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.
