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

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Date:   Wed, 5 Apr 2023 18:44:16 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "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 Wed, Apr 05, 2023, Huang, Kai wrote:
> 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.

Hrm, that's a bug in this patch.  Drat.  More below.

> __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?

ECREATE doesn't have a strict dependency on CPUID 0xD or XSAVE.  This exact scenario
is called out in the SDM:

  Legal values for SECS.ATTRIBUTES.XFRM conform to these requirements:
    * XFRM[1:0] must be set to 0x3.
    * If the processor does support XSAVE, XFRM must contain a value that would
      be legal if loaded into XCR0.
    * If the processor does not support XSAVE, or if the system software has not
      enabled XSAVE, then XFRM[63:2] must be zero.

So the above needs to be either

	xfrm & ~(vcpu->arch.guest_supported_xcr0 | XFEATURE_MASK_FPSSE)

or

	(xfrm & ~XFEATURE_MASK_FPSSE & ~vcpu->arch.guest_supported_xcr0)


I think I prefer the first one as I find it slightly more obvious that FP+SSE are
allowed in addition to the XCR0 bits.