KVM: VMX: Intercept guest reserved CR4 bits to inject #GP fault

Intercept CR4 bits that are guest reserved so that KVM correctly injects
a #GP fault if the guest attempts to set a reserved bit.  If a feature
is supported by the CPU but is not exposed to the guest, and its
associated CR4 bit is not intercepted by KVM by default, then KVM will
fail to inject a #GP if the guest sets the CR4 bit without triggering
an exit, e.g. by toggling only the bit in question.

Note, KVM doesn't give the guest direct access to any CR4 bits that are
also dependent on guest CPUID.  Yet.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200930041659.28181-5-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -4053,13 +4053,16 @@ void vmx_set_constant_host_state(struct vcpu_vmx *vmx)
 
 void set_cr4_guest_host_mask(struct vcpu_vmx *vmx)
 {
-	vmx->vcpu.arch.cr4_guest_owned_bits = KVM_POSSIBLE_CR4_GUEST_BITS;
+	struct kvm_vcpu *vcpu = &vmx->vcpu;
+
+	vcpu->arch.cr4_guest_owned_bits = KVM_POSSIBLE_CR4_GUEST_BITS &
+					  ~vcpu->arch.cr4_guest_rsvd_bits;
 	if (!enable_ept)
-		vmx->vcpu.arch.cr4_guest_owned_bits &= ~X86_CR4_PGE;
+		vcpu->arch.cr4_guest_owned_bits &= ~X86_CR4_PGE;
 	if (is_guest_mode(&vmx->vcpu))
-		vmx->vcpu.arch.cr4_guest_owned_bits &=
-			~get_vmcs12(&vmx->vcpu)->cr4_guest_host_mask;
-	vmcs_writel(CR4_GUEST_HOST_MASK, ~vmx->vcpu.arch.cr4_guest_owned_bits);
+		vcpu->arch.cr4_guest_owned_bits &=
+			~get_vmcs12(vcpu)->cr4_guest_host_mask;
+	vmcs_writel(CR4_GUEST_HOST_MASK, ~vcpu->arch.cr4_guest_owned_bits);
 }
 
 u32 vmx_pin_based_exec_ctrl(struct vcpu_vmx *vmx)
@@ -7249,6 +7252,8 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 		}
 	}
 
+	set_cr4_guest_host_mask(vmx);
+
 	/* Refresh #PF interception to account for MAXPHYADDR changes. */
 	update_exception_bitmap(vcpu);
 }