Re: [RFC PATCH v5 06/26] x86/cpu/intel: Allow SGX virtualization without Launch Control support

Jarkko Sakkinen
2021-02-16T02:16:49Z

On Sun, Feb 14, 2021 at 02:29:05AM +1300, Kai Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> The kernel will currently disable all SGX support if the hardware does
> not support launch control. Make it more permissive to allow SGX
> virtualization on systems without Launch Control support. This will
> allow KVM to expose SGX to guests that have less-strict requirements on
> the availability of flexible launch control.
>
> Improve error message to distinguish between three cases. There are two
> cases where SGX support is completely disabled:
> 1) SGX has been disabled completely by the BIOS
> 2) SGX LC is locked by the BIOS. Bare-metal support is disabled because
> of LC unavailability. SGX virtualization is unavailable (because of
> Kconfig).
> One where it is partially available:
> 3) SGX LC is locked by the BIOS. Bare-metal support is disabled because
> of LC unavailability. SGX virtualization is supported.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Co-developed-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
> v4->v5:
>
> - No code change.
>
> v3->v4:
>
> - Removed cpu_has(X86_FEATURE_SGX1) check in enable_sgx_any, since it logically
> is not related to KVM SGX series, per Sean.
> - Changed declaration of variables to be in reverse-christmas tree style, per
> Jarkko.
>
> v2->v3:
>
> - Added to use 'enable_sgx_any', per Dave.
> - Changed to call clear_cpu_cap() directly, rather than using clear_sgx_caps()
> and clear_sgx_lc().
> - Changed to use CONFIG_X86_SGX_KVM, instead of CONFIG_X86_SGX_VIRTUALIZATION.
>
> v1->v2:
>
> - Refined commit message per Dave's comments.
> - Added check to only enable SGX virtualization when VMX is supported, per
> Dave's comment.
> - Refined error msg print to explicitly call out SGX virtualization will be
> supported when LC is locked by BIOS, per Dave's comment.
>
> ---
> arch/x86/kernel/cpu/feat_ctl.c | 57 ++++++++++++++++++++++++++--------
> 1 file changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/feat_ctl.c b/arch/x86/kernel/cpu/feat_ctl.c
> index 27533a6e04fa..96c370284913 100644
> --- 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_sgx_any, enable_sgx_kvm, enable_sgx_driver;
> + bool enable_vmx;
> u64 msr;
>
> if (rdmsrl_safe(MSR_IA32_FEAT_CTL, &msr)) {
> @@ -114,13 +115,21 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c)
> return;
> }
>
> + enable_vmx = cpu_has(c, X86_FEATURE_VMX) &&
> + IS_ENABLED(CONFIG_KVM_INTEL);

It's less than 100 characters:

enable_vmx = cpu_has(c, X86_FEATURE_VMX) && IS_ENABLED(CONFIG_KVM_INTEL);

This is better:

enable_vmx = IS_ENABLED(CONFIG_KVM_INTEL) && cpu_has(c, X86_FEATURE_VMX);

You only want to evaluate cpu_has() if COHNFIG_KVM_INTEL is enabled.

> +
> /*
> - * 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.
> + * Separate out SGX driver enabling from KVM. This allows KVM
> + * guests to use SGX even if the kernel SGX driver refuses to
> + * use it. 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) &&
> + IS_ENABLED(CONFIG_X86_SGX);
> + 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 +145,18 @@ 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 +179,29 @@ 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);

Empty line before return statement.

> + return;
> + }
> +
> + /*
> + * VMX feature bit may be cleared due to being disabled in BIOS,
> + * in which case SGX virtualization cannot be supported either.
> + */
> + if (!cpu_has(c, X86_FEATURE_VMX) && enable_sgx_kvm) {
> + pr_err_once("SGX virtualization disabled due to lack of VMX.\n");
> + enable_sgx_kvm = 0;
> + }
> +
> + if (!(msr & FEAT_CTL_SGX_LC_ENABLED) && enable_sgx_driver) {
> + if (!enable_sgx_kvm) {
> + pr_err_once("SGX Launch Control is locked. Disable SGX.\n");
> + clear_cpu_cap(c, X86_FEATURE_SGX);
> + } else {
> + pr_err_once("SGX Launch Control is locked. Support SGX virtualization only.\n");
> + clear_cpu_cap(c, X86_FEATURE_SGX_LC);
> + }
> }
> }
> --
> 2.29.2
>
>

/Jarkko