linux-kernel - Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable

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Date:   Thu, 16 Jan 2020 00:49:19 +0200
From:   Liran Alon <liran.alon@...cle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization
 out of nested_enable_evmcs()



> On 15 Jan 2020, at 19:10, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> With fine grained VMX feature enablement QEMU>=4.2 tries to do KVM_SET_MSRS
> with default (matching CPU model) values and in case eVMCS is also enabled,
> fails.
> 
> It would be possible to drop VMX feature filtering completely and make
> this a guest's responsibility: if it decides to use eVMCS it should know
> which fields are available and which are not. Hyper-V mostly complies to
> this, however, there is at least one problematic control:
> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES
> which Hyper-V enables. As there is no 'apic_addr_field' in eVMCS, we
> fail to handle this properly in KVM. It is unclear how this is supposed
> to work, genuine Hyper-V doesn't expose the control so it is possible that
> this is just a bug (in Hyper-V).

Have you tried contacted someone at Hyper-V team about this?

> 
> Move VMX controls sanitization from nested_enable_evmcs() to vmx_get_msr(),
> this allows userspace to keep setting controls it wants and at the same
> time hides them from the guest.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> arch/x86/kvm/vmx/evmcs.c | 38 ++++++++++++++++++++++++++++++++------
> arch/x86/kvm/vmx/evmcs.h |  1 +
> arch/x86/kvm/vmx/vmx.c   | 10 ++++++++--
> 3 files changed, 41 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
> index 89c3e0caf39f..b5d6582ba589 100644
> --- a/arch/x86/kvm/vmx/evmcs.c
> +++ b/arch/x86/kvm/vmx/evmcs.c
> @@ -346,6 +346,38 @@ uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu)
>        return 0;
> }
> 
> +void nested_evmcs_filter_control_msr(u32 msr_index, u64 *pdata)
> +{
> +	u32 ctl_low = (u32)*pdata, ctl_high = (u32)(*pdata >> 32);

Nit: I dislike defining & initialising multiple local vars on same line.

> +	/*
> +	 * Enlightened VMCS doesn't have certain fields, make sure we don't
> +	 * expose unsupported controls to L1.
> +	 */
> +
> +	switch (msr_index) {
> +	case MSR_IA32_VMX_PINBASED_CTLS:
> +	case MSR_IA32_VMX_TRUE_PINBASED_CTLS:
> +		ctl_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
> +		break;
> +	case MSR_IA32_VMX_EXIT_CTLS:
> +	case MSR_IA32_VMX_TRUE_EXIT_CTLS:
> +		ctl_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
> +		break;
> +	case MSR_IA32_VMX_ENTRY_CTLS:
> +	case MSR_IA32_VMX_TRUE_ENTRY_CTLS:
> +		ctl_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
> +		break;
> +	case MSR_IA32_VMX_PROCBASED_CTLS2:
> +		ctl_high &= ~EVMCS1_UNSUPPORTED_2NDEXEC;
> +		break;
> +	case MSR_IA32_VMX_VMFUNC:
> +		ctl_low &= ~EVMCS1_UNSUPPORTED_VMFUNC;
> +		break;
> +	}
> +
> +	*pdata = ctl_low | ((u64)ctl_high << 32);
> +}
> +
> int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> 			uint16_t *vmcs_version)
> {
> @@ -356,11 +388,5 @@ int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> 	if (vmcs_version)
> 		*vmcs_version = nested_get_evmcs_version(vcpu);
> 
> -	vmx->nested.msrs.pinbased_ctls_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
> -	vmx->nested.msrs.entry_ctls_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
> -	vmx->nested.msrs.exit_ctls_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
> -	vmx->nested.msrs.secondary_ctls_high &= ~EVMCS1_UNSUPPORTED_2NDEXEC;
> -	vmx->nested.msrs.vmfunc_controls &= ~EVMCS1_UNSUPPORTED_VMFUNC;
> -
> 	return 0;
> }
> diff --git a/arch/x86/kvm/vmx/evmcs.h b/arch/x86/kvm/vmx/evmcs.h
> index 07ebf6882a45..b88d9807a796 100644
> --- a/arch/x86/kvm/vmx/evmcs.h
> +++ b/arch/x86/kvm/vmx/evmcs.h
> @@ -201,5 +201,6 @@ bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmcs_gpa);
> uint16_t nested_get_evmcs_version(struct kvm_vcpu *vcpu);
> int nested_enable_evmcs(struct kvm_vcpu *vcpu,
> 			uint16_t *vmcs_version);
> +void nested_evmcs_filter_control_msr(u32 msr_index, u64 *pdata);
> 
> #endif /* __KVM_X86_VMX_EVMCS_H */
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e3394c839dea..8eb74618b8d8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1849,8 +1849,14 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> 	case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
> 		if (!nested_vmx_allowed(vcpu))
> 			return 1;
> -		return vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
> -				       &msr_info->data);
> +		if (vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
> +				    &msr_info->data))
> +			return 1;
> +		if (!msr_info->host_initiated &&
> +		    vmx->nested.enlightened_vmcs_enabled)
> +			nested_evmcs_filter_control_msr(msr_info->index,
> +							&msr_info->data);
> +		break;

Nit: It seems more elegant to me to put the call to nested_evmcs_filter_control_msr() inside vmx_get_vmx_msr().

The patch itself makes sense to me and looks correct.
Reviewed-by: Liran Alon <liran.alon@...cle.com>

-Liran