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 12:12:03 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.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 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@...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:

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

> 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.