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:   Thu, 6 Apr 2023 03:02:29 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "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, 2023-04-05 at 18:44 -0700, Sean Christopherson wrote:
> 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.

The above check doesn't verify xfrm is a super set of 0x3.  I think we verify
that per SDM:

39.7.3 Processor Extended States and ENCLS[ECREATE]

The ECREATE leaf function of the ENCLS instruction enforces a number of
consistency checks described earlier. The execution of ENCLS[ECREATE] leaf
function results in a #GP(0) in any of the following cases:
  • SECS.ATTRIBUTES.XFRM[1:0] is not 3.
  • The processor does not support XSAVE and any of the following is true:
	— SECS.ATTRIBUTES.XFRM[63:2] is not 0.
	— SECS.SSAFRAMESIZE is 0.
  • The processor supports XSAVE and any of the following is true:
	— XSETBV would fault on an attempt to load XFRM into XCR0.
	— XFRM[63]=1.
	— The SSAFRAME is too small to hold required, enabled states ...


And in the ECREATE pseudo code, the relevant parts seem to be:

	(* Check lower 2 bits of XFRM are set *)
	IF ( ( DS:TMP_SECS.ATTRIBUTES.XFRM BitwiseAND 03H) ≠ 03H)
		THEN #GP(0); FI;

	IF (XFRM is illegal)
		THEN #GP(0); FI;

The first part is clear, but the second part is vague. 

I am not sure in hardware behaviour, whether XCR0 is actually checked in
ECREATE.  It's more likely XCRO is actually checked in EENTER.  

But I think it's just fine to also check against XCR0 here.