3 = kvm_read_cr3(vcpu) :
>> +		else
>> +			guest_cr3 = vcpu->kvm->arch.ept_identity_map_addr;
>>  		ept_load_pdptrs(vcpu);
>>  	}
>>  
> That what I am thinking, will think about it some more tomorrow.

OK, and I'll feed it into a local test.

Jan

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