Re: [PATCH] kvm: nVMX: VMWRITE checks VMCS-link pointer before VMCS field

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Jim Mattson <jmattson@google.com> writes:

> According to the SDM, a VMWRITE in VMX non-root operation with an
> invalid VMCS-link pointer results in VMfailInvalid before the validity
> of the VMCS field in the secondary source operand is checked.
>
> Fixes: 6d894f498f5d1 ("KVM: nVMX: vmread/vmwrite: Use shadow vmcs12 if running L2")
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 38 +++++++++++++++++++-------------------
>  1 file changed, 19 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 4aea7d304beb..146e1b40c69f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4864,6 +4864,25 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
>  	if (vmx->nested.current_vmptr == -1ull)
>  		return nested_vmx_failInvalid(vcpu);
>  
> +	if (!is_guest_mode(vcpu)) {
> +		vmcs12 = get_vmcs12(vcpu);
> +
> +		/*
> +		 * Ensure vmcs12 is up-to-date before any VMWRITE that dirties
> +		 * vmcs12, else we may crush a field or consume a stale value.
> +		 */
> +		if (!is_shadow_field_rw(field))
> +			copy_vmcs02_to_vmcs12_rare(vcpu, vmcs12);

But (unless I'm missing some pre-requisite patch) we haven't set 'field'
yet, it happens later when we do

field = kvm_register_readl(vcpu, (((vmx_instruction_info) >> 28) & 0xf));

> +	} else {
> +		/*
> +		 * When vmcs->vmcs_link_pointer is -1ull, any VMWRITE
> +		 * to shadowed-field sets the ALU flags for VMfailInvalid.
> +		 */
> +		if (get_vmcs12(vcpu)->vmcs_link_pointer == -1ull)
> +			return nested_vmx_failInvalid(vcpu);
> +		vmcs12 = get_shadow_vmcs12(vcpu);
> +	}
> +
>  	if (vmx_instruction_info & (1u << 10))
>  		field_value = kvm_register_readl(vcpu,
>  			(((vmx_instruction_info) >> 3) & 0xf));
> @@ -4889,25 +4908,6 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
>  		return nested_vmx_failValid(vcpu,
>  			VMXERR_VMWRITE_READ_ONLY_VMCS_COMPONENT);
>  
> -	if (!is_guest_mode(vcpu)) {
> -		vmcs12 = get_vmcs12(vcpu);
> -
> -		/*
> -		 * Ensure vmcs12 is up-to-date before any VMWRITE that dirties
> -		 * vmcs12, else we may crush a field or consume a stale value.
> -		 */
> -		if (!is_shadow_field_rw(field))
> -			copy_vmcs02_to_vmcs12_rare(vcpu, vmcs12);
> -	} else {
> -		/*
> -		 * When vmcs->vmcs_link_pointer is -1ull, any VMWRITE
> -		 * to shadowed-field sets the ALU flags for VMfailInvalid.
> -		 */
> -		if (get_vmcs12(vcpu)->vmcs_link_pointer == -1ull)
> -			return nested_vmx_failInvalid(vcpu);
> -		vmcs12 = get_shadow_vmcs12(vcpu);
> -	}
> -
>  	offset = vmcs_field_to_offset(field);
>  	if (offset < 0)
>  		return nested_vmx_failValid(vcpu,

-- 
Vitaly