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.
