linux-kernel - Re: [PATCH 1/3] KVM: VMX: Don't rely _only_ on CPUID to enforce XCR0 restrictions for ECREATE

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Message-ID: <626179c54707297736158da89ee634705cd6d62f.camel@intel.com>
Date:   Wed, 5 Apr 2023 10:52:59 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] KVM: VMX: Don't rely _only_ on CPUID to enforce XCR0
 restrictions for ECREATE

On Tue, 2023-04-04 at 17:59 -0700, Sean Christopherson wrote:
> Explicitly check the vCPU's supported XCR0 when determining whether or not
> the XFRM for ECREATE is valid.  Checking CPUID works because KVM updates
> guest CPUID.0x12.1 to restrict the leaf to a subset of the guest's allowed
> XCR0, but that is rather subtle and KVM should not modify guest CPUID
> except for modeling true runtime behavior (allowed XFRM is most definitely
> not "runtime" behavior).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/sgx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/sgx.c b/arch/x86/kvm/vmx/sgx.c
> index aa53c98034bf..362a31b19b0e 100644
> --- a/arch/x86/kvm/vmx/sgx.c
> +++ b/arch/x86/kvm/vmx/sgx.c
> @@ -175,7 +175,8 @@ static int __handle_encls_ecreate(struct kvm_vcpu *vcpu,
>  	    (u32)attributes & ~sgx_12_1->eax ||
>  	    (u32)(attributes >> 32) & ~sgx_12_1->ebx ||
>  	    (u32)xfrm & ~sgx_12_1->ecx ||
> -	    (u32)(xfrm >> 32) & ~sgx_12_1->edx) {
> +	    (u32)(xfrm >> 32) & ~sgx_12_1->edx ||
> +	    xfrm & ~vcpu->arch.guest_supported_xcr0) {

Perhaps this change is needed even without patch 2?

This is because when CPUID 0xD doesn't exist, guest_supported_xcr0 is 0.  But
when CPUID 0xD doesn't exist, IIUC currently KVM doesn't clear SGX in CPUID, and
sgx_12_1->ecx is always set to 0x3.  

__handle_encls_ereate() doesn't check CPUID 0xD either, so w/o above explicit
check xfrm against guest_supported_xcr0, it seems guest can successfully run
ECREATE when it doesn't have CPUID 0xD?


>  		kvm_inject_gp(vcpu, 0);
>  		return 1;
>  	}
> -- 
> 2.40.0.348.gf938b09366-goog
>