KVM: VMX: Fix host MSR_KERNEL_GS_BASE corruption

enter_lmode() and exit_lmode() modify the guest's EFER.LMA before calling
vmx_set_efer().  However, the latter function depends on the value of EFER.LMA
to determine whether MSR_KERNEL_GS_BASE needs reloading, via
vmx_load_host_state().  With EFER.LMA changing under its feet, it took the
wrong choice and corrupted userspace's %gs.

This causes 32-on-64 host userspace to fault.

Fix not touching EFER.LMA; instead ask vmx_set_efer() to change it.
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/vmx.c

@@ -1744,18 +1744,15 @@ static void enter_lmode(struct kvm_vcpu *vcpu)
 			     (guest_tr_ar & ~AR_TYPE_MASK)
 			     | AR_TYPE_BUSY_64_TSS);
 	}
-	vcpu->arch.efer |= EFER_LMA;
-	vmx_set_efer(vcpu, vcpu->arch.efer);
+	vmx_set_efer(vcpu, vcpu->arch.efer | EFER_LMA);
 }
 
 static void exit_lmode(struct kvm_vcpu *vcpu)
 {
-	vcpu->arch.efer &= ~EFER_LMA;
-
 	vmcs_write32(VM_ENTRY_CONTROLS,
 		     vmcs_read32(VM_ENTRY_CONTROLS)
 		     & ~VM_ENTRY_IA32E_MODE);
-	vmx_set_efer(vcpu, vcpu->arch.efer);
+	vmx_set_efer(vcpu, vcpu->arch.efer & ~EFER_LMA);
 }
 
 #endif