[RFC PATCH v6 06/25] x86/cpu/intel: Allow SGX virtualization without Launch Control support

[Kai Huang <kai.huang@intel.com>]

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>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
---
v5->v6:

 - No code change. Added Dave's Acked-by.

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);
+
 	/*
-	 * 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);
+		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