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

[Huang, Kai]

On Thu, 2023-04-06 at 12:12 -0700, Sean Christopherson wrote:
> On Thu, Apr 06, 2023, Huang, Kai wrote:
> > 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@xxxxxxxxxx>
> > > > > ---
> > > > >  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:
> 
> Oooh, right.  It's not that FP+SSE are always allowed, it's that FP+SSE must always
> be _set_.  So this?
> 
> 		xfrm & ~(vcpu->arch.guest_supported_xcr0 | XFEATURE_MASK_FPSSE) ||
> 		(xfrm & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE

Looks good.

I'll try to get some test done with this code change.

> 
> > 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.
> 
> ECREATE doesn't check XCR0, it checks that XFRM represents a legal XCR0 values
> for the platform, which in KVM is tracked as guest_supported_xcr0.

Yes agreed.