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:
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.
