linux-kernel - Re: [PATCH 4/5] KVM: VMX: Disallow PT MSRs accessing if PT is not exposed to guest

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Message-ID: <YSUALsBF8rKNPiaS@google.com>
Date:   Tue, 24 Aug 2021 14:20:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] KVM: VMX: Disallow PT MSRs accessing if PT is not
 exposed to guest

On Tue, Aug 24, 2021, Xiaoyao Li wrote:
> Per SDM, it triggers #GP for all the accessing of PT MSRs, if
> X86_FEATURE_INTEL_PT is not available.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4a70a6d2f442..1bbc4d84c623 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1010,9 +1010,16 @@ static unsigned long segment_base(u16 selector)
>  static inline bool pt_can_write_msr(struct vcpu_vmx *vmx)
>  {
>  	return vmx_pt_mode_is_host_guest() &&
> +	       guest_cpuid_has(&vmx->vcpu, X86_FEATURE_INTEL_PT) &&
>  	       !(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN);
>  }
>  
> +static inline bool pt_can_read_msr(struct kvm_vcpu *vcpu)
> +{
> +	return vmx_pt_mode_is_host_guest() &&
> +	       guest_cpuid_has(vcpu, X86_FEATURE_INTEL_PT);
> +}
> +
>  static inline bool pt_output_base_valid(struct kvm_vcpu *vcpu, u64 base)
>  {
>  	/* The base must be 128-byte aligned and a legal physical address. */
> @@ -1849,24 +1856,24 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  							&msr_info->data);
>  		break;
>  	case MSR_IA32_RTIT_CTL:
> -		if (!vmx_pt_mode_is_host_guest())
> +		if (!pt_can_read_msr(vcpu))

These all need to provide exemptions for accesses from the host.  KVM allows
access to MSRs that are not exposed to the guest so long as all the other checks
pass.  Same for the next patch.

Easiest thing is probably to pass in @msr_info to the helpers and do the check
there.

>  			return 1;
>  		msr_info->data = vmx->pt_desc.guest.ctl;
>  		break;