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

Dave Hansen
2021-01-26T16:28:17Z

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.

> /*
> - * 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.
*

> - 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?

> + 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.