Re: [RFC PATCH v3 07/27] x86/cpu/intel: Allow SGX virtualization without Launch Control support

Kai Huang
2021-01-27T07:35:55Z

On Tue, 26 Jan 2021 08:26:21 -0800 Dave Hansen wrote:
> On 1/26/21 1:30 AM, Kai Huang wrote:
> > --- a/arch/x86/kernel/cpu/feat_ctl.c
> > +++ b/arch/x86/kernel/cpu/feat_ctl.c
> > @@ -105,7 +105,8 @@ early_param("nosgx", nosgx);
> > void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> > {
> > bool tboot = tboot_enabled();
> > - bool enable_sgx;
> > + bool enable_vmx;
> > + bool enable_sgx_any, enable_sgx_kvm, enable_sgx_driver;
> > u64 msr;
> >
> > if (rdmsrl_safe(MSR_IA32_FEAT_CTL, &msr)) {
> > @@ -114,13 +115,22 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> > return;
> > }
> >
> > + enable_vmx = cpu_has(c, X86_FEATURE_VMX) &&
> > + IS_ENABLED(CONFIG_KVM_INTEL);
>
> The reason it's called 'enable_sgx' below is because this code is
> actually going to "enable sgx". This code does not "enable vmx". That
> makes this a badly-named variable. "vmx_enabled" or "vmx_available"
> would be better.

It will also try to enable VMX if feature control MSR is not locked by BIOS.
Please see below code:

"
> > - if (cpu_has(c, X86_FEATURE_VMX) && IS_ENABLED(CONFIG_KVM_INTEL)) {
> > + if (enable_vmx) {
> > msr |= FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX;
> >
> > if (tboot)
> > msr |= FEAT_CTL_VMX_ENABLED_INSIDE_SMX;
> > }
"

And if feature control MSR is locked, kernel cannot truly enable anything, but
can only print out msg in case BIOS disabled either VMX, or SGX, or SGX_LC, and
kernel wants to support that.

Does this make sense to you?

>
> > /*
> > - * Enable SGX if and only if the kernel supports SGX and Launch Control
> > - * is supported, i.e. disable SGX if the LE hash MSRs can't be written.
> > + * Enable SGX if and only if the kernel supports SGX. Require Launch
> > + * Control support if SGX virtualization is *not* supported, i.e.
> > + * disable SGX if the LE hash MSRs can't be written and SGX can't be
> > + * exposed to a KVM guest (which might support non-LC configurations).
> > */
>
> I hate this comment.
>
> /*
> * Separate out bare-metal SGX enabling from KVM. This allows
> * KVM guests to use SGX even if the kernel refuses to use it on
> * bare-metal. This happens if flexible Faunch Control is not
> * available.
> *

Thanks.

>
> > - enable_sgx = cpu_has(c, X86_FEATURE_SGX) &&
> > - cpu_has(c, X86_FEATURE_SGX_LC) &&
> > - IS_ENABLED(CONFIG_X86_SGX);
> > + enable_sgx_any = cpu_has(c, X86_FEATURE_SGX) &&
> > + cpu_has(c, X86_FEATURE_SGX1) &&
> > + IS_ENABLED(CONFIG_X86_SGX);
>
> The X86_FEATURE_SGX1 check seems to have snuck in here. Why?

Please see my reply to Sean's reply.

>
> > + enable_sgx_driver = enable_sgx_any &&
> > + cpu_has(c, X86_FEATURE_SGX_LC);
> > + enable_sgx_kvm = enable_sgx_any && enable_vmx &&
> > + IS_ENABLED(CONFIG_X86_SGX_KVM);
> >
> > if (msr & FEAT_CTL_LOCKED)
> > goto update_caps;
> > @@ -136,15 +146,18 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> > * i.e. KVM is enabled, to avoid unnecessarily adding an attack vector
> > * for the kernel, e.g. using VMX to hide malicious code.
> > */
> > - if (cpu_has(c, X86_FEATURE_VMX) && IS_ENABLED(CONFIG_KVM_INTEL)) {
> > + if (enable_vmx) {
> > msr |= FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX;
> >
> > if (tboot)
> > msr |= FEAT_CTL_VMX_ENABLED_INSIDE_SMX;
> > }
> >
> > - if (enable_sgx)
> > - msr |= FEAT_CTL_SGX_ENABLED | FEAT_CTL_SGX_LC_ENABLED;
> > + if (enable_sgx_kvm || enable_sgx_driver) {
> > + msr |= FEAT_CTL_SGX_ENABLED;
> > + if (enable_sgx_driver)
> > + msr |= FEAT_CTL_SGX_LC_ENABLED;
> > + }
> >
> > wrmsrl(MSR_IA32_FEAT_CTL, msr);
> >
> > @@ -167,10 +180,29 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> > }
> >
> > update_sgx:
> > - if (!(msr & FEAT_CTL_SGX_ENABLED) ||
> > - !(msr & FEAT_CTL_SGX_LC_ENABLED) || !enable_sgx) {
> > - if (enable_sgx)
> > - pr_err_once("SGX disabled by BIOS\n");
> > + if (!(msr & FEAT_CTL_SGX_ENABLED)) {
> > + if (enable_sgx_kvm || enable_sgx_driver)
> > + pr_err_once("SGX disabled by BIOS.\n");
> > clear_cpu_cap(c, X86_FEATURE_SGX);
> > + return;
> > + }
>
>
> Isn't there a pr_fmt here already? Won't these just look like:
>
> sgx: SGX disabled by BIOS.
>
> That seems a bit silly.

Please see my reply to Sean's reply.