[PATCH v2 2/4] KVM: nVMX: Don't stuff secondary execution control if it's not supported

[Sean Christopherson]

When stuffing the allowed secondary execution controls for nested VMX in
response to CPUID updates, don't set the allowed-1 bit for a feature that
isn't supported by KVM, i.e. isn't allowed by the canonical vmcs_config.

WARN if KVM attempts to manipulate a feature that isn't supported.  All
features that are currently stuffed are always advertised to L1 for
nested VMX if they are supported in KVM's base configuration, and no
additional features should ever be added to the CPUID-induced stuffing
(updating VMX MSRs in response to CPUID updates is a long-standing KVM
flaw that is slowly being fixed).

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fe5615fd8295..13d3f5eb4c32 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4459,6 +4459,16 @@ vmx_adjust_secondary_exec_control(struct vcpu_vmx *vmx, u32 *exec_control,
 	 * controls for features that are/aren't exposed to the guest.
 	 */
 	if (nested) {
+		/*
+		 * All features that got grandfathered into KVM's flawed CPUID-
+		 * induced manipulation of VMX MSRs are unconditionally exposed
+		 * to L1 if the feature is supported by KVM (for nested).  I.e.
+		 * KVM should never attempt to stuff a feature that isn't
+		 * already exposed to L1 for nested virtualization.
+		 */
+		if (WARN_ON_ONCE(!(vmcs_config.nested.secondary_ctls_high & control)))
+			enabled = false;
+
 		if (enabled)
 			vmx->nested.msrs.secondary_ctls_high |= control;
 		else
-- 
2.39.0.rc1.256.g54fd8350bd-goog