KVM: x86: Split kvm_is_valid_cr4() and export only the non-vendor bits

Split the common x86 parts of kvm_is_valid_cr4(), i.e. the reserved bits
checks, into a separate helper, __kvm_is_valid_cr4(), and export only the
inner helper to vendor code in order to prevent nested VMX from calling
back into vmx_is_valid_cr4() via kvm_is_valid_cr4().

On SVM, this is a nop as SVM doesn't place any additional restrictions on
CR4.

On VMX, this is also currently a nop, but only because nested VMX is
missing checks on reserved CR4 bits for nested VM-Enter.  That bug will
be fixed in a future patch, and could simply use kvm_is_valid_cr4() as-is,
but nVMX has _another_ bug where VMXON emulation doesn't enforce VMX's
restrictions on CR0/CR4.  The cleanest and most intuitive way to fix the
VMXON bug is to use nested_host_cr{0,4}_valid().  If the CR4 variant
routes through kvm_is_valid_cr4(), using nested_host_cr4_valid() won't do
the right thing for the VMXON case as vmx_is_valid_cr4() enforces VMX's
restrictions if and only if the vCPU is post-VMXON.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220607213604.3346000-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

arch/x86/kvm/svm/nested.c

@@ -325,7 +325,8 @@ static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,
 			return false;
 	}
 
-	if (CC(!kvm_is_valid_cr4(vcpu, save->cr4)))
+	/* Note, SVM doesn't have any additional restrictions on CR4. */
+	if (CC(!__kvm_is_valid_cr4(vcpu, save->cr4)))
 		return false;
 
 	if (CC(!kvm_valid_efer(vcpu, save->efer)))

arch/x86/kvm/vmx/vmx.c

@@ -3238,8 +3238,8 @@ static bool vmx_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
 	/*
 	 * We operate under the default treatment of SMM, so VMX cannot be
-	 * enabled under SMM.  Note, whether or not VMXE is allowed at all is
-	 * handled by kvm_is_valid_cr4().
+	 * enabled under SMM.  Note, whether or not VMXE is allowed at all,
+	 * i.e. is a reserved bit, is handled by common x86 code.
 	 */
 	if ((cr4 & X86_CR4_VMXE) && is_smm(vcpu))
 		return false;

arch/x86/kvm/x86.c

@@ -1081,17 +1081,23 @@ int kvm_emulate_xsetbv(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_xsetbv);
 
-bool kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
 	if (cr4 & cr4_reserved_bits)
 		return false;
 
 	if (cr4 & vcpu->arch.cr4_guest_rsvd_bits)
 		return false;
 
-	return static_call(kvm_x86_is_valid_cr4)(vcpu, cr4);
+	return true;
+}
+EXPORT_SYMBOL_GPL(__kvm_is_valid_cr4);
+
+static bool kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+{
+	return __kvm_is_valid_cr4(vcpu, cr4) &&
+	       static_call(kvm_x86_is_valid_cr4)(vcpu, cr4);
 }
-EXPORT_SYMBOL_GPL(kvm_is_valid_cr4);
 
 void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned long cr4)
 {

arch/x86/kvm/x86.h

@@ -434,7 +434,7 @@ static inline void kvm_machine_check(void)
 void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu);
 void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu);
 int kvm_spec_ctrl_test_value(u64 value);
-bool kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4);
+bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4);
 int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
 			      struct x86_exception *e);
 int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva);