linux-kernel - Re: [PATCH] KVM: nVMX: remove incorrect vpid check in nested invvpid emulation

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Date:	Wed, 25 Nov 2015 17:21:20 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>,
	Haozhong Zhang <haozhong.zhang@...el.com>
Cc:	kvm@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: nVMX: remove incorrect vpid check in nested invvpid
 emulation



On 25/11/2015 16:45, Bandan Das wrote:
> Haozhong Zhang <haozhong.zhang@...el.com> writes:
> 
>> This patch removes the vpid check when emulating nested invvpid
>> instruction of type all-contexts invalidation. The existing code is
>> incorrect because:
>>  (1) According to Intel SDM Vol 3, Section "INVVPID - Invalidate
>>      Translations Based on VPID", invvpid instruction does not check
>>      vpid in the invvpid descriptor when its type is all-contexts
>>      invalidation.
> 
> But iirc isn't vpid=0 reserved for root mode ? I think we don't want
> L1 hypervisor to be able do a invvpid(0).

The instruction simply "invalidates all mappings tagged with all
non-zero VPIDs", which in our case is all L0 mappings tagged with vpid02.

Paolo

>>  (2) According to the same document, invvpid of type all-contexts
>>      invalidation does not require there is an active VMCS, so/and
>>      get_vmcs12() in the existing code may result in a NULL-pointer
>>      dereference. In practice, it can crash both KVM itself and L1
>>      hypervisors that use invvpid (e.g. Xen).
> 
> If that is the case, then just check if it's null and return without
> doing anything.
> 
>> Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
>> ---
>>  arch/x86/kvm/vmx.c | 5 -----
>>  1 file changed, 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 87acc52..af823a3 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -7394,11 +7394,6 @@ static int handle_invvpid(struct kvm_vcpu *vcpu)
>>  
>>  	switch (type) {
>>  	case VMX_VPID_EXTENT_ALL_CONTEXT:
>> -		if (get_vmcs12(vcpu)->virtual_processor_id == 0) {
>> -			nested_vmx_failValid(vcpu,
>> -				VMXERR_INVALID_OPERAND_TO_INVEPT_INVVPID);
>> -			return 1;
>> -		}
>>  		__vmx_flush_tlb(vcpu, to_vmx(vcpu)->nested.vpid02);
>>  		nested_vmx_succeed(vcpu);
>>  		break;
> 
> I also noticed a BUG() here in the default. It might be a good idea to replace
> it with a WARN.
> 
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